Fabrication & Thermophysical Studies of Hexa Ferrites
By
Ghulam Asghar
CIIT/SP05-PPH-003/ISB
PhD Thesis
In
Physics
COMSATS Institute of Information Technology
Islamabad-Pakistan
Spring 2011
COMSATS Institute of Information Technology
Fabrication & Thermophysical Studies of Hexa Ferrites
A Thesis Presented to
COMSATS Institute of Information Technology, Islamabad
In partial fulfillment
of the requirement for the degree of
PhD Physics
By
Ghulam Asghar
CIIT/SP05-PPH-003/ISB
Spring, 2011
Fabrication & Thermophysical Studies of Hexa Ferrites
A Post Graduate Thesis submitted to the Department of Physics as partial
fulfillment of the requirement for the award of Degree of Ph. D. (Physics).
Name Registration No.
Ghulam Asghar CIIT/SP05-PPH-003/ISB
Supervisor
Dr. Muhammad Anis-ur-Rehman
Associate Professor, Department of Physics,
COMSATS Institute of Information Technology (CIIT),
Islamabad Campus.
June, 2011
Final Approval
This thesis titled
Fabrication & Thermophysical Studies of Hexa Ferrites
By
Ghulam Asghar
Registration No. CIIT/SP05-PPH-003/ISB
has been approved
for the COMSATS Institute of Information Technology, Islamabad
External Examiner:________________________________
Dr.
Supervisor: ___________________________________
Dr. M. Anis-ur-Rehman
Department of Physics/Islamabad
HoD:______________________________________
Dr. Ishaq Ahmed
HoD (Department of Physics/ Islamabad)
Dean, Faculty of Science: ______________________
Prof. Dr. Arshad Saleem Bhatti
Declaration
I Mr. Ghulam Asghar Reg. # CIIT/SP05-PPH-003/ISB, hereby declare that I have
produced the work presented in this thesis, during the scheduled period of study. I also
declare that I have not taken any material from any source except referred to wherever due
that amount of plagiarism is within acceptable range. If a violation of HEC rules on research
has occurred in this thesis, I shall be liable to punishable action under the plagiarism rules of
the HEC.
Date: _________________ Signature of the student:
___________________________
Ghulam Asghar
Reg. # CIIT/SP05-PPH-003/ISB
Certificate
It is certified that Mr. Ghulam Asghar Reg. # CIIT/SP05-PPH-003/ISB has carried
out all the work related to this thesis under my supervision at the Department of Physics,
COMSATS Institute of Information Technology, Islamabad and the work fulfills the
requirement for award of Ph. D degree.
Date: _________________
Supervisor:
_________________________
Dr. Muhammad Anis-ur-Rehman,
Associate Professor
Head of the Department:
_____________________________
Dr. Mahnaz Qadir Haseeb
Associate Professor
Department of Physics
Dedication
This dissertation is dedicated to my mother and father who prayed for me a lot.
Acknowledgements
All praise to Almighty Allah, the most gracious and merciful, whose blessings are
unlimited, and who blessed me with opportunity to pay my contribution in efforts to explore
some facts of his created striking and outstanding universe. Without the help and blessing of
my Allah, I was unable to complete my project. Countless prays for his holy prophet
Muhammad (P.B.U.H), who is forever, a light of guidance and wisdom for all humanity.
Special gratitude to my supervisor Dr. Muhammad Anis-ur-Rehman, for all kinds
of support he has provided me. I am sincerely obliged to him for providing the freedom with
respect to the research activities. It was his encouragement so that the project has been
completed. I have really no words to express my thoughts for him. God bless him all the
time in every walk of life.
I would like to pay my indebtedness to Prof. Dr. Arshad Saleem Bhatti, Dean of
Sciences, who remains always a source of courage for me. I am also grateful to Prof. Dr.
Sajid Qammar, Chairman of the Department of Physics for providing me research facilities
at CIIT, Islamabad and Dr. Ishaq Ahmad, for his continuous guidance.
I am very thankful to Dr. M. Ashraf Atta for his noble cooperation and to all my
teachers especially for their cooperation and healthy suggestions during study and research
work at COMSATS Institute of Information Technology Islamabad, Pakistan. Higher
Education Commission (HEC), Pakistan is highly acknowledged for providing financial
support through “Indigenous 5000 Scholarship Program”. This scholarship policy made it
possible for me to do this work. I also owe my profound thanks to Dr. Atta-ur-Rahman,
Ex. Chairman Higher Education Commission (HEC), Pakistan.
I would like to acknowledge Dr. M. Saif Ullah Awan, for providing facilities of some
of the characterization during my experimental work. My special thanks goes to my cousin
Ghulam Hasnain Tariq and Syed Nasir Khusro for their help in research work and to my other
lab fellows, Muhammad Akram, Ali Abdullah, Anwar-ul-Haq, M. Yasin Shami, Awais
Siddique Saleemi, Muhammad Mubeen, Muhammad Ali Malik, Ms. Zeb-un-Nisa, Ms.
Mariam Ansari, Ms. Khush Bakhat and Ms Fatima-Tuz-Zahra for helping me whenever I feel
stuck. I would also like to express my gratitude to Rafaqat Hussain, Niaz Ahmed Niaz,
Rizwan Ahmed Khan, Zahoor Ahmad, Muhammad Farooq Nasir and M. Hafeez.
I am deeply grateful to my family members, their love for me and providing me
encouragement at every step of life. I am really gratified to my daughter Areeba Naureen and
Inshraah Asghar who missed me very much whenever they wanted me with them; actually
their sacrifices are far above my words.
Ghulam Asghar
CIIT/SP05-PPH-003/ISB
Abstract
Fabrication & Thermophysical Studies of Hexa Ferrites
Strontium hexaferrite nano material with nominal composition SrFe12O19 is prepared
by wet chemical methods. The effect of variation in synthesis parameters such as molar ratio
of cations (Fe/Sr), volume rate of addition of precipitating agent and the pH of the solution on
the phase purity and particle size is studied to optimize them for the synthesis by co-
precipitation method. The effect of molar ratio of cations (Fe/Sr) on phase purity is studied by
using X-ray diffraction (XRD) patterns. It is observed from indexed XRD patterns that molar
ratio of cations does not affect the phase purity of strontium hexaferrites as there is no
impurity peak present in any sample and all patterns are almost similar. The effect of volume
rate of addition of precipitating agent on phase purity and surface morphology are analyzed
by using XRD diffraction patterns and scanning electron micrographs (SEM). The indexed
XRD patterns show that the increase in the volume rate of addition of precipitating agent
improves the phase purity and SEM micrographs show that the size of the particles also
decrease with the increase in the volume rate of addition of precipitating agent. The effect of
pH variation on structural and electrical properties of strontium hexaferrite is analyzed by
using X-ray diffractometer, scanning electron microscope, temperature dependent dc
resistivity measurement system and precision component analyzer. Indexed XRD patterns
show that the secondary phases are decreased with the increase in pH of the solution and
single phase strontium hexaferrite is obtained for pH=13. The pH of the solution also imparts
a significant effect on structural morphology of prepared hexaferrite samples. The SEM
micrographs with varying pH samples clearly indicate that most of the particles are of
hexagonal shape. It can also be seen that the particle size and their distribution also decrease
with the increase in the pH of the solution. The dc resistivity is also increased by increasing
pH and this may be due to increase in the grain boundaries.
The composition SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) is prepared in order to
increase the coercivity of strontium hexaferrites. Results obtained indicate that Cr doping
causes the formation of secondary phases. It is also observed that for X ≤ 0.6, both dielectric
constant and coercivity is increased while saturation magnetization is decreased. The increase
in coercivity was due to variation in particle size and impurity phases which acted as pinning
centers. The decrease in saturation magnetization is because of the replacement of cation
(Fe3+) having high magnetic moment (5µB) on octahedral sites with cation (Cr3+) having
smaller magnetic moment (3µB). Another composition SrFe12-2xCrxZnxO19 with (X=0.0, 0.2,
0.4, 0.6, 0.8) is prepared with co-precipitation method in order to reduce the dielectric loss
tangent. The results show that Cr-Zn doping causes increase in the particle size and decrease
in dielectric loss tangent and make the strontium hexaferrite useful for high frequency
applications. The hysteresis loops of the Cr-Zn doped samples reveal that both coercivity and
saturation magnetization is decreased with increase in doping concentration. The same
composition SrFe12-2xCrxZnxO19 with x=0.0, 0.2, 0.4, 0.6, 0.8 is synthesized with WOWS sol-
gel method (WOWS stands for Without Water and Surfactants; a new simplified sol-gel
method developed in our lab). The structural and dielectric measurements results obtained
from the samples prepared with WOWS sol-gel method are better than the results obtained
from the same composition prepared with co-precipitation.
In some cases, the materials with high loss as well as high dielectric constant may be
desired in applications such as electromagnetic (EM) wave absorbing coatings. To achieve
these properties, reduction of oxygen from sintered SrFe12O19 is made. This treatment resulted
in the increase in the concentration of Fe2+ ions and free iron atoms and hence in the increase
in both dielectric constant and dielectric loss and making the material useful for microwave
absorption.
Table of Contents
Chapter 1 ............................................................................................................. 1
Introduction ......................................................................................................... 1
1. Introduction ..................................................................................................... 2
1.1 Nano science .............................................................................................. 2
1.2 Types of ferrites ............................................................................................. 3
1.2.1 Soft ferrites ............................................................................................. 3
1.2.2 Hard ferrites ............................................................................................ 4
1.3 Classification of hexaferrites ......................................................................... 4
1.4 M-type hexaferrites ....................................................................................... 5
1.5 Structural properties of strontium hexaferrites (SrM) ................................... 6
1.6 Magnetic properties of M-type strontium hexaferrites ................................. 8
1.6.1 Source of magnetism .............................................................................. 8
1.6.2 Classification of magnetic materials ....................................................... 9
1.6.3 Ferrimagnetism in ferrites ....................................................................... 9
1.6.4 Superexchange interaction .................................................................... 11
1.6.5 Magnetocrystalline anisotropy.............................................................. 12
1.6.6 Magnetic structure of M-type strontium hexaferrite (SrM).................. 12
1.6.7 Literature review about magnetic properties of SrM ............................ 14
1.7 The dc electrical properties of M-type strontium hexaferrites ................... 16
1.7.1 Temperature dependent dc electrical resistivity ................................... 16
1.8 Frequency dependent dielectric properties of strontium hexaferrites ......... 17
1.8.1 Electronic polarization .......................................................................... 18
1.8.2 Ionic polarization .................................................................................. 18
1.8.3 Orientation polarization ........................................................................ 19
1.8.4 Hyperelectronic polarization ................................................................ 19
1.8.5 Space charge polarization ..................................................................... 20
1.8.6 The dielectric constant .......................................................................... 21
1.8.7 The dielectric loss ................................................................................. 24
1.8.8 Literature review about frequency dependent electrical properties of
SrM ................................................................................................................ 25
1.8.9 Ferromagnetic resonance and Snoek’s Limit ....................................... 26
1.9 Mechanical properties of ferrites ................................................................. 31
1.10 Thermal transport properties ..................................................................... 32
1.11 Chemical stability ...................................................................................... 32
1.12 Applications of M-type hexaferrites ......................................................... 32
1.13 Motivation and objectives ......................................................................... 34
Chapter 2 ........................................................................................................... 38
Characterization Techniques ............................................................................. 38
2. Characterization Techniques ......................................................................... 39
2.1 X-ray diffraction .......................................................................................... 39
2.2 Scanning Electron Microscopy (SEM) ....................................................... 42
2.3 Frequency dependent ac measurements ...................................................... 44
2.4 Temperature dependent dc resistivity measurements .................................. 45
2.5 Magnetic hysteresis ..................................................................................... 48
Chapter 3 ........................................................................................................... 51
Synthesis techniques .......................................................................................... 51
3. Synthesis techniques ...................................................................................... 52
3.1 Solid state reaction method ...................................................................... 52
3.2 Wet chemical method .............................................................................. 52
3.2.1 Precursor methods ............................................................................. 53
3.2.2 Spray-drying ...................................................................................... 53
3.2.3 Freeze-drying method ....................................................................... 53
3.2.4 Microemulsion method ..................................................................... 53
3.2.5 Hydrothermal method ....................................................................... 54
3.2.6 Sol-gel method .................................................................................. 54
3.2.7 WOWS sol-gel method ..................................................................... 55
3.2.8 Co-precipitation method ................................................................... 55
Chapter 4 ........................................................................................................... 58
Optimization of synthesis parameters for phase purity ..................................... 58
4. Optimization of synthesis parameters for phase purity ................................. 59
4.1 Synthesis .................................................................................................. 59
4.2 Effect of variation in molar ratio (Fe/Sr) on structural properties of SrM
....................................................................................................................... 60
4.2.1 Conclusion ........................................................................................ 61
4.3 Volume rate of addition of precipitating agent on structural properties of
SrM ................................................................................................................ 62
4.3.1 Conclusion ........................................................................................ 64
4.4 Effect of variation in pH on structural and electrical properties of
strontium hexaferrites (SrM) ......................................................................... 64
4.4.1 Effect of pH on the structural properties of SrM .............................. 65
4.4.2 Effect of pH on dc electrical properties of SrM ................................ 69
4.4.3 Effect of pH on frequency dependent ac electrical measurements of
SrM ............................................................................................................. 71
4.4.4 Conclusion ........................................................................................ 75
Chapter 5 ........................................................................................................... 76
Structural, electrical and magnetic properties of Cr doped strontium
hexaferrites ........................................................................................................ 76
5. Structural, electrical and magnetic properties of Cr doped strontium
hexaferrites ........................................................................................................ 77
5.1 Structural properties of Cr doped SrM .................................................... 77
5.2 Frequency dependent ac electrical properties of SrM ............................. 78
5.2.1 The dielectric constant (ε') ................................................................ 80
5.2.2 The dielectric loss tangent (tanδ) ...................................................... 82
5.2.3 The dielectric loss factor () ........................................................... 83
5.2.4 The ac conductivity (ac) .................................................................. 84
5.3 Temperature dependent dc electrical properties of SrM ......................... 85
5.4 Magnetic properties of Cr doped SrM ..................................................... 86
5.5 Conclusion ............................................................................................... 89
Chapter 6 ........................................................................................................... 91
Structural, electrical and magnetic properties of Cr-Zn doped strontium
hexaferrites prepared by co-precipitation method .......................................... 91
6. Structural, electrical and magnetic properties of Cr-Zn doped strontium
hexaferrites prepared by co-precipitation method ............................................. 92
6.1 Structural properties of Cr-Zn doped SrM .............................................. 92
6.2 Frequency dependent ac electrical properties of Cr-Zn doped SrM ........ 95
6.2.1 The dielectric constant () ................................................................ 97
6.2.2 The dielectric loss tangent (tan) ...................................................... 99
6.2.3 The dielectric loss factor () ......................................................... 100
6.2.4 The ac conductivity (ac) ................................................................ 100
6.3 Temperature dependent dc electrical properties of Cr-Zn doped SrM .. 101
6.4 Magnetic properties of Cr-Zn doped SrM ............................................. 102
6.5 Conclusion ............................................................................................. 105
Chapter 7 ......................................................................................................... 106
Structural and electrical properties of Cr-Zn doped strontium hexaferrites
prepared by WOWS sol-gel method ............................................................... 106
7. Structural and electrical properties of Cr-Zn doped strontium hexaferrites
prepared by WOWS sol-gel method ............................................................... 107
7.1 Structural studies ................................................................................... 107
7.2 Dielectric properties ........................................................................... 109
7.2.1 The dielectric constant () .............................................................. 109
7.2.2 The dielectric loss tangent (tan) .................................................... 111
7.2.3 The ac conductivity (ac) ................................................................ 111
7.3 Conclusion ............................................................................................. 113
7.4 Comparison ............................................................................................ 114
7.4.1 Structural properties ........................................................................ 114
7.4.2 Dielectric properties ........................................................................ 115
7.4.3 Conclusion ...................................................................................... 117
Chapter 8 ......................................................................................................... 118
Oxygen reduced strontium hexaferrite for microwave absorbing coatings .... 118
8. Oxygen reduced strontium hexaferrite for microwave absorbing coatings 119
8.1 Reduction procedure .............................................................................. 119
8.2 Effect of oxygen reduction on structural properties of strontium
hexaferrites (SrM) ........................................................................................ 120
8.3 Frequency dependent ac electrical properties of oxygen reduced
strontium hexaferrites .................................................................................. 120
8.4 The dielectric loss tangent (tanδ) ........................................................... 120
8.6 Temperature dependent dc electrical properties of sintered SrFe12O19
before and after reduction ............................................................................ 124
8.7 Conclusion ............................................................................................. 124
9 Conclusions .................................................................................................. 125
9.1 Future work ............................................................................................... 127
10. References ................................................................................................. 128
LIST OF FIGURES
Fig 1.1: A typical hysteresis loop of soft and hard magnet. .......................................... 3
Fig 1.2: Chemical composition diagram of the ferrimagnetic ferrites [17]. .................. 4
Fig 1.3: Unit cell of the M-type hexaferrite. .................................................................. 7
Fig 1.4: Magnetic moment ordering in (a) diamagnetics, (b) paramagnetics, (c)
ferromagnetic, (d) antiferromagnetics and (e) ferrimagnetics ..................................... 10
Fig 1.5 Schematic diagram of polarization mechanisms and their frequency response
...................................................................................................................................... 20
Fig 1.6 Different polarization mechanisms in the absence and presence of an external
electric field [88]. ......................................................................................................... 21
Fig 1.7 The precession motion of magnetic moment around the applied magnetic
field. ............................................................................................................................. 27
Fig 2.1: Geometrical illustration of Bragg’s law [147]................................................ 39
Fig 2.2: PANalytical X'pert pro MPD X-ray diffractometer ....................................... 42
Fig 2.3: JEOL JSM-6700F scanning electron microscope (SEM) .............................. 43
Fig 2.4: (a) Two-terminal and (b) four-terminal resistance measurement
arrangements[148] ....................................................................................................... 46
Fig 2.5: Circuit diagram of the apparatus used for dc electrical resistivity
measurements ............................................................................................................... 47
Fig 2.6: The dc magnetometer (RIKEN DENSHI) ...................................................... 49
Fig 2.7: A typical hysteresis loop along with data obtained from the dc magnetometer
...................................................................................................................................... 50
Fig 3.1: Schematic diagram for the chemical co-precipitation method. ...................... 57
Fig 4.1: Indexed XRD patterns of SrFe12O19 for different molar ratios (MR=Fe/Sr) . 61
Fig 4.2: Indexed XRD patterns of the samples with different volume rate of addition
of the precipitating agent ............................................................................................. 62
Fig 4.3: SEM micrographs of the samples with different volume rate of addition of the
precipitating agent ........................................................................................................ 63
Fig 4.4: Indexed patterns of XRD of samples of SrFe12O19 for different values of pH
(- SrFe2O4, # -α-Fe2O3) .............................................................................................. 66
Fig 4.5: SEM micrographs of the sintered samples of SrFe12O19 for different values of
pH ................................................................................................................................. 68
Fig 4.6: Plot of ln of dc electrical resistivity of SrFe12O19 for different values of pH as
a function of temperature ............................................................................................. 70
Fig 4.7: Plot of drift mobility versus temperature of SrFe12O19 for different values of
pH as a function of temperature ................................................................................... 70
Fig 4.8: The plot of dielectric constant (έ) as a function of ln of frequency of
SrFe12O19 for different values of pH ............................................................................ 72
Fig 4.9: The plot of dielectric loss tangent (tanδ) as a function of ln of frequency of
SrFe12O19 for different values of pH ............................................................................ 73
Fig 4.10: The plot of dielectric loss factor (ε'') as a function of log of frequency of
SrFe12O19 for different values of pH ............................................................................ 74
Fig 5.1: Indexed XRD patterns of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .............. 78
Fig 5.2: Plot of dielectric constant (ε') as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 80
Fig 5.3: Plot of activation energy and dielectric constant at 3MHz versus Cr
concentration (X) ......................................................................................................... 81
Fig 5.4: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 83
Fig 5.5: Plot of dielectric loss factor () as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 84
Fig 5.6: Plot of ac conductivity (σac) as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ..................................................................... 85
Fig 5.7: Plot of lnρ as a function of 1/kBT for SrFe12-xCrxO19
(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit. ................................................... 86
Fig 5.8: Hysteresis loops of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ....................... 87
Fig 5.9: Plot of coercivity and saturation magnetization versus Cr concentration of
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .................................................................... 88
Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ...... 94
Fig 6.2: SEM micrographs of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............. 96
Fig 6.3: Plot of grain size (nm) as a function of Cr-Zn concentration (X) for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................. 97
Fig 6.4: Plot of dielectric constant (ε') as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................. 98
Fig 6.5: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for
SrFe12-xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................... 98
Fig 6.6: Plot of dielectric loss factor () as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ........................................................... 100
Fig 6.7: Plot of ac conductivity (σac) as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............................................................ 101
Fig 6.8: Plot of lnρ as a function of 1/kBT for SrFe12-2xCrxZnxO19
(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit. ................................................. 102
Fig 6.9: Hysteresis loops of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) .............. 103
Fig 6.10: Plot of coercivity (Hc) and saturation magnetization (Ms) versus
Cr-Zn concentration (X) of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) ............... 104
Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
prepared by WOWS sol-gel method. ......................................................................... 108
Fig 7.2: Plot of dielectric constant (ε') as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method
.................................................................................................................................... 110
Fig 7.3: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method
.................................................................................................................................... 110
Fig 7.4: Plot of ac conductivity (σac)as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with sol-gel method. .......... 112
Fig 7.5: Plot of experimental and theoretically calculated dielectric constant () for
the sample sample X=0.6 (SrFe12-2xCrxZnxO19) as a function of frequency. ............. 112
Fig 8.1: Experimental setup for oxygen reduction ..................................................... 119
Fig 8.2: Indexed XRD patterns of sintered strontium hexaferrite samples before and
after reduction ............................................................................................................ 121
Fig 8.3: Dielectric constant () of sintered SrFe12O19 before and after reduction .... 122
Fig 8.4: Dielectric loss tangent (tanδ) of sintered SrFe12O19 before and after reduction
.................................................................................................................................... 122
Fig 8.5: Plot of temperature dependent dc electrical resistivity of sintered SrFe12O19
before and after reduction .......................................................................................... 123
LIST OF TABLES
Table 1.1: Ferrimagnetic oxides in BaO–MeO–Fe2O3 ternary phase [18] .................... 5
Table 1.2: Number of Fe3+ ions with their type, spin and geometry [40] .................... 13
Table 4.1: List of the chemicals used with their specification ..................................... 59
Table 4.2: Lattice constants (a & c), crystallite size (D114), cell volume (V), X-ray
density (ρx), bulk density (ρm), % α-Fe2O3 and particle size ....................................... 67
Table 4.3: Dielectric loss (tanδ), dielectric constant (έ), dc electrical resistivity (ρdc)
and drift mobility (μd) of SrFe12O19 for different values of pH ................................... 71
Table 5.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk
density (ρm), porosity, activation energy (ΔE), dc electrical resistivity (ρdc), dielectric
constant (), dielectric loss tangent (tanδ), ac conductivity( σac), coercivity (Hc) and
saturation magnetization (Ms) of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8). .............. 79
Table 6.1: Lattice parameters (a & c), average crystallite size (Dav), X-ray density
(ρx), bulk density (ρm), porosity, activation energy, dc electrical resistivity (ρdc),
dielectric constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the
prepared samples of SrFe12-2xCrxZnxO19 X 0.0, 0.2, 0.4, 0.6, 0.8. ............................ 93
Table 7.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk
density (ρm), % porosity, dielectric constant (ε'), dielectric loss tangent (tanδ) and ac
conductivity (σac) of the prepared samples of SrFe12-2xCrxZnxO19 X
0.0, 0.2, 0.4, 0.6, 0.8 by WOWS sol-gel method. ..................................................... 109
LIST OF ABBREVIATIONS
B Magnetic Induction
Dav Average crystallite size
∆E Activation energy
EMI Electromagnetic interface
έ dielectric constant
ε0 permittivity of free space
FE-SEM Field Effect Scanning Electron Microscopy
FMR Ferromagnetic resonance
H Magnetic field strength
Hc Coericivity
HA Magnetocrystalline anisotropy
M Magnetization
μd drift mobility
Ms Saturation magnetization
Mr Remanent magnetization
ρx X-ray density
ρm Bulk density
ρdc dc electrical resistivity
SrM M-type Strontium hexaferrite
SEM Scanning Electron Microscope
SMPS Switched-mode power supply
σac ac conductivity
tan δ dielectric loss tangent
TEM Transmission Electron Microscopy
WOWS Without Water and Surfactants, a new simplified sol-gel method
XRD X-Ray diffraction
Y Youngs modulus
YIG Y3Fe5O12 Yittrium iron oxide
Chapter 1 Introduction
1. Introduction
1.1 Nano science
The properties of a single atom of an element are different from the bulk material of the
same element. As the size of the particle decreases, the surface to volume ratio increases. The
properties of a material are greatly affected by this surface to volume ratio. A particle of size
50nm has only 5% of its atoms on its surface whereas a particle of 5nm has 50% of its atoms
on its surface.
Nano-science deals with the preparation and characterization of such materials with any
one dimension in the range of 1-100nm. At nanometer scale, the material exhibits properties
that are very much different from the bulk [1].
Nano-science has brought a revolution in the field of science. Nanotechnology is being
used in many fields of science such as solid state physics, chemistry, medical science,
biotechnology and materials engineering.
Nano structures have got much attention of researchers because of its wide range of
applications. A material comprised of nanometer-scale particles have high fraction of grain
boundaries and hence surface defects and large surface to volume ratio. These properties
determine the properties of a synthesized material and differentiate them from a bulk material
[2, 3].
In recent years, ferrites have got much attention of scientists due to their great scientific
and technological applications such as magnetic media, memory cores, high frequency
devices, catalysis and gas sensors [4-9]. In Italian language, iron is named as ferry. Ferrites
are iron containing complex oxides with different structures and possess interesting
electronic, magnetic, surface reactivity and optical properties in nano size [10]. As a
consequence of these numerous applications, new synthesis techniques have been developed
for the production of nano-ferrites [8-14].
The commercial preparation of various types of ferrites was not started until the
beginning of 20th century. The commercial ferrites could not make their room in the world
because their magnetic properties were inferior to those of the ferromagnetic alloys [4].
Synthesis of new ferrites with enhanced existing properties started in 1950’s due to rapid
expansion of their applications in the devices such as radio, television, carrier telephony,
computer circuitry and microwave devices.
1.2 Types of ferrites
The ferrites are usually classified into two types.
1.2.1 Soft ferrites
1.2.2 Hard ferrites
1.2.1 Soft ferrites
These are the magnetic materials having narrow hysteresis loops possessing moderate
saturation magnetization with very small coercivity (smaller than 1 kAm-1 [15]. Soft ferrites
are widely used in the cores of transformers and switched-mode power supply (SMPS) [16].
Spinel ferrites and garnets are one of the examples of soft ferrites.
Fig 1.1: A typical hysteresis loop of soft and hard magnet.
1.2.2 Hard ferrites
Hard ferrites are known as permanent magnets due to high coercivity (greater than 10
kAm-1). Hard ferrites are commonly used in electric motor and radios etc [15, 16].
Hexaferrites are one of the examples of hard ferrites.
The difference in the hysteresis loop of soft and hard ferrite is shown in the figure 1.1.
It is clear from the figure that soft magnetic materials have very narrow coercive field while
hard magnetic material have large coercive field.
1.3 Classification of hexaferrites
Depending on the chemical composition, hexaferrites are classified into following
six types [17]
(a) M-type hexaferrites, (b) W-type hexaferrites, (c) X-type hexaferrites, (d) Y-type
hexaferrites, (e) Z-type hexaferrites, (f) U-type hexaferrites.
The different compositions of the hexagonal compounds are shown in figure 1.2
as a part of ternary phase diagram for the BaO-MeO-Fe2O3 system.
Fig 1.2: Chemical composition diagram of the ferrimagnetic ferrites [17].
Where Me represents a divalent ion among the first transition elements. The stacking
orders of cubic and hexagonal basic units determine the type of composition such as X, W, U,
Z and Y-type hexaferrites. These types are given in table 1.1 [17].
Table 1.1: Ferrimagnetic oxides in BaO–MeO–Fe2O3 ternary phase [18]
Symbol Composition Crystallographic
build up
No. of molecules
/unit cell
c-axis
(Å)
M BaFe12O19 RSR*S* (MM*) 2M 23.2
X Ba2Me2Fe28O46 MM*S 3MeX 84.0
W Ba2Me2Fe16O27 MSM*S* 2MeW 32.8
U Ba2Me2Fe36O60 MM*Y* MeU 38.1
Z Ba2Me2Fe24O41 MYMY 2MeZ 52.3
Y Ba2Me2Fe12O22 3TS 3MeY 43.5
* Represents rotation at 180
1.4M‐typehexaferrites
M-type hexaferrites are a type of magnetic oxide with chemical formulae BaO.6Fe2O3
(BaM), SrO.6Fe2O3 (SrM) and PbO.6Fe2O3 (PbM). M-type hexaferrites possess higher
coercivity (400 kAm-1) than any other type of ferrite [19]. This family of ferrites have been a
subject of continuous interest for several decades due to the fact that these compounds have
been the work horse of the permanent magnet market [20] and for passive microwave
components, microwave absorber, magnetic recording media, electronic devices, medicine
and magneto-optical recording [21-24]. This material can easily be prepared into powder form
and converted into desired shape. The present work is on M-type strontium hexaferrites due to
its better magnetic properties, high Curie temperature, better chemical and thermal stability,
large uniaxial magnetocrystalline anisotropy, large electrical resistivity, high corrosion
resistance and easy handling because of its non-toxicity.
1.5Structuralpropertiesofstrontiumhexaferrites(SrM)
These M-type hexagonal magnetic oxides were initially developed by Went et al..
[25]. The synthesis of strontium hexaferrite was made for the first time by Adelsk¨old [26].
He also found that the crystal structure of strontium hexaferrite is similar to that of
magnetoplumbite. Later his determination was confirmed by the structural refinement of
strontium hexaferrites [27, 28]. The space group of this structure is P63/mmc. This structure
has hexagonal symmetry. Its ‘a’ axis is called minor axis and ‘c’ axis is considered as major
and preferred axis. This preferred axis positively contributes to M-type ferrites as permanent
magnetic material. The formula of magnetoplumbite structure is MFe12O19 where M is Ba, Sr,
or Pb.
The magnetoplumbite structure contains two formula units per unit cell. The unit cell
of magnetoplumbite structure contains ten layers of oxygen ions shown in figure 1.3. These
layers form spinel blocks S followed by R block containing Ba or Sr ion followed by S* and
R* blocks. S* and R* blocks are similar to S and R blocks but have a rotation over 1800 around
the c-axis. The layer in which Ba or Sr ion is present is hexagonally packed with two oxygen
layers at each side. The four oxygen layers between those containing the Ba or Sr ion are
cubically packed. This stacking of layers gives a rise to an overlap, of cubically and
hexagonally packed sections in the structure. The basal plane containing the barium ion is a
mirror plane of R block and consequently the block preceding and succeeding the R block
must be rotated over 1800 with respect to each other. This is also the reason that the
elementary cell of M-type structure contains 10 not the 5 oxygen layers. Five layers of oxygen
form one formula unit. Such two formula units results in one unit cell. The crystallographic
structure can be described as RSR*S* as given by Braun [29].
The unit cell of this structure contains 2 M ions (M= Ba+2, Sr+2 and Pb+2), 24 Fe3+ ions
and 38 O2- ions. The 24 iron (Fe) atoms are dispersed on five interstitial sites of the ten layers
mentioned above. Three octahedral (B) sites containing twelve ‘k’ sites, two ‘a’ sites and four
‘fiv’ sites, one tetrahedral (A) four ‘fvi’ sites and one bipyramidal (C) site contains two ‘b’ sites
[30].
Fig 1.3: Unit cell of the M-type hexaferrite.
The Fe3+ ions present at 2a sites are octahedrally corresponding with equal Fe–O
distances while the Fe3+ ions present at 4f2 and 12k sites are octahedrally corresponding with
different Fe–O distances ranging from 0.185 to 0.237 nm. The Fe3+ ions present at 4f1 are
tetrahedrally corresponding with oxygen and the Fe3+ ions present at 2b sites are coordinated
by five oxygen ions. The structure also contains short Fe–Fe distances. The Fe3+ ions present
at 4f2 sites have about 0.27 nm distances
between each other while the distance between Fe3+ ions at 12k sites is about 0.29–0.30 nm
[28].
1.6MagneticpropertiesofM‐typestrontiumhexaferrites
1.6.1Sourceofmagnetism
The magnetic moment, produced due to the movement of electrons in an atom, is
considered as fundamental source of magnetism. It can be related with the current loop.
Consider a current loop of area A and the current flowing through it is I then magnetic
moment ‘µ’ is given by
μ IA1.1
The unit of magnetic moment is A-m2.
Origin of magnetism is the motion of electron, spin motion and orbital motion. Both of
these motions are responsible for net magnetic moment on an electron and each electron
behaves like a tiny magnet. The nucleus of an atom also have magnetic moment but negligible
as compared to electron. These electrons respond to external magnetic field and produce
magnetization which is net magnetic dipole moment per unit volume of the material.
M 1.2
where ‘µ’ is the magnetic moment, ‘ν’ is volume and ‘M’ is magnetic dipole moment.
Therefore any atom with at least one electron should have some magnetic behavior. But
only those atoms or molecule show magnetic behavior which have unpaired electrons due to
Pauli’s exclusion principle.
The magnetic induction ‘B’ is given by
B B μ M1.3
where ‘Bext’ and ‘µ0’ are the strength of external magnetic field and permeability of free space
respectively. The magnetic field strength ‘H’ is given by
HBμ
1.4
Putting the value of ‘Bext’ in the above equation
B μ H M 1.5
The relationship between magnetization ‘M’ and the applied field ‘H’ is defined as
χMH1.6
where ‘χ’ is the magnetic susceptibility of the material.
1.6.2Classificationofmagneticmaterials
When a material is exposed to the field of an external magnet, its response determine
the magnetic properties of that material. Depending on the response to external magnetic field
(bulk magnetic susceptibility), materials are categorized into five groups and are named as
diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials.
Some of the materials with their response to external magnetic field are given in figure 1.4.
1.6.3Ferrimagnetisminferrites
According to Neel, the net magnetic moment of unpaired electrons (with spin up and
down) of the atoms present on the interstitial sites of ferrites is different. As a result, a net
magnetic moment appears due to the difference in number of magnetic ions or their magnetic
moments present on two type of interstitial sites [31].
Strontium hexaferrites (M-type ferrites) are ferrimagnetic in nature and its structure is
magnetoplumbite, the name after the naturally occurring of PbFe19O12. This family of ferrites
is named as hexaferrite due to its six fold symmetrical uniaxial crystallographic structure. In
many respects, hexaferrites are similar spinel ferrites in many respects due to the fact that they
Fig 1.4: Magnetic moment ordering in (a) diamagnetics, (b) paramagnetics, (c) ferromagnetic, (d) antiferromagnetics and (e) ferrimagnetics
possess the same ratio of octahedral to tetrahedral sites. The major difference is that M-type
ferrites include one extra site named as trigonal bipyramid site and one big site holding
divalent Pb, Ba, or Sr ions. Hexagonal ferrites are famous for their permanent magnet
applications. The trigonal bipyramid site containing Fe3+ ion is responsible for strong uniaxial
or planar anisotropy of M-type hexa ferrites. M-type hexaferrite structure is formed by
stacking of alternate building blocks beginning with a spinel block, named as S aligned with
the c axis along a <111> body diagonal. Due to this the ferromagnetic spin arrangements are
aligned with only one easy (or hard) magnetic direction. The spinel structure contains three
fold symmetry, in this direction. When S block is systematically rotated by 1800, six fold
symmetry is obtained. There are blocks which contain the layers having large cations, Ba2+
(radius 1.36Å), Sr2+ (1.16Å), Pb2+ (1.18Å) or combinations thereof. The layers in these blocks
also contain trigonal bipyramid sites. These blocks are named as R (or R*) [32].
1.6.4Superexchangeinteraction
When two magnetic dipoles having moment ~ 1µB separated by 1 Å have energy
~10-23 J at 1 K temperature. At high temperature (1000K) this interaction becomes very weak.
For long range magnetic order, exchange interaction phenomenon takes place. Number of
solids such as oxides has magnetic ground states. An exchange interaction between non-
adjacent magnetic ions mediated by a non-magnetic ion is called superexchange interaction. It
is a phenomenon in which two electrons from a double negative ion (such as oxygen) in a
solid go to different positive ions and couple with their spins, giving rise to a strong
antiferromagnetic coupling between the positive ions, which are too far apart to have a direct
exchange interaction is superexchange. In insulating materials such as metal oxides, the
distance between metal ions is large so antibonding state, according to Hund’s rule for
parallel spin alignment, due to mutual repulsion of electrons could not be established. So the
direct exchange between them does not contribute to the magnetic properties. Ferrites
contains cation ions at the interstitial sites of anion (oxygen) based crystal structure. Kramers
(1934) reported exchange mechanism between non neighboring cations ions mediated by
anions (oxygen) ions to discuss magnetic properties of ferrites. With some addition to
Kramers theory for indirect exchange, Neel postulated a new theory for antiferromagnetic
oxides and afterwards for ferrites. Mathematically, the theory of indirect exchange was
explained by Anderson in 1950. Anderson named this theory as superexchange. In this
mechanism of interaction, p orbital of oxygen filled in its ground state, exchange an electron
with the adjacent 3 orbitals of magnetic ions. To make a coupling with cation, electron of
oxygen ion should have a spin opposite to that of cation. This would result in leaving the
other spin of oxygen ion orbital free to couple with the unpaired spin of cation ion preferably
located opposite to the original cation. This is the cause of the stability of antiparallel
alignment of two cations adjacent to oxygen ion.
The ferrites contain two different lattice sites. In case of antiferromagnetic materials,
the moment of two different lattice sites are equal. In case of ferrites the moments are not
equal. This resulted in a net moment. This net moment is obtained from the difference in the
moments on two sites. This phenomenon is named as uncompensated antiferromagnetism or
ferrimagnetism [19, 33].
.
1.6.5Magnetocrystallineanisotropy
All magnetic materials (ferromagnetic or ferrimagnetic) possess one direction or more
than one direction in which their magnetic moments prefer to be oriented. This direction is
considered as easy axis for that material. The energy required to change the orientation of
magnetic moment from easy axis is called magnetocrystalline anisotropy energy. This
magnetocrystalline energy is expressed as
Ek = K1sin2ɵ + K2sin4 ɵ + …… , 1.7
where K1 and K2 are anisotropy constants and ɵ is the angle between magnetization and the
easy axis
The values of these anisotropy constants are strongly affected by the temperature
[34]. The c-axis is considered as easy axis for M-type hexaferrites. The cause of high
magnetocrystalline anisotropy in M-type hexaferrite could not be explained on the basis of
spin dipole-dipole alignment mechanism. It is due to the fact that all of the magnetic ions
(Fe3+) are S-state (L=0) and therefore the possibility of first-order spin-orbit or John-Teller
stabilizations do not exists. One possible reason might be the trigonal bipyramidal crystal
field which might stabilize the d5 electrons into a low-spin [17].
1.6.6MagneticstructureofM‐typestrontiumhexaferrite(SrM)
According to Gorter [35], M-type hexaferrites are ferrimagnetic in nature. The M-type
hexaferrites are of hexagonal structure. It contains 64 ions per unit cell on 11 different
symmetry sites [36]. The unit cell of this structure contains and 2 M ions (M= Sr2+, Pb2+ and
S2+), 24 Fe3+ ions and 38 O2-ions. The 24 Fe3+ ions are distributed over five distinct sites i.e.
12k, 2a, 4f2, 4f1 and 2b. Out of these five, three sites 2a, 4f2 and 12k are octahedral, 4f1 is
tetrahedral and 2b is trigonal bipyramid site which is surrounded by five oxygen atoms [37].
The oxygen ions are present at 4e, 4f, 6h and 12k sites form a closed pack lattice [38]. The M
ions are present at 2d sites [39]. The site occupancy, the number and spin of the 12 Fe3+ ions
is given in table 1.2. The table shows that there are 6 Fe3+ ions at 12k site with spin up, 1 ion
at each 4f2 and 4f1 site with spin down and 1 ion in 2a and 2b site each having spin up. So
there are 8 Fe3+ ions with spin up and 4 with spin down. So the net moment obtained is of 4
Fe3+ present in formula units. One Fe3+ ion has the magnetic moment of 5μB and so the net
magnetic moment of one formula unit is 20 μB.
Table 1.2: Number of Fe3+ ions with their type, spin and geometry [40]
Site type 12K 2a 4f1 4f2 2b
Geometry Octahedral Octahedral Tetrahedral Octahedral Trigonal bipyramidal
Fe3+ ions 6 1 2 2 1
Spin Up Up Down Down Up
1.6.7LiteraturereviewaboutmagneticpropertiesofSrM
M-type hexaferrites are famous for their application as permanent magnets. These
materials cover 90wt% of the annual production of permanent magnets. These magnets are
manufactured about 300 and 500 tons per year in Europe [41]. This family of ferrites are of
also much importance due to their application for high density perpendicular recording media
[42]. Recently IBM has reported a magnetic tape composed of barium hexaferrite
nanoparticles possessing 15 times higher recording density than the commercially available
tapes in market [43].
The magnetic properties of a material depend upon number of factors such as the
chemical composition, microstructural parameters, impurities, and temperature etc. Thus the
scientists paid much attention to the synthesis of strontium hexaferrite having fine particle
size with narrowed distribution and minimum agglomeration[44]. The magnetic properties of
ferrites are greatly influenced by the nature of cation and their distribution in the structure.
One formula unit of strontium hexaferrite contain 12 iron atoms which are present on five
distinct sites: three parallel (12k, 2a, and 2b) and two antiparallel (4f1 and 4f2) [30, 45]. The
site occupancy, the number and spin of the 12 Fe3+ ions is given in table 1.2. The table shows
that there are 6 Fe3+ ions at 12k site with spin up, 1 ion at each 4f2 and 4f1 site with spin down
and 1 ion in 2a and 2b site each having spin up. So there are 8 Fe3+ ions with spin up and 4
with spin down. The magnetic moment of 4 Fe3+ ions with spin up and 4 Fe3+ ions with spin
down cancel each other’s effect and only the net moment of 4 Fe3+ ions with spin up is left in
one formula units. As the magnetic moment of one Fe3+ ion is 5μB so the total moment of 4
Fe3+ ions with spin up is 20 μB per formula unit. The saturation magnetization of strontium
hexaferrite depends upon the cation distribution on these different interstitial sites.
The magnetic properties of strontium hexaferrite can be tailored by replacing ferric
ion (Fe3+) with the ion of other element having same or different valency. If Fe3+ ion with spin
down is replaced by such an ion whose unpaired electrons are less than that of Fe3+ ion then
the number of unpaired electrons with the spin up would be increases. As a result net
magnetic moment will increase because the net magnetic moment of one formula unit of
strontium hexaferrite arises because of the difference of the spin up and spin down electrons.
This increase in net magnetic moment will result in the increase in saturation magnetization.
Similarly if Fe3+ ion with spin up is replaced by such an ion whose unpaired electrons are less
than that of Fe3+ ion then the number of unpaired electrons with the spin up would be
decreased. As a result net magnetic moment will decrease and hence saturation magnetization
will decrease. Except very few cases, saturation magnetization decreases with the replacement
of Fe3+ ion with any other cation on the interstitial sites.
The coercivity mainly depends upon magneto crystalline anisotropy and
microstructural parameters such as grain size, stresses in the crystal lattice, porosity,
vacancies, spin canting and impurities. The high magneto crystalline anisotropy results in
high coercivity. Strontium hexaferrite (M-type hexaferrite) is considered as hard magnet due
to their high magneto crystalline anisotropy. The decrease in grain size (single domain) and
increase in porosity and impurities increases the coercivity of the material and vice versa. The
intrinsic coercivity of strontium hexaferrite is also very high (about 7 kOe). The coercivity of
this material has been has been increased as high as 13kOe and decreased to few hundred
oestered according to its requirement for different applications [46].
Strontium hexaferrites possess better magnetic properties than barium hexaferrites
because of smaller ionic radius of strontium than barium. Strontium hexaferrites have also an
advantage over barium ferrite due to problems in the synthesis of barium ferrite in better form
[47].
A lot of research has been made to investigate and improve the magnetic properties of
strontium hexaferrites due to their large number of application in the market based on their
magnetic properties. Different researchers used different synthesis techniques, [44, 48-54]
with different annealing temperatures to investigate and tailor the magnetic properties of
strontium hexaferrites for different applications. Numerous magnetic studies has been carried
out with the substitution of Fe3+ or Sr2+ ions with elements such as Sm [55], Mn, Co, Zr [56],
Al [57], Gd [58], Si-Ca [59], Ba [60], La-Co [61], Al-Cr [62], Co–Nd [63], Zr–Ni [64], Al–
Ga [65], Zn, Ti, Ir [66], Zn-Nb [67], Zn, Co, Ti [68], La, Sm, Nd [69]. Some researchers
added different oxides SiO2, CaO [70] or used core (SrFe12O19) - shell (SiO2) model [71],
Fe/Sr ratio [72], some prepared composites by using different oxides such as SiO2, Fe2O3,
Bi2O3, H3BO3 and SrCO3 [73, 74] and studied the magnetic properties for different
applications.
1.7 The dc electrical properties of M-type strontium hexaferrites
Most of the ferrites are considered as high resistive materials. The resistivity of these
materials depends upon the microstructural parameters, dopants and their site occupancy,
annealing temperature and time. The electrical conduction in strontium hexaferrites is
discussed by using Verwey model [75]. The unit cell of strontium hexaferrites contains 24 Fe
atoms distributed on three different interstitial sites named as octahedral, tetrahedral and
trigonal bipyramedal sites explained above. It is the number of ferrous ions, formed due to the
reduction of oxygen during annealing, present on the octahedral sites that play a dominant
role in the process of conduction as well as dielectric polarization [76]. In an ideal crystal
lattice of strontium hexaferrite, only ferric (Fe3+) ions are present on interstitial sites. But
during heat treatment, a number of ferric (Fe3+) ions are transformed to ferrous (Fe2+) ions due
to small reduction and gives a rise to oxygen vacancies. According to Verwey model, the
electrical conduction in ferrites is mainly due to hopping of electrons between ions of the
same element present in more than one valence state, distributed randomly over
crystallographic equivalent lattice sites [77]. The distance between cation ions present on
interstitial sites of ferrites is different. It is smaller for the cations present on octahedral B
sites than those present on octahedral B and tetrahedral A sites. The probability of electron
hopping between tetrahedral A and octahedral B sites much smaller as compared to the
hopping between (B)–(B) sites. As only Fe3+ ions are present on tetrahedral A site so electron
hopping between (A)–(A) sites is not possible. If any Fe3+ ion is transformed to Fe2+ ion
during processing then it preferentially occupy (B) sites only. In ferrites, the conduction is due
to the hopping of electrons between the ions of same element having different oxidation state
(Fe2+ to Fe3+) present octahedral B sites [78, 79].
1.7.1Temperaturedependentdcelectricalresistivity
The dc electrical resistivity of these ferrites could be increased by decreasing the
concentration of Fe2+ ions by any method. The resistivity of nano materials increases with the
decrease in grain size. The decrease in grain size results in the increase in grain boundaries
which acts as poorly conducting medium. Hence the dc resistivity could be controlled by
varying the grain size of the synthesized material. The impurities present in the crystal
structure produce the stress in the lattice which usually results in the increase in resistivity.
The moisture present in the material provides conducting path to charge carriers and hence
decreases the resistivity.
The resistivity of these materials lies in the range of the semiconductors (several
ohms to several mega ohms) at different temperatures. Also their resistivity decreases with the
increase in temperature. On the basis of these properties, ferrites are considered as
semiconductors. The energy required by charge carrier to overcome the barriers while moving
from one point to another is called activation energy.
1.8Frequencydependentdielectricpropertiesofstrontiumhexaferrites
Two equal and opposite charges separated by a very small distance as compared with
the distance to an observer form an electric dipole. An electric dipole µ is given by
μ Qd 1.8
where ‘Q’ and ‘d’ is the charge and the distance between the two charges respectively.
The direction electric dipole moment µ is taken from the negative to the positive charge and
unit is C-m. Another unit of electric dipole moment is debye which is equal to 3.33 x10-30 C-
m.
When a high resistive (dielectric) material is exposed to an applied electric field, the
displacement of the charge inside the material takes place without transferring to electrodes
and give a rise to the formation of electric dipoles and as a whole the material is said to be
polarized and this process is known as polarization. The ability of the material to respond to
the field is called its polarizability (α).
There are five different types of polarization mechanisms named as electronic
polarization, ionic polarization, orientation polarization, interfacial polarization (Maxwell-
Wagner) and hyper electronic polarization. At lower frequencies (< 106 Hz), all polarization
mechanisms prevail. The total polarizability α due to all of these polarization mechanisms
may be expressed as α = αe + αi + αo+ αs.
1.8.1Electronicpolarization
When an atom is placed in an electric field, a deformation in the spherical charge
distribution of negative charge around the nucleus takes place. This leads to the electronic
polarizability αe which is the measure of the ease with which the charge centers may be
dislocated. The dipole moment produced due to this polarization is independent of frequency
and is directly related to the strength of the applied electric field. This polarization mechanism
is present in all materials.
The electronic polarizability αe can be calculated by making a supposition that the
atom is of perfect spherical shape. The electronic polarizability αe for one atom can be
calculated by
α 4πε R 1.9
where 0 is permittivity of free space and ‘R’ is the radius of atom [80].
1.8.2Ionicpolarization
In ionic materials, in addition to deformation of electronic cloud around the nucleus, a
displacement in the cations and anions also takes place. This displacement of ions gives a rise
to Ionic polarizability ‘αi’. It is more pronounced in weakly bonded molecules and prevails in
the frequency range (1012 - 1013 Hz).
Ionic polarizability ‘αi’ can be found by using the equation [80]
α qy. d
1.10
where ‘q’ is the charge; ‘y’ is Young’s Modulus and ‘d0’ is the separation between ions.
1.8.3Orientationpolarization
There are materials such as polar liquids, gases or polymers whose molecules possess
dipole moment even when no external field is present. The moments of these materials have
random directions. When these materials are placed in an external electrical field, their dipole
moments become align and leads to the dipolar polarizability αd.
Debye proposed a model to calculate orientation polarizability αd for the polar
materials with small dipole moment and field strength. Debye relation for orientation
polarizability αd is given by [81]
α μ3kT
1.11
where ‘µ’, ‘k’ and ‘T’ are the dipole moment, Boltzmann constant, temperature respectively.
The polarization produced due to the orientation of polar molecules is greater than
electronic polarization.
1.8.4Hyperelectronicpolarization
Hyperelectronic polarization occurs in some long polymeric molecules Pohl et al. [82,
83]. This polarization appears at low frequencies and is because of the pliant interaction of
charge pairs of excitons, located on long, polarizable polymers. The movement of charge
pairs to long range in large molecules will cause large polarization. The hyperelectronic
polarization is much greater to that of the electronic polarization in the band of frequency
ranging from several kHz to several MHz.
1.8.5Spacechargepolarization
Space charge polarization is also known as interfacial polarization. When the external
field is applied, the charge carriers move through the material and stop at grain boundaries,
cracks and defects. This movement causes large scale distortion inside the material and leads
to space charge polarizability αs. This interfacial polarization prevails up to (103 Hz). The
interfacial polarization is responsible for high dielectric response in many materials such as
polymers and ceramics [84]. The contribution of this polarization mechanism to the total
polarization is large at low frequency region (Hz to kHz). The high dielectric constant at
lower frequencies is mainly because of interfacial polarization [85]. The degree of the
polarization depends on the grain size and its morphology, number of grain boundaries and
defects as well as the difference in the conductivity between crystalline and amorphous
region [84, 86, 87]. Space charge polarization mechanism is very complicated and no
satisfactory models are available to calculate the interface polarizability yet. However
Maxwell-Wagner model is usually used to discuss polarization mechanism in polycrystalline
materials.
Fig 1.5 Schematic diagram of polarization mechanisms and their frequency response
Fig 1.6 Different polarization mechanisms in the absence and presence of an external electric field [88].
1.8.6Thedielectricconstant
Dielectric constant is the response of a material to an applied electric field. When a
highly resistive (dielectric) material is placed in a static electric field, it gets polarized
instantaneously and the dielectric constant is considered as real number. In a polarized
dielectric material, negative charge gather towards positive electrode and positive charge
gather towards negative electrode providing external electrical field. This displacement of
charge in a dielectric material gives a rise to electric dipoles. These electric dipoles, produced
due to the displacement of charge, create their own electric field which is opposite to the
applied field [89].
When the material is placed in an alternating electric field, the dielectric constant
varies with the variation in frequency. At lower frequencies, space charge polarization
mechanism is dominant as compared to other mechanisms. At relatively higher frequencies
(MHz), space charge polarization mechanism vanishes and dipole polarization mechanism
becomes dominant. In this way, at ultra high frequencies, the dielectric constant due to only
electronic polarization prevails.
When a dielectric material is subjected to an alternating field, the orientation of the
dipoles changes as the field reverses its direction. At lower frequencies of the applied field,
the orientation of dipoles can easily follow the applied field. At relatively higher frequencies,
the orientation of dipole starts lagging the applied field due to inertial effects and spatially
oriented defects and the dielectric constant becomes a complex quantity.
The complex dielectric constant is given by
ε∗ ε jε 1.12
where ‘*’, ‘’and ‘’ are complex, real part and imaginary part of dielectric constant
respectively.
The space charge polarization results in the high dielectric constant. Maxwell-Wagner
two layer model [90] is used to discuss the dielectric constant obtained because of space
charge polarization. According to this model, interfacial polarization takes place because of
two layers of the dielectric material. One thick layer acts as resistive medium while second
thin layer act as insulating medium. Using Maxwell-Wagner two layer model, one can derive
complex dielectric constant which is given by,
ε∗ εε ε1 jωτ
jσωε
1.13
where ‘’, ‘s’, ‘dc’, ‘’ and ‘0’ are dielectric constant for electronic polarization, dielectric
constant at dc field, conductivity, relaxation time and dielectric constant of free space
respectively and =2f.
The real part of dielectric constant obtained from above equation is given by
ε εε ε1 ω τ
1.14
For hexaferrites, the space charge polarization is discussed in term of charge hopping
mechanism. The hopping polarizability α depends upon the width and height of the potential
barrier between two sites and is given by [80],
αq r3kT
P A → B P B → A 1.15
where ‘q’ is the charge, ‘r’ is the distance between A and B sites, ‘k’ is the Boltzmann
constant, ‘T’ is the temperature, and P A → B P B → A is the average product of two
hopping probabilities.
1.8.7Thedielectricloss
The dielectric loss is the amount of energy dissipated in a material due to electrical
conduction, dielectric relaxation, dielectric resonance and loss from non-linear processes [30].
The dielectric loss in a material also takes place due to delay between the electric field and the
electric displacement vectors [91]. There are two types of dielectric losses namely intrinsic
and extrinsic losses. Intrinsic dielectric losses depend on the crystal structure, frequency of
applied field, temperature and the interaction of the phonon with the ac electric field [92, 93].
Extrinsic losses depends upon crystal imperfections such as impurities, microstructure
defects, dislocations, vacancies, porosity, grain boundaries, microcracks, random crystallite
orientation, dopant atoms etc. The extrinsic losses could be reduced by controlling the above
parameters during synthesis and other different processing. The crystals with different
symmetry groups and defects cause different losses at particular frequency and temperature
[92].
Energy losses in a dielectric material are mainly because of two factors.
1. Conduction of free carriers
2. Relaxation effects
The dielectric loss due to conduction of free carriers is given by
tanδ4πσωε
1.16
where =2f, is dielectric constant and is conductivity.
If a graph of log (tanδ) versus log ω is drawn then it would be a straight line. If dielectric loss
factor ε is proportional to 1/, the conduction is frequency independent and is considered as
dc energy loss.
Different impurities and defects present in dielectric material show relaxation effects.
The dielectric loss due to these relaxation effects could be found by using the equation
tanδ 1.17
These losses (due to relaxation effects) are different from that of conduction losses. The plot
of relaxation loss against frequency is not a straight line but it shows a maximum peak at
particular frequency.
The total loss could be obtained by adding the loss due to the conduction of free
charge carriers and due to relaxation effects and is given by
tanδ4πσωε
ε ε ωτ1 ω τ
1.18
At lower frequencies (kHz), the contribution of the first term on right hand side becomes
predominant and the contribution of second term is negligible.
In ferrites the dielectric losses are discussed using Koop’s phenomenological theory on the
basis of Maxwell-Wagner two layer model. According to Koop’s, the grain in a bulk material
acts as a resistor and grain boundary acts as thin insulating layer [94].
1.8.8LiteraturereviewaboutfrequencydependentelectricalpropertiesofSrM
When a highly resistive material is placed in a circuit for electrical isolation, then that
material is named as insulator and when the same material is placed in an electrical field then
that material is named as dielectric. When a high resistive (dielectric) material is placed in an
electric field, the displacement of the charge inside the material takes place without
transferring to electrodes and as a whole the material is said to be polarized and this process is
known as polarization. At lower frequencies (< 106 Hz), all polarization mechanisms such as
electronic polarization (atomic polarization), orientation polarization (dipole polarization),
ionic polarization, interfacial polarization (Maxwell-Wagner) prevail.
Ferrites are considered as highly resistive materials and are being widely used in the
devices operating at higher frequencies. In microwave devices, both intrinsic and extrinsic
losses are of significant importance. Intrinsic losses mainly arise due to the fundamental
interactions in ferromagnetic materials with in the magnetic system. The extrinsic losses are
developed because of the crystal imperfections (depends on the synthesis technique and
conditions), porosity, grain boundaries, surface roughness, polycystallinity (random local
anisotropy), slow and fast relaxing impurities (rare earth slow relaxers, Fe3+, Fe2+ hopping,
etc.).
In microwave devices, conduction, dielectric and magnetic losses are also of much
importance. These losses determine the performance of the device. The off resonance losses
in ferrite based microwave devices such as circulators and micro strip tunable filters plays
very important role in their performance [95].
The high-frequency ferrites (spinels, garnets and hexaferrites) are magnetically
anisotropic and gyromagnetic in nature. These characteristics originate from the precessional
motion of the magnetic moments. For a particular direction of biasing magnetic field, the
precessional motion of moments gives a sense of rotation in one direction. When the biasing
field is reversed, sense of rotation is also reversed. The frequency of the precessional motion
is proportional to the strength of biasing field. The strength of this biasing field depends on
the magnetocrystalline anisotropy field, demagnetizing field and applied magnetic field in
ferrites. When the sense of rotation of precessional motion of magnetic moments and applied
field (circularly polarized EM wave) is same then the interaction between them would be very
strong. Reversing the direction of EM wave (applied field) reverses the sense of rotation. The
strong interaction would be observed only in one direction of propagation of EM wave. Such
a direction dependent interaction of EM wave in ferrites enabled them to be used for
circulators, isolators and other non-reciprocal devices. The interaction in EM wave and
ferrite can be tailored by using variable applied field. This characteristic of ferrites allows
them to be used as phase shifters, filters and other tunable devices. The above interaction
becomes strongest at ferromagnetic resonance (FMR). At FMR, a strong absorption of
incident microwave energy (attenuation of a wave) takes place.
1.8.9FerromagneticresonanceandSnoek’sLimit
The ferromagnetic resonance is produced due to the precessional motion of a
ferromagnetic material when exposed to an external magnetic field. The external magnetic
field produces a turning effect in magnetization. This turning effect of
Fig 1.7 The precession motion of magnetic moment around the applied magnetic field.
external magnetic field results in the precessional motion of the magnetic moment. The
frequency of precessional motion is dependent on the orientation of the material and applied
magnetic field strength shown in figure 1.8.
This precession motion can be described by the Landau-Lifshitz-Gilbert equations
γM H∝
1.19
where ‘ϒ’ is the gyromagnetic factor of 28GHz/T, Heff is the effective DC field which
includes anisotropic field, demagnetization field and external applied field. ‘α’ is the damping
factor.
When a small ac field along with effective field is applied along the magnetic
moment direction to find ferromagnetic resonance (FMR), the mean magnetic moment M will
precess very close to Heff. In non-dissipated condition, the above equation becomes
∂M∂t
γM H 1.20
In the absence of external field, magnetization rotation dominates. This resonance is only
because of the internal anisotropic field given by:
ω γH γ2KM
1.21
where ‘K1’ is the crystalline anisotropy of material
According to Snok’s law, the product of permeability and ferromagnetic resonance
frequency (FMR) is a constant and is proportional to the saturation magnetization and is given
by:
ω μ 12γ3
4πM 1.22
As the saturation magnetization of a material has particular value so the permeability of that
material is also limited to specific value. This Snoek’s limit shows that the saturation
magnetization permeability and permeability of a material are directly related to each other
[96].
Ferromagnetic resonance frequency of ferrites is strongly affected by their
magnetocrystalline anisotropy field (HA). As the magnetocrystalline anisotropy field (HA) of
spinel ferrites is very small so their ferromagnetic resonance (FMR) frequency falls near or
below 1GHz. This limits the operating frequency of devices based upon spinel ferrites to C, S
and X-bands. The ferrites used in these devices are biased by permanent magnets. The field of
these permanent magnets is used to saturate the ferrite and to shift FMR frequency to higher
frequency thus increases the limit of operating frequency of devices. Spinel devices becomes
untenable above X-band frequency.
The garnets, another type of ferrites, possess excellent structural and chemical
stability. The garnet (Y3Fe5O12 (YIG)) is being widely used in the devices operating at
microwave frequency due to its low ferromagnetic resonance (FMR) line width, 0.6Oe [97],
which results in very small microwave loss. YIG is biased by using permanent magnets. The
operating frequency of YIG based devices is below 1–2GHz.
The magnetic damping due to ferromagnetic resonance (FMR) determines the total
loss in microwave devices. The line width of ferromagnetic resonance determines the
magnetic loss. The line width of single crystal is much narrow (low magnetic loss) than that
of polycrystalline materials (high magnetic loss). The ferromagnetic resonance line width of
M-type hexaferrites is very large. This characteristic of ferrite is used to develop various types
of microwave absorbing devices [98]. The operating frequency of ferrites is determined by
their ferromagnetic resonance (FMR) frequency.
At higher operating frequencies, the ferrites having large magnetocrystalline
anisotropy (M-type hexa ferrites) are used. M-type strontium hexaferrites generates very low
residual losses at high frequency application (GHz) than spinel (soft) ferrites due to their high
magneto crystalline anisotropy [99]. The magnetocrystalline anisotropy (HA) of M-type
hexaferrites is approximately 1000 times greater than that of spinel ferrites [17, 100, 101].
The zero field FMR frequency of these materials is about 36GHz. These materials also
required external magnetic field to saturate. The strength of the biasing field required to shift
ferromagnetic resonance (FMR) frequency to high frequency is substantially lower due to
high magnetocrystalline anisotropy.
M-type hexaferrite based devices could be operated at frequencies Ka-band which is far
below their resonance frequency.
Ferrite based devices can operate up to Ka-band (for below resonance operation). The
properties of M-type hexaferrites are greatly influenced by the distribution of cations present
at different interstitial sites of their crystal structure. The magnetocrystalline anisotropy (HA)
of these ferrites could be varied by substituting Fe, present on interstitial sites, with different
cations. The HA could be increased by doping Al, Cr and Ga [102]. This leads to increases
the operating frequency of device application upto and including U, E, and W bands [103,
104]. Conclusively the operating frequency of M-type hexaferrites and their substituted
compositions could be tailored from 1 to 100GHz. The large magnetocrystalline anisotropy
field (HA) also results in high remnant magnetization. Ferrite based microwave devices
require permanent magnet to bias the magnetic material to be operated at particular frequency.
The use of these permanent magnets hinders in the reduction of size and weight of devices.
The M-type hexaferrites are used as self-biased materials due to their large remnant
magnetization. So these materials are also useful for the reduction in size and weight of
devices.
The performance of ferrite based devices strongly depends on the ferromagnetic
resonance frequency (FMR) line width [105]. Hexaferrites are considered as strong candidate
for the devices operating at high frequencies because of their large magnetocrystalline
anisotropy field. This field could be used to bias these ferrite based materials.
The pollution produced by electromagnetic waves can be controlled by developing
such materials which could absorb these waves. Conducting materials could be used to shield
the electromagnetic waves but the reflection produced by these materials pollutes the
atmosphere. Keeping this problem in view, scientists paid considerable attention to develop
such materials which could absorb electromagnetic energy. Depending on the working
frequency (low or high), a variety of absorber materials are synthesized to suppress
electromagnetic waves [106-109]. Metallic magnetic material could not be used at relatively
higher frequencies due to their high eddy current losses. For high frequency electromagnetic
wave absorption, magneto-dielectric materials were developed. The development of
magnetic–dielectric absorbers received considerable attention due to their non zero complex
permittivity (r = - j) and permeability (µr = µ- jµ). The thickness of electromagnetic
wave absorbing material decreases by (µ)1/2 times [110]. Ferrite (spinel or hexagonal)
materials are used as magnetic fillers to prepare composites for different frequency band
width and thickness [23, 111].
Strontium hexaferrites (M-type ferrites) are highly resistive (108 -cm) with good
magnetic properties. These are low density materials and find a number of applications in
market due their economical production and high microwave magnetic losses, dc resistivity
and chemical stability [112-114]. Different researchers have reported their work on
hexaferrites for microwave applications [115-119].
A low dielectric loss is necessary for high performance devices as it may result in
higher efficiency and lower noise [120] which is especially important for high frequency
applications (e.g. in MHz). So if the dielectric losses of strontium hexaferrites are reduced to
very low value by controlling its microstructural properties and Fe2+ ions concentration then
the material would be more useful for high frequency applications.
M-type strontium hexaferrites also have lot of potential to find its frequency
dependent application due to their tunable electrical properties but their electrical properties
has not been reported too much in the literature. A very small work has been reported by
different scientists on the electrical properties of strontium hexaferrites. The scientists
working on strontium hexaferrites used different synthesis methods [56, 76] and dopants such
as Mn, Co, Ti [121, 122], SiO2 [123], Si-Ca [59], Ba [60], Pb [124], Ba-Ce-Ni [125], Al-Cr
[62], Zr–Ni [64], Al–Ga [65], Zr-Cu [126], Ca [127] and investigated their dielectric
properties in different frequency regions (MHz and GHz). But still there is a plenty of room to
improve their frequency dependent electrical properties by varying synthesis conditions and
doping elements with different concentration.
1.9 Mechanical properties of ferrites
The mechanical strength of a material is related to porosity. Strontium hexaferrites
have large porosity (low density) due to which their mechanical strength is low however their
compressive strength is high [33]. The mechanical properties such as hardness, tensile and
flexural strength of strontium hexaferrites have not been studied so far to much extent.
1.10 Thermal transport properties
This property though becomes very important when this material is used in heat generating
devices. Thermal properties of these materials could not get too much attention of the
researchers. The thermal conductivity, coefficient of linear thermal expansion and specific
heat are of great importance. Strontium hexaferrites are ceramic materials and have poor
thermal conductivity. Haberey et al. [128] reported that the linear thermal coefficient of
strontium hexaferrite is its axis dependent. It is 14.0× 106 K-1 when the sample is placed
parallel to c-axis and 10.0×106 K-1 when placed perpendicular to the c-axis. Specific heat
capacity, thermal diffusivity and thermal conductivity mainly depend upon microstructural
properties, temperature and chemical composition. According to the measurements of Hussain
and Maqsood [129] heat capacity per unit volume of strontium hexaferrite is 2.73 MJ m-3 K-1,
thermal diffusivity is 1.132 mm2 S-1 and thermal conductivity is 2.69 Wm-1 K-1. When the
temperature of strontium hexaferrite is raised, its heat capacity per unit volume decreases
while thermal diffusivity and thermal conductivity increases.
1.11Chemicalstability
Along with the above characteristics, these materials also show good chemical
stability. M-type hexaferrites are found to be stable in week acids such as citric acid, acetic
acid (CH3COOH) and phenol solutions. These are also stable in alkalis such as NH3, NaOH,
NaCl, KOH and other chemicals. But M-type hexaferrites are soluble in strong acids such as
H3PO4, H2SO4, HCl, HNO3 and HF.
1.12ApplicationsofM‐typehexaferrites
The demand of a material in market depends upon a number of factors such as its
physical properties and production cost. Before 1930s, AlNiCo alloys were used as hard
magnets. These materials were very brittle and their production methods were limited [130].
Hexaferrites, which are also considered as hard ferrites, were discovered around 1950s [131].
Very soon this material became work horse of permanent magnet market due to their low cost
production. As permanent magnets are hard and brittle so their machining is difficult. To
overcome this problem, bonded magnets are used. The most common bonded magnets are M-
type hexaferrites. These materials are widely used in different moving-coil instruments. These
instruments works on the principle that when a current carrying coil is placed in an external
magnetic field (permanent magnet field), a turning effect is produced. These permanent
magnets are also used in equipments used in research apparatus, industrial machinery and
consumer products. They are also used in various biomedical apparatus, generators, industrial
motors, power tools, audio/video equipment, printers, copiers, personal computers,
automobiles and household appliances [132].
The strontium hexaferrites (M-type hexaferrites) also find their applications in the
magnetic recording media-tapes and disks because of its large saturation magnetization.
Presently, most of the magnetic memories consist of islands which are magnetized into digital
bits. The beauty of this technology is that it provides more dense recording at relatively low
cost.
From the viewpoint of microwave devices operating at high frequency, ferrites,
because of their high dc electrical resistivity (low eddy current losses) and excellent magnetic
property, are widely used as dielectric material as compared to the metallic magnetic
materials. The working frequency of soft ferrites (spinel ferrite (MeFe2O4)) is limited to 1GHz
with wide bandwidth absorption. M-type hexaferrites are more useful for high frequency
(several GHz) absorption application due to their high magneto crystalline anisotropy which
results to very low residual losses in comparison with soft ferrite [99, 133].
At frequencies, above 1 GHz, electrical energy cannot be transmitted through wires.
It is transmitted in the form of electromagnetic waves through space or contained in wave-
guides. These electromagnetic waves (microwaves) are controlled by magnetic components.
These magnetic components possess excellent dielectric properties at microwave frequencies.
These materials transmit electromagnetic waves with small losses. These components are
called phase shifters, circulators and Faraday rotators. These components are widely used in
space communication, in controlling satellites, aircrafts and radars [33].
In different systems such as satellite communication system, wireless communication
system, radar and precise guidance system, microwaves of high frequency (several gigahertz)
are used. Electromagnetic interface (EMI) in gigahertz range is a big issue and miniaturization
of devices is also of great interest [134]. The development of microwave absorbers to reduce
back scattering has got much attention of scientists [135]. These materials are of great
importance for microwave absorber coatings on military aircrafts for radar jamming. M-type
hexaferrites are famous for their large magneto crystalline anisotropy. The working frequency
(GHz) of these materials can be varied by tailoring anisotropy field [136, 137].
1.13Motivationandobjectives
M-type hexaferrites (MFe12O19 where M=Ba, Pb or Sr) find many technologically
demanding and challenging applications because of their high dc electrical resistivity, high
Curie temperature, better chemical and thermal stability, high corrosion resistance and large
uniaxial magnetocrystalline anisotropy. M-type hexaferrites are being widely used in
automobile industry, dc electric motors, data storing devices, magneto-optic recording media,
door catchers, loud speakers, plasteferrite, injection-molded pieces, microwave devices and a
lot more [106, 138-140]. In the present work, strontium hexaferrite is selected due to its better
electrical and magnetic properties, high Curie temperature than other M-tpe hexaferrite
family, easy to handle because of its non-toxicity.
The properties of this ferrite are largely dependent on the processing routes used for
its fabrication [18, 42, 141]. A number of synthesis methods have been used to prepare
hexaferrites. Some methods are complex and expensive while some of them are resulted in
the formation of secondary phases and required very high annealing temperature for longer
time to reduce the impurity phases. Hussain et al. [124] prepared M-type hexaferrite by
standard ceramic method and annealed the samples at 900C for 10 h and even then
secondary phases were not removed. Kikuchi et al. [61] prepare M-type hexaferrite with
polymerizable complex method. He annealed his samples at temperature above 900C for 24
h to get single phase hexaferrite. Narang et al. [60] prepared M-type hexaferrites by
conventional ceramic method and annealed the sample at 1250C for 20 h but impurity phase
was not removed. Ghasemi et al. [56] prepared M-type hexaferrites by sol–gel method. The
viscous residue was heated at 200C for 20 h to get dried gel. This dried gel was annealed at
1000 C for 1 h. The XRD indicates the presence of small amount of impurity phase. Jacobo
et al. [63] prepared hexaferrites by using self combustion technique and annealed the powder
at 1100C for 2h but could not get a rid of impurity phases. Iqbal et al. [64] synthesized M-
type strontium hexaferrites by co-precipitation technique. The samples were annealed at
925C for 1h and got single phase strontium hexaferrite powder. Charalampos et al. [142]
annealed barium hexaferrites, prepared by co-precipitation method, at 920C for 2h. This
literature survey has motivated us to use co-precipitation and sol-gel method for the synthesis
of strontium hexaferrites because these are very simple methods and require low annealing
temperature for shorter time for phase purity. These wet chemical methods also provide
greater chemical homogeneity, greater reactivity, high purity and fine particle size with
narrow distribution [33].
The phase purity and particle size of hexaferrites, prepared by co-precipitation
method, are strongly dependent on different synthesis parameters such as volume rate at
which the precipitating agent is added and the pH of the solution. Different synthesis
parameters and post synthesis treatments [63, 127, 142-144] have been used by researchers to
control impure phases. In this thesis work, we analyzed that how the synthesis parameters
(molar ratio of cations, volume rate at which the precipitating agent is added and the pH of the
solution) affect the phase purity and particle size of strontium hexaferrite material. For this
purpose, molar ratio of cations (Fe/Sr) is varied from 12 to 08 with step size of 1, volume rate
at which the precipitating agent is added (NaOH(aq)) is varied from 30ml/min to 2000ml/min
and pH is varied from 13 to 08 with step size of 1.
Strontium hexaferrites (M-type hexaferrites) are known to be the work horse of
permanent magnet market due to their high magnetocrystalline anisotropy. A lot of work has
been done to tailor the magnetic properties of these materials for different applications such as
permanent magnetic for dc motors, data storing devices and bio-medical applications.
The electrical properties of strontium hexaferrite materials are also of much
importance but have not been reported too much in the literature. The operating frequency
limit of ferrite based devices is strongly affected by their magnetocrystalline anisotropy field
(HA). The magnetocrystalline anisotropy (HA) of M-type hexaferrites is approximately 1000
times greater than that of spinel ferrites [17, 101, 131]. The magnetocrystalline anisotropy
(HA) and hence operating frequency limit of M-type hexaferrites based microwave devices
such as circulators, isolators and other non-reciprocal devices, phase shifters, microwave
absorbing devices could be tailored by using different dopants.
Özgür et al.[102] reported that Cr causes the increase in magnetocrystalline
anisotropy of M-type hexaferrites. Keeping this observation in view, the composition SrFe12-
xCrxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 is prepared by simple co-precipitation method and
the structural, electrical and magnetic properties these Cr varying sample are studied. This
composition has been prepared by co-precipitation method and its dielectric properties have
not been reported yet.
A low dielectric loss is necessary for high performance devices as it may result in
higher efficiency and lower noise [120] which is especially important for high frequency
applications. So if the dielectric loss of strontium hexaferrites is reduced to very low value by
controlling its microstructural properties and Fe2+ ions concentration then the material would
be more useful for high frequency applications.
Angeles et al. [145] reported that Zn2+ ions preferentially occupy tetrahedral A and
may also occupy bipyramidal C sites where it replaces Fe3+. For charge neutrality, the Fe2+
ions present on octahedral B site [127, 146] are converted into Fe3+ ions. This results in the
decrease in the Fe2+ ions concentration responsible for dielectric loss tangent. Tetrahedral site
occupancy of Zn2+ leads the ferrites to increase their saturation magnetization.
To increase both coercivity and saturation magnetization (permeability), Cr and Zn
doped strontium hexaferrite having composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 is synthesized by co-precipitation and WOWS sol-gel
method (WOWS stands for Without Water and Surfactants). The effect of this composition on
structural, electrical and magnetic properties has been studied. Strontium hexaferrites with
this composition and with the above synthesis techniques has been studied for the first time to
best of our knowledge.
Strontium hexaferrites are being widely used as microwave absorbing coatings for
radar jamming purpose. For such type of coatings, the material should be non toxic and
possess high chemical and thermal stability [112-114]. The material should have high
dielectric constant and high permeability to reduce the thickness of coating layer and high
microwave magnetic loss.
It is reported in literature that when oxygen is reduced from crystalline strontium
hexaferrite at elevated temperatures (800-900)C, then some of the Fe3+ ions present on
interstitial sites becomes free iron atoms which causes the increase in saturation
magnetization. It was expected that it would also result in the increase in carrier concentration
and hence dielectric constant.
Keeping this in view, single phase strontium hexaferrites is hydro-nitroginated at
850C for one hour and its effect on dielectric properties is studied. The study of dielectric
properties of oxygen reduced strontium hexaferrites has not been reported so far to best of our
knowledge.
Chapter2CharacterizationTechniques
2.CharacterizationTechniques
2.1X‐raydiffraction
To study the structural properties of a material, X-ray diffraction technique is
commonly used. When a beam of fast moving electrons, travelling in evacuated tube, strikes
on the surface of a material (target), X-rays are produced. These X-rays are considered as
characteristics of that material. Most of the X-ray diffractometer contain Cu as target material.
The X-rays, generated from Cu, strikes on the surface of the material under testing.
If ‘d’ is the distance between the planes (considered as the characteristics of a
material) and ‘’ is the wave length of X-rays then according to Bragg’s law.
= 2dsinɵ 2.1
where ‘ɵ’ is the angle between the plane of the crystal and incident X-ray beam.
Fig 2.1: Geometrical illustration of Bragg’s law [147]
For constructive interference this path difference should be the integral multiple of
the wavelength so the above equation becomes
n= 2dhkl sinɵ 2.2
where ‘hkl’ are Miller indices
Using the data of the peaks, obtained from XRD patterns of polycrystalline hexagonal
structure, different structural parameters such as lattice constant (a & c), crystallite size (D),
cell volume (V) and X-ray density (ρx) can be calculated by using the following formulae.
The lattice parameters (a & c) of hexagonal crystal structure can be calculated by using the
formula
1 43
2.3
where ‘hkl’ are Miller indices and ‘d’ is the interplaner spacing.
The crystallite size ‘D’ is calculated by using Full Width at Half Maximum (FWHM),
obtained from the diffraction peaks in Scherrer’s formula given by
D0.89Cosɵ
2.4
where ‘’ is wave length, ‘’ is FWHM and ‘ɵ’ is the Bragg angle
The volume of unit cell can be calculated from formula
V √32a c2.5
X-ray density (theoretical density) is calculated by the formula
ρ 2.6
where ‘n’ is number of formula units per unit cell, ‘Mm’ is the molar mass, ‘V’ is the volume
of unit cell and ‘NA’ is the Avogadro’s number.
Porosity is calculated by the formula
P 1ρρ
2.7
In the present work, variation in structural parameters and phase formation of the
synthesized material was examined by X-ray diffraction machine (PANalytical X'pert pro
PMD) with Cu-Kα as the radiation source. The machine was operated at 40kV and 30mA.
An image of PANalytical X'pert pro MPD X-ray diffractometer is shown in figure 2.2.
2.2ScanningElectronMicroscopy(SEM)
X-ray diffraction provides information about the internal structure of crystal such as
interplaner spacing, lattice constant and hence provides the information about the type of
crystal structure. But XRD does not provide the image of the particles of a material. To get
direct image of particle, electron microscopy is used. The principle of electron microscope is
similar to that of optical microscope and the only difference is of wave length and hence
resolution. The wave length of electron beam used in electron microscope is much smaller
(about 5 orders of magnitude) than that of visible light. This short wave length results in high
resolution and provides much clear scans of short structures.
Fig 2.2: PANalytical X'pert pro MPD X-ray diffractometer
In scanning electron microscope, the secondary and back scattered electrons emitted
from the surface of the material under testing are received by a detector and amplified. The
variation in emitted and back scattered electrons is converted in to SEM image. This provides
highly resolved details of small structures. This technique helps a lot in understanding the
minute variation in the surface morphology of a material.
When the electron beam strikes the surface of a material under testing, the surface gets
charged and its electric field defocuses the electron beam. To avoid this problem, the surface
of the sample should be conducting or made conducting by a very thin coating of a
conducting material. This coating layer is grounded to ground any charge accumulated on
surface.
In the present work, the surface morphology and grain size of the prepared material
was studied by the field emission scanning electron microscope (FE-SEM) JEOL JSM-6700F.
An image of JEOL JSM-6700F scanning electron microscope (SEM) in our department is
shown in figure 2.3.
Fig 2.3: JEOL JSM-6700F scanning electron microscope (SEM)
2.3Frequencydependentacmeasurements
Frequency dependent electrical properties are measured by precision component
analyzer (6400B). This system provides both two probe and four probe methods to take
different measurements in the frequency range 20Hz to 3MHz. The ac measurement derive
level of this instrument varies from 1mV to 10V rms. This instrument measures real and the
imaginary parts of an impedance vector. Then it converts them into the required parameters
automatically. Impedance is the measure of the total opposition to the flow of an alternating
current at particular frequency offered by the circuit or device. It is an important parameter to
characterize an electronic component or instrument. The real part of impedance vector is
named as resistance R and imaginary part is named as reactance X. In terms of rectangular
coordinates, the impedance vector is expressed as Z=R+jX. The reciprocal of the impedance
(1/Z) gives admittance (Y). The expression for admittance becomes Y=G+jB where G
represents conductance and B susceptance. The SI unit of impedance is ohm (Ω) and
admittance is siemen (S).
The quality factor Q determines the purity of reactance. This term is more often used
for inductors. For capacitors, the term dissipation factor (tanδ) is commonly used. The
dissipation factor (tanδ) is usually expressed in terms of tanδ and is defined as the ratio of the
energy dissipated by the component to the energy stored in a component.
The values of capacitance (C) obtained from precision component analyzer at
different frequencies are used to calculate dielectric constant () using the formula:
εCdAε
2.8
where ‘d’, ‘A’ and ‘0’ are the thickness, area of pellet and is the permittivity of free space
respectively.
Dissipation factor (D) obtained from precision component analyzer at different
frequencies is taken as dielectric loss tangent (tanδ).
Dielectric loss factor (ε´´) is calculated by using the equation
ε ε tanδ2.9
Frequency dependent ac conductivity (σac) is calculated by using the equation
σ ε ε tanδ2.10
2.4Temperaturedependentdcresistivitymeasurements
During literature survey it was observed that the electrical properties of M-type hexaferrites
yet could not get too much attention of scientists although their electrical properties are of
much importance. These materials have lot of applications in market (especially in high
frequency devices) because of their large dc resistivity and good dielectric properties.
The dc electrical resistivity is usually measured by two methods.
1. Two probe method
2. Four probe method
Usually two probe method is used for the materials having electrical resistivity much higher
than the resistivity of contact probs. The figure 2.4 (b) shows that the resistivity of the probes
also adds up in the resistivity of the material but as their resistivity is much smaller than the
resistivity of the material so can easily be ignored.
Secondly two probe method is better to use for resistivity measurements at high temperatures
(400 C). On the other hand, four probe method, shown in figure 2.4 (b), is used where the
material have very low electrical resistivity. In this method, the resistivity of contacts is not
included in the resistivity of the material under consideration. Usually silver
paste is used for connections which becomes unstable at high temperatures.
Fig 2.4: (a) Two-terminal and (b) four-terminal resistance measurement arrangements[148]
Fig 2.5: Circuit diagram of the apparatus used for dc electrical resistivity measurements
Strontium hexaferrites are considered as very high resistive material so two probe
method is used for their temperature dependent resistivity measurements. The circuit diagram
of dc resistivity measurement system is shown in figure 2.4. The sample is sandwiched
between two electrodes. The sample along with electrodes is placed in a tube furnace. The
furnace is connected with ac power supply and its temperature is controlled by a power
regulator. The electrodes are connected with dc source. The pressure contacts are made
between electrodes and pellet. A volt meter and a sensitive ammeter are connected in parallel
and series with the circuit respectively. The variation in current due to change in temperature,
with step size of 5C, is noted. The values thus obtained are used to calculate temperature
dependent dc resistivity and activation energy of the samples.
The dc resistivity ‘ρ’ is calculated by using the formula
ρ RAL2.11
where ‘R’ is the resistance, ‘L’ is thickness and ‘A’ is the area of cross-section of pellet.
The activation energy is calculated the Arrhenius relationship given by
ρ ρ exp∆Ek T
2.12
where ‘ρ’, ‘ρ0’, ‘ΔE’, ‘kB’ and ‘T’ are resistivity at temperature T, resistivity extrapolated to
1/T = 0, constant, activation energy, Boltzmann’s constant and temperature respectively
[123].
The drift mobility (µd) is calculated by using equation
μ1neρ
2.13
where ‘e’ is charge of electron, ‘ρ’ is resistivity and n is the concentration of charge carriers given by the equation
nN D P
M2.14
where ‘NA’ is the Avogadro’s number, ‘DB’ is the bulk density, ‘PFe’ is
the number of iron atoms and ‘M’ is the molecular weight of the chemical formula.
2.5Magnetichysteresis
The dc magnetometer (RIKEN DENSHI) was used to obtain hysteresis loop and
hence different magnetic parameters. The electromagnet can generate the field up to 3 tesla.
The North and South Pole faces have the diameter (ф = 50 mm) and can be separated up to
100 mm. The dc magnetometer provides both MH-loop and BH-loop in a single step with
controlled sweep rates. The image of the dc magnetometer (RIKEN DENSHI) is shown in
figure 2.6.
The magnetic parameters such as coercivity Hc is taken directly from loop table while
Ms is calculated by using formula:
M 2.15
The Ms thus obtained is divided by density of sample in order to get its values in emu/g
Fig 2.6: The dc magnetometer (RIKEN DENSHI)
Fig 2.7: A typical hysteresis loop along with data obtained from the dc magnetometer
Chapter 3
Synthesis techniques
3. Synthesis techniques
The properties of a material are greatly dependent upon microstructure of nano
particles. The microstructural stress, grain size, , vacancies, phase purity, impurities, porosity,
surface defects etc. are largely affected by the synthesis and post synthesis technique
followed. These parameters could be controlled to large extent by choosing appropriate
synthesis technique along with suitable preparation conditions. Scientists have used different
synthesis techniques to prepare strontium hexaferrites.
There are two different routes to synthesis a material.
1. Solid state reaction method
2. Wet chemical methods
3.1 Solid state reaction method
In solid state reaction method, individual oxides are mixed in stoichiometric
quantities and are ground well to obtain homogeneous mixture. The grinding is made by
different methods such as ball milling, mortar and pestle etc. The ground powder is subjected
to annealing usually at elevated temperatures for longer time. This results in large particle size
with wide particle size distribution. Appearance of impurity phases also remain a big
issue[59, 149, 150].
3.2 Wet chemical method
Wet chemical method is also considered as non conventional method which includes
1. Precursor method
2. Spray-drying method
3. Freeze-drying method
4. Microemulsion method
5. Hydrothermal method
6. Sol-gel method
7. WOWS sol-gel method
8. Co-precipitation method
3.2.1 Precursor methods
The precursor method provides a precise stoichiometry for the synthesis of ferrites. In
this method the precursors, containing the reactants in the required stiochiometry, are
decomposed upon heating and ferrites are formed.
3.2.2 Spray-drying
Precipitates of a material could be produced by the evaporation of solvent from
concentrated solution of cations. In order to keep the particle size small, the concentrated
solution is converted into fine droplets using high pressure and solvent is removed by a
stream of hot gas. The particles obtained are converted into a compact powder and annealed
to get required phase of material.
3.2.3 Freeze-drying method
This method also involves the conversion of concentrated solution of cations into fine
droplets but these droplets are rapidly freezed by passing them through a bath at very low
temperature such as liquid nitrogen or ice-acetone. The particles obtained are dried by
sublimation of the ice in vacuum.
3.2.4 Microemulsion method
Microemulsion is the dispersion of two immiscible liquids particles stabilized by a
surface active coating [151]. It is the mixture of oil, water and surfactant. The oil is mixture of
olefins and hydrocarbons. In this technique, the surfactants formed an interface between oil
and water. The surfactant forms hydrophobic tails dissolved in oil and the hydrophilic head
dissolved in the aqueous phase. The nature of surfactant plays very important role in the
synthesis of nano materials. Water oil microemulsion method has been used by the scientists
in synthesizing metals, halides and oxides [152-156]. Different surfactants have been used by
the scientists for the synthesis of M-type hexaferrites [151, 157-160]. Microemulsion is high
cost technique [161]. Microemulsion technique involves the probability of absorption of
surfactants at the surface of nano particles which acts as impurity [162]. This method yields
very small material and faces reproducibility problems [163, 164].
3.2.5 Hydrothermal method
It is the technique used to crystallize a material by using aqueous solution at high
temperature and pressure. Hydrothermal synthesis takes place in a closed container in which
water is used as solvent. When this container is heated, a very high pressure is produced in the
container and the reaction which takes place at such elevated pressure is known as
hydrothermal reaction. When the temperature of a liquid increases from its critical point, the
liquid is said to supercritical and the fluid behaves both like liquid as well as gas. The surface
tension of that liquid becomes negligible and dissolve the compound easily. The critical
temperature and pressure of water is 374 C and 218 atm respectively. Different compounds
such as SrFe12O19 have been synthesized by this method [53, 165-168]. But the reaction rate is
slow and in order to increase the reaction rate ultrasonic, electric field or microwaves are used
which increases their cost. The hydrothermal system in which ultrasonics are used for heat
treatment is called ultrasonic-hydrothermal. Similarly the hydrothermal system using
microwaves and electric fields for heating are called microwave-hydrothermal and
electrochemical-hydrothermal respectively[169-171].
3.2.6 Sol-gel method
This method is also known as chemical solution deposition method. This method is
widely used for the synthesis of nano materials starting from chemical solutions (sol) called
precursors. Most of the precursors are metal chlorides and alkoxides which refer to various
forms of hydrolysis and polycondensation reactions. The sol is converted into a solid called
gel. This gel contains some solvent which is removed by a drying process. The rate of
removal of solvent determines porosity and microstructure of final product. The gel obtained
is converted into required material after heat treatment. This heat treatment improves the
crystallinity and densification of the material. In this method pH, temperature and
composition of the solution determine the particle size [157]. The gel formed by this method
takes long time to dry. It also requires high annealing temperature for long time to get phase
purity.
3.2.7 WOWS sol-gel method
The WOWS sol-gel method has number of advantages over other sol-gel methods.
This method does not require surfactants or templates [172].
In WOWS sol-gel method, stoichiometric amounts of different precursors are
dissolved in ethylene glycol. The molar ratio between metal salts to ethylene glycol is kept at
1:14 to dissolve the metal salts homogeneously. The pH of solution was 1. The solution is
stirred for 30 min at room temperature in order to prepare homogeneous solution. The
temperature of this solution is raised to 100oC with continuous stirring. By doing so, a thick
gel is formed. The gel so obtained is heated to 300oC. At 300oC, the gel dried and was burned
slowly and transformed into fine powder.
The proposed reaction for all the nitrates with ethylene glycol may be like following
3.2.8 Co-precipitation method
In co-precipitation reactions, the solution of the salts, required for final compound,
are prepared. Usually water soluble salts are used but the salts which are not soluble in water,
are dissolved in acids. The solutions of all the precursors are mixed and a precipitating agent
(basic solution such as solution of NaOH, KOH, NH4OH etc) is added. With the addition of
precipitating agent, nucleation (precipitates) sites are developed and growth of grains and
their agglomeration starts to become more thermodynamically stable particles [173]. The
number of nucleation sites, their growth and agglomeration depends upon pH, temperature,
stirring speed and time of solution [174]. Faster the nucleation process, the smaller would be
the particle size with narrow size distribution [163]. In co-precipitation reaction, chlorides,
OFe
CH2
CH2
OH
OH
CH2
CH2
OFe
OH
+ Fe(NO3).9H2O
CH2
CH2
OFe
nitrates or acetates are formed which are removed during washing process and hydroxides are
removed during heat treatment. The synthesis of our work is based on co-precipitation method
because it has number of advantages such as it provides [33]
1. Greater homogeneity
2. Greater reactivity
3. High purity - no grinding
4. Fine particle size with narrow distribution
5. Elimination of calcinations
This method is also famous for its low cost and provides reproducible results. Co-
precipitation provides low porosity, greater homogeneity of small grain sizes and cation
distribution in them. Because of these excellent properties, we used co-precipitation
method for the synthesis of our material (M-type strontium hexaferrites). WOWS sol-gel is
also used to compare the results obtained with co-precipitation method. The steps followed
for the synthesis of strontium hexaferrites by co-precipitation method are shown in figure 3.1.
Fig 3.1: Schematic diagram for the chemical co-precipitation method.
Chapter 4
Optimization of synthesis parameters for phase purity
4. Optimization of synthesis parameters for phase purity
Strontium hexaferrite is synthesized by chemical co-precipitation technique. Different
synthesis parameters such as molar ratio (Fe/Sr), volume rate at which the precipitating agent
is added and pH of solution is optimized. The effect of variation in molar ratio (Fe/Sr) on the
phase purity of strontium hexaferrite is observed from X-ray diffraction patterns. Different
volume rate of addition of precipitating agent are used and their effect on phase purity and
structural morphology is studied by using X-ray diffraction patterns and scanning electron
micrographs (SEM images). The pH of the solution plays a major role in the formation of a
precipitates. The pH of the solution was varied and its effect on phase purity, structural
morphology and electrical properties is studied
4.1 Synthesis
In the present work, the chemicals used for the synthesis of different sample series
were of analytical grade and their details are given below in table 4.1.
Table 4.1: List of the chemicals used with their specification
S. No. Compound Formula Molar mass % Purity Company
01 Strontium nitrate Sr(NO3)2 211.63 g 99.0 Merck
02 Ferric nitrate nine
hydrate Fe(NO3)3.9H2O 404.00 g 98.0 Merck
03 Chromium nitrate
six hydrate Cr(NO3)3.6H2O 400.21 g 98.0 Uni-Chem
04 Zinc nitrate nine
hydrate Zn(NO3)3.9H2O 297.40 g 98.0
Sigma-
Aldrich
05 Sodium hydro-
oxide NaOH 40.00 g 99.0
Sigma-
Aldrich
For the compositions of strontium hexaferrite synthesized by co-precipitation
technique [143], all the salts (Fe(NO3)3.9H2O & Sr(NO3)2) were dissolved in de-ionized water
with required molarities. The solutions of the precursors were mixed and heated on hot plate
with violent stirring. When the temperature of the solution was reached to 70C, calculated
amount of 2M NaOH solution added abruptly and maintained the pH of the solution at
130.03. This final solution was stirred for one hour to get fine particle size with greater
homogeneity. The precipitates thus obtained were washed well with de-ionized water. The
washed precipitates were dried at 105C in an electric oven. The dried precipitates were
crushed in to fine powder and then annealed at 925C in a programmed furnace. The
temperature was increased at the rate of 5C/min. The annealed powder was converted in
pellets by using uniaxial press and then sintered at 910C in furnace for 20min. These
sintered pellets were used for different characterizations. All the composition of strontium
hexaferrites with different dopants such Cr and Cr-Zn were synthesized under the similar
conditions mentioned above.
4.2 Effect of variation in molar ratio (Fe/Sr) on structural properties of SrM
The chemical formula of M-type strontium hexaferrite is SrFe12O19. According to this
formula, the molar ratio between Fe and Sr is 12. But researchers used different molar ratio in
order to get single phase material [143, 175]. In order to investigate the effect of molar ratio
on phase purity, the molar ratio is varied from 12 to 08 with step size of one and its effect on
phase purity is studied by X-ray diffraction analysis. Indexed XRD patterns of molar ratio
varying samples are shown in figure 4.1.
Fig 4.1: Indexed XRD patterns of SrFe12O19 for different molar ratios (MR=Fe/Sr)
It can be seen from the figure that all XRD patterns are almost similar and no
impurity peak is present. It shows that variation in molar ratio (Fe/Sr) in the range 12-08 does
not affect phase purity of strontium hexaferrite. We used theoretical molar ration (Fe/Sr) =12
for the synthesis of all compositions.
4.2.1 Conclusion
A systematic study is made to know that how molar ratio (Fe/Sr) affects the phase
purity of hexaferrites. Different samples with varying molar ratio in the range 12-08 has been
prepared and characterized. This was required as one can find only a scattered study in the
already reported like Chen et al. [[143]] used molar ratio (Fe/Sr) greater than 12. Ataie et al.
[176] prepared strontium hexaferrites by co-precipitation method and used molar ratio (Fe/Sr)
8 to get phase pure hexaferrite material. Iqbal et al. [[65]] synthesized strontium hexaferrites
by chemical co-precipitation method and used 11 molar ratio (Fe/Sr) and got single phase
material. The effect of variation in molar ratio (Fe/Sr) on phase purity is studied by using
XRD patterns. The indexed XRD patterns indicate that almost all the patterns are similar and
no impurity peak is detected. Thus variation in molar ratio in the range 12-08 does not affect
the phase purity of strontium hexaferrites.
4.3 Volume rate of addition of precipitating agent on structural
properties of SrM
During optimization of different synthesis parameters, it is observed that volume rate
at which the precipitating agent is added, also greatly affect the formation of phase pure
material. Number of samples was prepared by varying only volume rate at which the
precipitating agent is added and its effect on phase purity and structural morphology was
observed by using X-ray diffraction patterns (XRD) and scanning electron microscope
(SEM). Indexed XRD patterns of the samples with different volume rate at which the
precipitating agent is added, are shown in figure 4.2.
The XRD patterns clearly indicate that as the volume rate at which the precipitating
agent is added, decreases, the impure phase increases. It is concluded from the above results
that abrupt addition of precipitating agent results in more pure phases as compared to drop by
drop addition.
The effect of volume rate of addition of the precipitating agent on structural morphology was observed from SEM micrographs shown in figure 4.3. The figure shows the variation in particle size with the change in volume rate at which the precipitating is added.
30 40 50 60 70
160ml/min
2000ml/min
30ml/min
*
**
*
*
(4 4
0)
(0 1
4)
(0 2
12
)
(1 1
12
)(0
0 1
4)
(0 3
2)
(1 2
4)
(0 1
11
)
(0 1
10
)
(0 2
5)
(0 2
3)
(0 1
8)
(1 1
4)
(0 1
7)
(1 1
2)
(0 0
8)
Inte
nsity
(a.
u.)
2
*
(Deg.)
- Fe2O3
A
B
C
Fig 4.2: Indexed XRD patterns of the samples with different volume rate of addition
of the precipitating agent
Fig 4.3: SEM micrographs of the samples with different volume rate of addition of the precipitating agent
Clearly the particle size obtained is smaller in case of abrupt addition of precipitating agent as
compared to drop by drop addition. It is concluded from these results that abrupt addition of
precipitating agent is useful to obtain single phase strontium hexaferrite with small particle
size. So abrupt addition of precipitating agent is used in the synthesis of all composition
presented in this work.
4.3.1 Conclusion
During optimization of different synthesis parameters, it was observed that volume
rate of addition of precipitating agent also imparts a major effect on phase purity of strontium
hexaferrites. The effect of volume rate of addition of precipitating agent on phase has been
studied by using XRD and SEM micrographs. The indexed XRD patterns indicate that the
increase in volume rate of addition of precipitating agent improves the phase purity. The SEM
micrographs show that the particle size decreases with the increase in volume rate of addition
of precipitating agent. This may be due to the reason that the high volume rate of addition of
precipitating agent produces more nucleation sites than that with slow addition. As the
number of nucleation sites increases, the agglomeration of particles decreases. So high
volume rate of addition of precipitating agent is good to get phase pure and small particle
sized strontium hexaferrite material. This parameter has not been studied so far to the best of
our knowledge and its addition to the literature would be very useful for the scientists
working on strontium hexaferrites.
4.4 Effect of variation in pH on structural and electrical properties of strontium hexaferrites (SrM)
In co-precipitation technique, pH of the solution is the key factor which imparts its
large influence on the particles homogeneity, shape, size and their distribution and hence on
the electrical properties of the synthesized material. Keeping these effects in view, pH varying
samples (from 13 to 8 with step size of one) of strontium hexaferrites are synthesized in order
to investigate its effect on phase purity, particle size with its distribution and on electrical
properties.
4.4.1 Effect of pH on the structural properties of SrM
The structural properties of strontium hexaferrite samples, with different pH are
investigated from XRD patterns and SEM micrographs. The indexed XRD patterns of pH
varying samples are given in figure 4.4. Indexed XRD patterns show that as pH is decreased
from 13, extra phase of α-Fe2O3 started appearing and increases with the further decrease in
pH. Parameters such as crystallite size (D), lattice parameters a (Å) and c (Å), cell volume (V)
and X-ray density (ρx) are calculated from indexed XRD patterns and are given in table 4.2.
Structural morphology is studied from SEM micrographs and is given in figure 4.3. The
micrographs of pH varying samples of strontium hexa-ferrite shows that the particle size
increases with the decrease in pH and most of the particles have hexagonal structure with
diameter in the range of 0.4–3.5µm.
The particle size distribution also decreases on increasing the pH of the solution and
becomes very narrow for pH=13. This behavior is different as reported by Hessien et al.
[144]. The particle size distribution of pH varying samples is given in table 4.2. As the pH
increased, the rate of nucleation sites formation increased which resulted in decrease in
agglomeration.
Fig 4.4: Indexed patterns of XRD of samples of SrFe12O19 for different values of pH
(- SrFe2O4, # -α-Fe2O3)
Table 4.2: Lattice constants (a & c), crystallite size (D114), cell volume (V), X-ray density (ρx), bulk density (ρm), % α-Fe2O3 and particle size
pH=13 pH=12 pH=11 pH=10 pH=09 pH=08
Lattice Constant a (Å)
c (Å)
5.82(5)
23.12(4)
5.88(2)
23(6)
5.86(3)
22.84(9)
5.87(2)
23.03(4)
5.89(2)
23.04(2)
5.86(3)
23.06(4)
Crystallite size (D114 )
(nm)
52 52 70 59 69 59
Volume V (Å3) 677(13) 687(7) 690(10) 686(7) 693(5) 686(10)
X-ray Density ρx (g/cm3) 5.21 5.13 5.11 5.14 5.09 5.14
Bulk Density ρm (g/cm3) 3.27±0.02 3.18±0.03 3.39±0.02 3.53±0.02 3.29±0.03 3.63±0.04
% age of α-Fe2O3 0 5 10 16 20 24
Particle Size (µm) 0.6-0.9 0.5-1.2 0.5-1.5 0.5-2.0 0.8-3.0 0.4-3.5
Fig 4.5: SEM micrographs of the sintered samples of SrFe12O19 for different values of pH
4.4.2 Effect of pH on dc electrical properties of SrM
Temperature dependent dc electrical resistivity measurements
The variation in electrical current was measured due to the variation in temperature at
constant voltage by using two probe method shown in figure 2.4 (a). The data thus obtained
was used to calculate dc electrical resistivity of samples with varying pH. Figure 4.6 clearly
indicates that the dc resistivity decreases with the increase in temperature so M-type ferrites
behave like semiconductors [17]. This is due to the fact that the kinetic energy of the electrons
increases with the rise in temperature. The rate of hopping of electrons from one octahedral
site to the other increases and hence the resistivity decreases. Moreover the decrease in
resistivity with the decrease in pH is because of variation in particle size. As the particle size
increases, the number of grain boundaries decreases which acts as resistive medium and hence
resistivity decreases. The activation energy obtained from the slope of linear fit of the plot of
resistivity measurements also decreases with the increase in pH. It is mainly due to the
decrease in grain boundaries.
Effect of temperature on drift mobility of charge carriers is shown in figure 4.7. The
graph of drift mobility verses temperature shows that it increases with the rise in temperature.
This is because the electrons start moving easily from one interstitial site to another due to the
increase in temperature. The values of activation energy and drift mobility at 603K are given
in the table 4.3.
15 20 25 30 35 408
10
12
14
16
18
20
22
24
pH=08pH=09pH=10pH=11pH=12pH=13Linear fit
ln
(
-cm
)
1/kBT (eV)-1
Fig 4.6: Plot of ln of dc electrical resistivity of SrFe12O19 for different values of pH as a function of temperature
350 400 450 500 550 600 650 700 750
0.0
5.0x104
1.0x105
1.5x105
2.0x105
2.5x105
pH=08
d
V-S
c
m-2
Temperature (K)
0
1x104
2x104
3x104
4x104
5x104
d
V-S
c
m-2
pH=09pH=10pH=11pH=12pH=13
Fig 4.7: Plot of drift mobility versus temperature of SrFe12O19 for different values of pH as a function of temperature
Table 4.3: Dielectric loss (tanδ), dielectric constant (έ), dc electrical resistivity (ρdc) and drift mobility (μd) of SrFe12O19 for different values of pH
.
pH=13 pH=12 pH=11 pH=10 pH=09 pH=08
tanδ (1kHz) 1.64(2) 2.05(3) 2.36(03) 2.70(2) 3.65(2) 4.00(4)
tanδ (3MHz) 0.061(1) 0.063(1) 0.082(1) 0.142(1) 0.305(1) 0.341(1)
ε´ (1kHz) 211.0(3) 158.0(2) 146.0(2) 128.0(2) 117.0(2) 111.0(2)
ε´ (3MHz) 30.0(1) 28.0(1) 26.0(1) 23.0(1) 20.8(1) 14.6(1)
ρdc (Ω-cm) at
603K 1.66x107 1.20x107 5.90 x106 1.12 x107 8.28x105 4.96x104
4.4.3 Effect of pH on frequency dependent ac electrical measurements of SrM
Dielectric constant
The dielectric properties of ferrites could be explained by Maxwell–Wagner two layer
model in section 1.8.6. The first layer is composed of large number of grains that acts as
conducting layer at higher frequencies and the other layer consists of grain boundaries that act
as highly resistive medium at lower frequencies. The polarization and conduction mechanism
in ferrites are similar i.e. by electron exchange between ferrous (Fe2+) and ferric (Fe3+) ions
[126].
The charge polarization takes place by electron hoping between ferrous (Fe2+) and
ferric (Fe3+) ions. As the applied field frequency increases, it becomes more difficult for the
electron to hope from ferrous (Fe2+) and ferric (Fe3+) ions with the alternating frequency, the
net displacement of charge in one direction decreases and hence dielectric constant decreases.
It is observed that at relatively lower frequencies dielectric constant is high. It might be
because of moisture, voids, dislocations, density and impurities [177]. The Figure 4.8 shows
that the dielectric constant (ε') depends upon the pH value of the samples. It decreases with
the decrease in pH and this effect is prominent at low frequencies. This is because of the grain
size and
6 8 10 12 140
40
80
120
160
200
240
280
pH=08pH=09pH=10pH=11pH=12pH=13
Die
lect
ric C
onst
ant
(')
ln f
Fig 4.8: The plot of dielectric constant (έ) as a function of ln of frequency of SrFe12O19 for different values of pH
secondary phase (α-Fe2O3) which increases with the decrease in pH. The concentration of Fe2+
ions decreases due to which the polarization decreases and hence dielectric constant decreases
[178]. This could also be explained by Koops [179] model according to which the dielectric
constant at low frequency is because of grain boundaries. These grain boundaries acts as high
resistive medium and thus contribute to high dielectric constant. As the decrease in pH results
in increase in the grain size and decrease in grain boundaries and hence decrease in dielectric
constant [180].
Dielectric loss tangent (tanδ)
The figure 4.9 shows that the dielectric loss (tan δ) for all samples is greater at low
frequencies and decreases rapidly with the increase in frequency. At lower frequencies, high
dielectric loss may be because of impurities, crystal defects and moisture. The decrease in
dielectric loss with increasing frequency is because of charge polarization. As the applied
field frequency increases, the polarization lags the alternating field. The net polarization
decreases and hence dielectric loss decreases. The figure 4.9 also shows that at lower
frequencies, dielectric losses increases with the decrease in pH and becomes nearly uniform at
higher frequencies. This is because of conduction losses due to the electron hoping between
Fe2+ and Fe3+ ions [177]. Figure 4.5 indicates that resistivity of the samples decreases with the
decrease in pH and hence conductivity increases. As the decrease in pH results increase in the
grain size and decrease in grain boundaries. Thus conductivity increases and conduction
losses increases. The values of dielectric constant (ε') and dielectric loss tangent (tanδ) of
samples of SrFe12O19 for different values of pH is given in table 4.3.
6 8 10 12 14
0
1
2
3
4
5
6
7
pH=08pH=09pH=10pH=11pH=12pH=13
Die
lect
ric L
oss
Tan
gent
(ta
n)
ln f
Fig 4.9: The plot of dielectric loss tangent (tanδ) as a function of ln of frequency of SrFe12O19 for different values of pH
The dielectric loss factor (ε'') is calculated by using equation 2.12 [181]. The plot of
dielectric loss factor (ε'') as a function of frequency for all samples is shown in the figure
4.10. The trend is similar to dielectric loss (tan δ) as shown in the figure 4.8. At lower
frequencies, high dielectric loss factor (ε'') may be because of impurities, crystal defects and
moisture. Hudson et al. has shown that the dielectric losses in ferrites are mainly because of
conduction mechanism due to space charge polarization. As the frequency of the applied
electric field increases, the charges could not follow the alternating field. As a result the
polarization decreases and hence
6 8 10 12 14
0
200
400
600
800
1000
pH=08pH=09pH=10pH=11pH=12pH=13
Die
lect
ric
Loss
Fac
tor
('')
ln f
Fig 4.10: The plot of dielectric loss factor (ε'') as a function of log of frequency of SrFe12O19 for different values of pH
dielectric loss decreases. The figure 4.10 also shows that at lower frequencies, dielectric
losses factor (ε'') increases with the decrease in pH and becomes nearly uniform at higher
frequencies. This is because of the similar reason as explained above in dielectric loss (tan δ).
These results are consistent with dc resistivity measurements shown in figure 4.6. As the
decrease in pH results in increase in the grain size and decrease in grain boundaries. Thus
conductivity increases and conduction losses increases and hence dielectric loss factor (ε'')
increases.
4.4.4 Conclusion
pH of the solution plays a key role in controlling microstructural parameters and
phase purity of a material synthesized by co-precipitation method. In order to study the effect
of pH on structural and electrical properties of strontium hexaferrites, pH of the solution is
varied from 13 to 08. Indexed XRD patterns indicate that as pH increases the impurity phase
of α-Fe2O3 decreases and a single phase strontium hexaferrite is obtained for the sample
prepared by keeping pH=13. SEM micrographs show that the increase in pH results in the
decrease in particle size as well as its distribution. This may be due to the high pH of the
solution results in the increase in the number of nucleation sites and hence decreases the
possibility of agglomeration. The temperature dependent dc electrical resistivity
measurements indicate that dc resistivity increases with the increase in pH of the solution.
The increase in resistivity is due to the decrease in particle size which results in the increase
in the grain boundaries acting as highly resistive medium. The frequency dependent dielectric
loss decreases with the increase in pH of the samples. This behavior is also attributed to the
decrease in grain size. As the grain size decreases, the resistive area (grain boundaries)
increases and hence dielectric loss decreases. The decreasing trend of dielectric loss is
consistent with the variation in dc resistivity of samples prepared by varying pH. Both
structural and electrical measurements show that high pH of the solution is useful for the
synthesis of strontium hexaferrites.
Chapter 5
Structural, electrical and magnetic properties of Cr doped strontium hexaferrites
5. Structural, electrical and magnetic properties of Cr doped strontium hexaferrites
In ferrites, the conduction is because of the electron hopping from Fe2+ and Fe3+ ion
present on octahedral B sites [78, 79]. The presence of this Fe2+ also contributes a lot to the
dielectric properties of ferrites. It is reported in the literature that Cr3+ ions preferentially
occupy octahedral B sites [182] where it replaces Fe3+ ions. It impedes the motion of charge
carriers and results in tne increase in coercivity. The aim of the present work is to study the
effect of Cr substitution on the interstitial sites of strontium hexaferrite on frequency
dependent dielectric properties such as dielectric constant, dielectric loss tangent and
frequency dependent ac conductivity which has not been reported. In this work, effect of Cr
substitution on structural, electrical (temperature and frequency dependent) and magnetic
properties of strontium hexaferrite has been discussed.
5.1 Structural properties of Cr doped SrM
The crystallographic structure and phase formation of the composition SrFe12-
xCrxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 sintered at 940C is studied by using powder X-ray
diffraction data. All the peaks of X-ray diffraction (XRD) patterns shown in figure 5.1 are
identified by using ICDD patterns with reference code 01-080-1197. The indexed XRD
patterns show that all the compositions have major phase of strontium hexaferrite material.
The secondary phase of α-Fe2O3 showed an increasing trend with the increase in Cr content.
All the XRD patterns confirmed the successful substitution of Cr cations on the interstitial
sites of strontium hexaferrite lattice as there was no separate peak of these substituted cations
detected except for X=0.8. In this composition, a peak of Cr2O3 was appeared indicating that
the composition become over saturated with Cr and Cr formed its oxide instead of replacing
Fe3+ ions on interstitial sites of strontium hexaferrites. Parameters calculated from indexed
XRD patterns are given in table 5.1.
25 30 35 40 45 50 55 60 65 70
(1 1
2)
(0 0
8)
2 (Deg.)
Inte
nsity
(a.
u.)
0.4
0.6
0.8 (2 0
14
)
(2 1
8)
Cr2O3
#
(2 0
2)
*
(1 0
4)
(0 2
13)
(0 2
12
)
(1 1
12)
(0 0
14)
(0 3
2)
(0 2
7)
(0 1
10
)
(0 2
5)
(0 2
3)
(0 1
8)
(1 1
4)
(0 1
7)
*
-Fe2O3#
0.0
0.2
Fig 5.1: Indexed XRD patterns of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
( α-Fe2O3, # Cr2O3)
5.2 Frequency dependent ac electrical properties of SrM
For solids, Maxwell-Wagner two layer model is being commonly used to discuss
their dielectric properties. According to Maxwell-Wagner two layer model, the grain in a bulk
material acts as a resistor and grain boundary acts as thin insulating layer discussed earlier.
The dielectric properties are measured at room temperature in the frequency range (10kHz-
3MHz) using precision component analyzer.
Table 5.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk
density (ρm), porosity, activation energy (ΔE), dc electrical resistivity (ρdc), dielectric
constant (), dielectric loss tangent (tanδ), ac conductivity( σac), coercivity (Hc) and
saturation magnetization (Ms) of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8).
0.8 0.6 0.4 0.2 0.0 SrFe12-xCrxO19
5.878(2)
23.019(3)
3.92
5.888(5)
23.027(5)
3.91
5.889(2)
23.043(3)
3.91
5.888(2)
23.039(3)
3.91
5.884(2)
23.040(4)
3.92
Lattice Constant
a(Ȧ)
c(Ȧ)
c/a
58 39 61 81 44 Crystallite Size (D114) (1nm)
5.12 5.11 5.10 5.10 5.10 X-ray Density ρx (g/cm3)
3.02(2) 2.92(3) 3.00(2) 2.72(1) 2.88(2) Bulk Density ρm (g/cm3)
41 43 41 47 43 % Porosity
0.453(8) 0.444(8) 0.923(6) 0.391(4) 0.94(1) Activation Energy (eV)
1.92×106 4.23×105 2.23×108 4.11×105 1.78×108 DC resistivity ρdc (Ω-cm) at 200oC
59.0(1) 120.0(2) 37.0(1) 94.0(2) 23.0(1) Dielectric constant (ε') at 3MHz
0.240(1) 0.300(1) 0.210(1) 0.370(1) 0.320(1) Dielectric loss (tanδ)
at 3MHz
2.34×10-3 6.06×10-3 1.29×10-3 5.9×10-3 1.29×10-3 ac conductivity ac (S-m-1) at 3MHz
5.85 6.32 6.07 5.96 5.37 Coercivity Hc (kOe)
15.82 21.31 36.09 35.64 47.55 Saturation Magnetization Ms
(emu/g)
In ferrites, as described earlier, the polarization mechanism is similar to their
conduction mechanism i.e. because of the electron hopping from Fe2+ and Fe3+ ion present on
octahedral B sites. The presence of this Fe2+ contributes a lot to the dielectric properties of
ferrites. It is reported in the literature that Cr3+ ions preferentially occupy octahedral B sites
[182] where it replaces Fe3+ ions and increases the concentration of Fe2+ ions by transforming
Fe3+-Fe2+ for higher Cr content.
9 10 11 12 13 14 150
50
100
150
200
250
300
350
400
450 SrFe12-x
CrxO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Co
nst
an
t (
')
ln f
Fig 5.2: Plot of dielectric constant (ε') as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
5.2.1 The dielectric constant (ε')
The graph shown in figure 5.2 reveals that the dielectric constant ε' for all
compositions decreases sharply at lower frequencies and then becomes fairly constant for
higher frequencies. The dielectric polarization mechanism in ferrites is similar to their
conduction mechanism. The electron hoping on octahedral sites easily follow an external field
with low frequency and hence more the displacement of charge could take place inside the
grains and pile up at poorly conducting grain boundaries and hence causes large dielectric
constant. At high frequency, the hoping of the charge could not follow the applied field and
only local charge polarization could take place and hence net dielectric constant decreases.
0.0 0.2 0.4 0.6 0.80.0
0.5
1.0
1.5A
ctiv
atio
n e
ne
rgy
(eV
) Activation Energry Dielectric Constant at 3MHz
Cr Concentration
0
20
40
60
80
100
120
140
160
180
200
Die
lect
ric
con
sta
nt ( )
Fig 5.3: Plot of activation energy and dielectric constant at 3MHz versus Cr
concentration (X)
The figure 5.3 indicates that ε' have a wave like trend with the increase in doping content.
This could be explained on the basis of random distribution of Cr3+ ions on octahedral (2a and
12k) sites. As the polarization in ferrites is mainly due to hopping of electrons between ions
of the same element present in more than one valence state and the hopping probability
depends upon the separation between the ions involved and the activation energy [183]. The
distance between two iron atoms present on 12k sites is smaller than on any other site due to
small bond angle [184] so the presence of Fe2+ ions on 12k plays major role on the electrical
properties of strontium hexa-ferrites. It can be seen that dielectric constant increases for
X=0.2. It is possible that Cr3+ (0.755 Ȧ) ions may substitute Fe2+ (0.77 Ȧ) ions present on
octahedral (2a) site due to their similar ionic radii and for charge neutrality, Fe3+ ions present
on octahedral (12k) site transform into Fe2+ ions. As a result, concentration of Fe2+ ions on
12k sites increases and consequently polarization increases and hence dielectric constant
increases. It may also be due to the decrease in activation energy given in table 5.1. For
X=0.4, the decrease in dielectric constant may be due to the reason that Cr3+ ions may replace
Fe3+ present on 12k sites. The presence of Cr3+ ions on 12k sites impedes the motion of charge
carriers from Fe2+ to Fe3+ ions [185, 186] and thus increases the activation energy and
decreases the dielectric constant. For X=0.6, the increase in dielectric constant may be
attributed to the increase in Fe2+ ions concentration as higher Cr content causes the
transformation of Fe3+ - Fe2+ ions [187]. This may also be due to decrease in the activation
energy. For X=0.8, the decrease in dielectric constant, along with random distribution of Cr3+
ions, may also be due to the increase in activation energy.
5.2.2 The dielectric loss tangent (tanδ)
The general trend of the dielectric loss tangent (tan) shown in figure 5.4 could be explained
in terms of conduction losses which are the main source of dielectric losses. The major
source of conduction losses is the concentration of Fe2+ on octahedral sites. A relation
between conduction and dielectric losses was discussed by Iwauchi et al. [188]. As charge
polarization is similar to conduction mechanism in ferrites. In lower frequency region, the
dispersion in dielectric loss tangent is more than at high frequency region. It is due to the fact
that lower the frequency, more the time available for the displacement of the charge inside the
grains, more would be the conduction and higher would be the dielectric losses. As the
frequency increases, the electron hopping could not follow the applied frequency, the
conduction inside the grain decreases and hence dielectric loss tangent decreases. The figure 4
also reveals that the dielectric loss tangent has similar trend to that of dielectric constant with
the increase in doping concentration. This is mainly due to the variation in Fe2+ ion
concentration on 12k sites due to the substitution of Cr3+ on octahedral sites (explained
earlier) and activation energy. The compositions, where Fe2+ ion are more on 12k sites and
low activation energy, have higher dielectric losses and vice versa. Initially the dielectric loss
tangent increases due to the increase in Fe2+ ion concentration on 12k site, decrease in the
activation energy and high porosity (high moisture). Similarly it decreases due increase in
Cr3+ concentration on 12k site which impedes the motion of charge carriers, increase in the
activation energy and due to decrease in porosity (low moisture). The again increase in
dielectric loss tangent (X=0.6) may be attributed to the increase in Fe2+ ions concentration
discussed above. It decreases for X=0.8 due to increase in activation energy. The values of
dielectric loss tangent obtained from this composition of strontium hexaferrite are much
smaller than reported in [24, 65] so making this composition more suitable for high frequency
applications.
9 10 11 12 13 14 150
2
4
SrFe12-x
CrxO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Lo
ss T
an
ge
nt
(ta
n
ln f
Fig 5.4: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
5.2.3 The dielectric loss factor ()
The dielectric loss factor () [76] is calculated by using formula given in equation
2.12. The plot of the dielectric loss factor () shown in figure 5.5 indicates that it decreases
initially with the increase in frequency and then becomes fairly constant at higher frequencies.
It is because the hoping of electron could not follow the applied frequency and consequently
the dielectric loss factor () decreases. The variation in dielectric loss factor () with the
increase in Cr doping is due to the random distribution of Cr3+ ion on octahedral (2a and 12k)
sites, change in activation energy and Fe2+ ions concentration, explained in detail in the
previous section.
9 10 11 12 13 14 150
400
800
1200
1600
2000SrFe
12-xCr
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Lo
ss F
act
or
'')
ln f
Fig 5.5: Plot of dielectric loss factor () as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
5.2.4 The ac conductivity (ac)
The ac conductivity (ac) is calculated by using formula given in equation 2.13.
Figure 5.6 reveals that ac remains approximately constant at lower frequencies and then starts
increasing at higher frequencies. The hoping frequency of electron is small at lower
frequencies and it increases with the increase in frequency. This is attributed to the fact that
the required energy correlated with forward-backward hoping is only a fraction of the energy
necessary to activate long range diffusive conduction [189]. The variation in ac conductivity
with doping concentration is due to the variation in activation energy, Fe2+ ion concentration
and site occupancy of Cr ions.
9 10 11 12 13 14 150.0000
0.0008
0.0016
0.0024
0.0032
0.0040
0.0048
0.0056 SrFe12-x
CrxO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
ac (
S-m
-1)
ln f
Fig 5.6: Plot of ac conductivity (σac) as a function of ln of frequency for
SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
5.3 Temperature dependent dc electrical properties of SrM
The temperature dependent dc electrical resistivity was measured in the range of
373K-623K. Plot of ln of dc electrical resistivity as a function of 1/kBT is shown in figure
5.7. The dc electrical resistivity decreases with the increase in the temperature. This is due to
the fact that the kinetic energy of the electrons increases with the rise in temperature. The rate
of hopping of electrons from one octahedral site to the other increases, the drift mobility of
charge carriers increases and hence the resistivity decreases. The dc electrical resistivity of
ferrites mainly depends upon porosity, particle size and cations distribution on octahedral
sites. Higher porosity and smaller particle size cause the increase in dc electrical resistivity.
The Fe2+ ion concentration on octahedral site contributes a lot to the dc electrical resistivity.
Higher the concentration more would be the charge carriers available for conduction and
smaller would be the resistivity. The plot also indicates that the resistivity first decreases and
then increases with the increase in Cr concentration. This is due to variation in Fe2+
concentration on octahedral 12k site. The initial decrease (X=0.2) may also be due to the
increase in crystallite size which results in the decrease in grain boundaries (highly resistive
medium) and decrease (X=0.4) due to decrease in crystallite size and increase in activation
energy. For X=0.6, the dc resistivity decreases although the crystallite size decreases. This is
attributed to the increase in Fe2+ ion concentration and decrease in activation energy. The
resistivity again increases (X=0.8) due to the increase in activation energy and random
distribution of Cr3+ ion. The variation in dc electrical resistivity with increase in Cr content is
consistent with the variation in activation energy and dielectric measurements. Activation
energy was calculated from the slope of the linear plots of lnρ versus reciprocal of the
temperature shown in figure 5.7.
18 20 22 24 26 28 30 3210
12
14
16
18
20
22 X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Linear Fit
ln
cm
)
1/kBT (eV)-1
Fig 5.7: Plot of lnρ as a function of 1/kBT for SrFe12-xCrxO19
(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit.
5.4 Magnetic properties of Cr doped SrM
The magnetic parameters such as coercivity (Hc), saturation magnetization (Ms) and
remanence (Mr) calculated from hysteresis loops of the composition SrFe12-xCrxO19 with X
0.0, 0.2, 0.4, 0.6, 0.8 are shown in figure 5.8. The saturation magnetization (Ms) decreases
with the increase in Cr concentration. It is due to the substitution of Cr3+ cations on different
interstitial sites. In M-type
Fig 5.8: Hysteresis loops of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
hexaferrite, 24 Fe3+ ions are distributed on five different interstitial sites. There are three
octahedral (2a, 12k and 4f2), one tetrahedral (4f1), and one trigonal bipyramid (2b) site. The
sites (2a, 12k and 2b) are parallel and (4f1 and 4f2) are antiparallel. The iron atoms present on
these sites are coupled by super exchange interactions through the O2- ions, form the
ferrimagnetic structure[190, 191]. The M-type hexa-ferrite (magnetoplumbite structure)
contains two formula units per unit cell. 12 Fe3+ ions are arranged with eight spins in the up
direction and four in the down direction, giving a net moment of 4 Fe3+ ions per formula unit
times 5µB per ion, which gives a total of 20µB per formula unit [33].
0.0 0.2 0.4 0.6 0.8
5.4
5.6
5.8
6.0
6.2
6.4
Ms
(em
u/g
)
Hc (k
Oe
)
Cr concentration
Hc
16
20
24
28
32
36
40
44
48
-O- Ms
Fig 5.9: Plot of coercivity and saturation magnetization versus Cr concentration of SrFe12-xCrxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
According to ferrimagnetic theory [192], magnetism in ferrite originates from the net
magnetic moment of ions with spin up and spin down in interstitial sites. Figure 5.9 indicates
that saturation magnetization decreases with the increase in doping content. The saturation
magnetization mainly depends upon the distribution of Cr3+ ions on interstitial sites and
magnetic dilution with the substitution of Fe3+ ions by the lower magnetic moment ions. As
Cr3+ ions with magnetic moment (3 µB) preferentially occupy 12k and 2a sites with spin up so
net magnetic moment decreases and hence saturation magnetization decreases [182]. This
decrease may also be due to the appearance of non magnetic phase α-Fe2O3. Figure 5.9 also
indicates that coercivity (Hc) first increases for X0.6 and then decreases. The increase in
coercivity is attributed to the continuous increase in non magnetic phase α-Fe2O3 which acts
as pinning centers for domain wall motion. This increase could also be explained by using the
equation [193] given below:
H2KM
5.1
where ‘HA’ is the magnetocrystalline anisotropy field, ‘K1’ is constant and ‘Ms’ is saturation
magnetization.
According to equation 5.1, the decrease in saturation magnetization (Ms) results in an increase
in the magnetocrystalline anisotropy and hence in intrinsic coercivity.
Different factors such as Cr concentration, grain size and porosity contribute to the variation
in coercivity but different concentration, different factors are dominant. A sharp increase in
coercivity for X=0.2 is due to increase in the porosity and non magnetic phase α-Fe2O3. It
increases to very small value for X=0.4 due to decrease in crystallite size and increase in α-
Fe2O3 and increases sharply for X=0.6 due to sharp increases in porosity and decrease in
crystallite size. For X=0.8, the coercivity decreases due to decrease in porosity.
5.5 Conclusion
Cr doped strontium hexaferrites with composition SrFe12-xCrxO19 (X=0.0, 0.2, 0.4,
0.6, 0.8) has been prepared with co-precipitation method. The indexed XRD patterns indicate
that Cr doping causes the formation of secondary phases. For X=0.8, a peak of Cr2O3 is
appeared indicating the solubility limit of Cr in strontium hexaferrites. Hysteresis loops of the
samples with different Cr concentration indicate an increasing trend in coercivity. The
increase in coercivity is mainly due to increase in impurity phases. These impurity phases acts
as pinning centers and resist the domain wall motion.
Abbas et al. [194] has synthesized Cr doped strontium hexaferrites with composition
SrFe12-xCrxO19 (X=0.0, 0.1, 0.3, 0.5) by solid state reaction method. The sintering temperature
used by them is greater than 1200 ºC for 2h while the similar composition synthesized by us
with co-precipitation method is sintered at much lower temperature (940 ºC) for one hour.
The coercivity obtained by the above authors for X=0.5 is around 4.3 kOe which is much
smaller than that obtained in our case (>6 kOe) for similar composition. So Cr doped
strontium hexaferrite synthesized by co-precipitation method is better than that of solid state
reaction method with respect to both energy (sintering temperature) and magnetic properties.
As the increase in the coercivity results in the increase in the limit of the operating frequency
of the devices so Cr doped strontium hexaferrite synthesized by co-precipitation method is
more useful for the devices operating at high frequency.
Chapter 6
Structural, electrical and magnetic properties of Cr-Zn doped strontium hexaferrites prepared by
co-precipitation method
6. Structural, electrical and magnetic properties of Cr-Zn doped strontium hexaferrites prepared by co-precipitation method
In most of the electronic devices operating at high frequency, materials with very low
dielectric loss are required. One of the major sources of dielectric loss is the concentration of
charge carriers. In ferrites, the charge carriers are provided by Fe2+ ions. It is reported that
when Zn2+ being a divalent ion is doped in M-type hexaferrites, it preferentially occupy
tetrahedral (A) site where it replaces Fe3+ ions. For the charge neutrality, Fe2+ ions present on
octahedral (B) are changed to Fe3+ ions and hence the concentration of Fe2+ ions decreases.
Keeping this thing in view, Zn2+ was doped to decrease the concentration of Fe2+ ions in the
structure and hence to decrease the dielectric losses. It is reported that Cr (after certain
concentration) impedes the motion of charge carriers and previous study indicates that it
increases the coercivity and hence increases the operating limit of high frequency devices. So
keeping these observed results in view, Cr was also doped to reduce dielectric losses and
making the material useful for high frequency applications.
In the present work, Cr and Zn ions are substituted on the interstitial sites of strontium
hexaferrite and their effect on structural, electrical and magnetic properties of strontium
hexaferrites has been studied.
6.1 Structural properties of Cr-Zn doped SrM
The phase formation studies of the composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 are made by using powder X-ray diffraction data. All the
peaks of X-ray diffraction (XRD) patterns shown in figure 6.1 are identified by using ICSD
patterns with reference code 01-080-1197. All the XRD patterns confirmed the successful
substitution of Cr and Zn cations on the interstitial sites of strontium hexaferrite lattice as
there was no separate peak of these substituted cations detected. The impurity phase of α-
Fe2O3 is vanished for higher concentration of zinc. It might be due to the lower activation
energy of zinc ferrites. The parameters calculated from XRD patterns are given in table 6.1.
Table 6.1: Lattice parameters (a & c), average crystallite size (Dav), X-ray density
(ρx), bulk density (ρm), porosity, activation energy, dc electrical resistivity (ρdc), dielectric
constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the prepared samples of
SrFe12-2xCrxZnxO19 X 0.0, 0.2, 0.4, 0.6, 0.8 .
.
0.8 0.6 0.4 0.2 0.0 SrFe12-2xCrxZnxO19
5.883(9)
23.036(9)
3.91
5.884(2)
23.037(9)
3.91
5.884(3)
23.028(8)
3.91
5.882(8)
23.028(2)
3.91
5.884(2)
23.040(4)
3.92
Lattice Constant a(Å)
c(Å)
c/a
32 42 30 48 44 Crystallite Size (D114) (nm)
5.11 5.11 5.11 5.11 5.10 X-ray Density ρx (g/cm3)
2.49(2) 2.51(1) 2.58(2) 2.65(3) 2.88(2) Bulk Density ρm (g/cm3)
51 50 49 48 43 % Porosity
0.64(1) 0.67(1) 0.76(1) 0.84(1) 0.94(1) Activation Energy (eV)
3.65×106 5.5×106 1.75×107 3.08×107 1.78×108 DC resistivity ρdc (Ω-cm) at 200oC
8.6(1) 11.1(1) 16.5(1) 18.9(1) 23.0(1) Dielectric constant (ε') at 3MHz
0.020(1) 0.080(1) 0.150(1) 0.200(1) 0.320(1) Dielectric loss (tanδ)
at 3MHz
2.44×10-5 1.48×10-4 4.14×10-4 6.32×10-4 1.25×10-3 ac conductivity ac (S-m-1) at 3MHz
Structural morphology was studied by using scanning electron microscopy. The SEM
micrographs figure 6.2 showed that most of the particles are of hexagonal shape and their size
has increasing trend with the increase in Cr-Zn concentration. The average particle size for all
compositions was estimated by using diagonal method and its variation with Cr-Zn
concentration is given in the figure 6.3.
25 30 35 40 45 50 55 60 65 70
Inte
nsity
(a.
u.)
X=0.0
2 (Deg.)
X=0.2
X=0.4
X=0.6
X=0.8
(1 0
4)
*
(0 3
2)
(0 0
14)
(1 1
12)
(0 2
12)
(0 3
8)
(0 2
7)
(0 1
10)
(0 2
5)
(0 2
4)
(0 2
3)
(0 2
1)(1
1 4
)(0
1 7
)(1
12)(0
0 8
)
*
-Fe2O
3
Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
( α-Fe2O3)
6.2 Frequency dependent ac electrical properties of Cr-Zn doped SrM
For solids, Maxwell-Wagner two layer model is being commonly used to discuss
their dielectric properties. According to Maxwell-Wagner two layer model, the grain in a bulk
material acts as a resistor and grain boundary acts as thin insulating layer discussed earlier.
The dielectric properties are measured at room temperature in the frequency range (10kHz-
3MHz) using precision component analyzer.
Fig 6.2: SEM micrographs of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
0.0 0.2 0.4 0.6 0.8
200
300
400
500
Gra
in S
ize
(n
m)
Cr-Zn concentration (X)
Fig 6.3: Plot of grain size (nm) as a function of Cr-Zn concentration (X) for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
In ferrites, as described earlier, the polarization mechanism is similar to their
conduction mechanism i.e. because of the electron hopping from Fe2+ and Fe3+ ion present on
octahedral B sites. The presence of this Fe2+ also contributes a lot to the dielectric properties
of ferrites. It is reported in the literature that Cr3+ ions preferentially occupy octahedral B sites
[182] where it replaces Fe3+ ions and increases the concentration of Fe2+ ions by transforming
Fe3+-Fe2+ for higher Cr content. Zn2+ ions preferentially occupy tetrahedral A and may also
occupy bipyramidal C sites [145] where it replaces Fe3+. For charge neutrality, the Fe2+ ions
present on octahedral B site [127, 146] are converted into Fe3+ ions.
6.2.1 The dielectric constant ()
The graph shown in figure 6.4 reveals that the dielectric constant ε' for all
compositions decreases sharply at lower frequencies and then becomes fairly constant for
higher frequencies. The dielectric polarization mechanism in ferrites is similar to their
conduction mechanism. At low frequency, the electron could easily follow the applied field
and the time available for electrons to hope from one site to other sites is more. Hence more
the
9 10 11 12 13 14 150
20
40
60
80
100 SrFe12-2x
CrxZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Co
nst
an
t
')
ln f
Fig 6.4: Plot of dielectric constant (ε') as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
9 10 11 12 13 14 15
0.0
0.4
0.8
1.2
1.6
2.0SrFe
12-2xCr
xZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Lo
ss T
an
ge
nt
(ta
n
ln f
Fig 6.5: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for
SrFe12-xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
displacement of charge could take place inside the grain and hence causes large dielectric
constant. At high frequency, the hoping of the charge could not follow the applied field and
only local charge polarization could take place and hence net dielectric constant decreases.
The graph also indicates that ε' has a decreasing trend with the increase in doping contents.
This is due to the fact that in ferrites, the space charge polarization directly depends upon Fe2+
ion concentration in a grain. As Zn2+ ions have strong preference to occupy tetrahedral (A)
sites, so concentration of Fe2+ ions on tetrahedral site decreases as explained earlier. So
electric polarization decreases and consequently ε' decreases. This may also be due to the
reason that the Cr ions do not participate in the conduction process but impedes the
transformation of Fe2+ - Fe3+ ions [185, 186].
6.2.2 The dielectric loss tangent (tan)
The general trend of the dielectric loss tangent (tan) shown in figure 6.5 could be
explained in terms of conduction losses which are the main source of dielectric losses. A
relation between conduction and dielectric losses was discussed by Iwauchi et al. [188].
According to Iwauchi, charge polarization in ferrites is similar to their electrical conduction
mechanism. In lower frequency region, the dielectric loss tangent is more than at high
frequency region. It is due to the fact that lower the frequency, more the time available for the
displacement of the charge inside the grains which acts as a conductive medium, more would
be the conduction and higher would be the dielectric losses. As the frequency increases, the
electron hopping could not follow the applied frequency, the conduction inside the grain
decreases and hence dielectric loss tangent decreases. The figure 6.5 also shows that the
dielectric loss tangent also decreases with the increase in doping concentration. This may be
due to the decrease in Fe2+ ion concentration, which is responsible for conduction losses,
because of increase in Zn2+ content as explained earlier. The values of dielectric loss tangent
obtained from this composition of strontium hexa-ferrite are much smaller than reported in
[24, 65] so making this substitution (X=0.2 to 0.8) more suitable for high frequency
applications.
6.2.3 The dielectric loss factor ()
The dielectric loss factor () is calculated by using equation 2.12. The plot of the
dielectric loss factor () shown in figure 6.6 indicates that it decreases initially with the
increase in frequency and then becomes fairly constant at higher frequencies. It is because the
hoping of electron could not follow the applied frequency and consequently the dielectric loss
factor () decreases. The decrease in dielectric loss factor () with the increase in Cr-Zn
doping is due to the decrease in Fe2+ ion concentration.
9 10 11 12 13 14 15
0
40
80
120
160
200 SrFe12-2x
CrxZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Lo
ss F
act
or
'')
ln f
Fig 6.6: Plot of dielectric loss factor () as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
6.2.4 The ac conductivity (ac)
The ac conductivity (ac) is calculated by using the equation 2.13. Figure 6.7 shows
that ac increases up to the relaxation frequency and then decreases and becomes constant for
a range of frequencies and then starts increasing at higher frequencies. The hoping frequency
of electron is small at lower frequencies and it increases with increase in frequency. This is
attributed to the fact that the required energy correlated with forward-backward hoping is only
a fraction of the energy necessary to activate long range diffusive conduction [189].
.
9 10 11 12 13 14 15
0.0000
0.0004
0.0008
0.0012
0.0016SrFe
12-2xCr
xZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
ac (
S-m
-1)
ln f
Fig 6.7: Plot of ac conductivity (σac) as a function of ln of frequency for
SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
6.3 Temperature dependent dc electrical properties of Cr-Zn doped
SrM
The temperature dependent dc electrical resistivity was measured in the range of
373K-623K. Plot of ln of dc electrical resistivity as a function of 1/kBT is shown in figure
6.8. The dc electrical resistivity decreases with the increase in the temperature due to increase
in drift mobility of charge carriers. The plot also indicates that resistivity has a decreasing
trend with the increase in Cr-Zn concentration. This mainly because of increase in the grain
size. As the grain size increases, grain boundaries (which acts as highly resistive medium)
decrease and consequently resistivity decreases. Activation energy was calculated from the
slope of the linear plots of lnρ versus reciprocal of the temperature using Arrhenius relation
shown in figure 6.8.
18 20 22 24 26 28 30 32
10
12
14
16
18
20
22
24 SrFe12-2x
CrxZn
xO
19
ln d
c (
-cm
)
1/kBT (eV)-1
X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Linear Fit
Fig 6.8: Plot of lnρ as a function of 1/kBT for SrFe12-2xCrxZnxO19
(X=0.0, 0.2, 0.4, 0.6, 0.8). Line shows the linear fit.
The values of the activation energy as given in table 1, also decreases with the increase in
doping content and that is due to increase in the grain size.
6.4 Magnetic properties of Cr-Zn doped SrM
The magnetic parameters such as coercivity (Hc), saturation magnetization (Ms) and
remanence (Mr) are calculated from hysteresis loops of the composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 are shown in figure 6.9.
Fig 6.9: Hysteresis loops of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
The saturation magnetization (Ms) also decreases with the increase in Cr-Zn concentration. It
is due to the substitution of Cr-Zn cations on different interstitial sites already discussed.
0.0 0.2 0.4 0.6 0.8
4.4
4.6
4.8
5.0
5.2
5.4
Ms
(em
u/g
)
Hc (k
Oe
)
Cr-Zn concentration (X)
28
32
36
40
44
48
Fig 6.10: Plot of coercivity (Hc) and saturation magnetization (Ms) versus
Cr-Zn concentration (X) of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8)
The Figure 6.10 indicates that saturation magnetization decreases with the increase in doping
content. It may be due to following reasons. According to ferrimagnetic theory [192],
magnetism in ferrite originates from the net magnetic moment of ions with spin up and spin
down in interstitial sites. The variation in saturation magnetization mainly depends upon the
distribution of Cr3+ ions on interstitial sites and then magnetic dilution with the substitution of
Fe3+ ions by the lower magnetic moment ions. It is reported that Cr3+ ions with magnetic
moment (3 µB) preferentially occupy 12k and 2a sites with spin up. This results in the
decrease in the total magnetic moment of cations with spin up. As the net magnetic moment
of one formula unit is due to the difference of the magnetic moments of cations with spin up
and spin down so net moment decreases and hence saturation magnetization decreases [195].
On the other hand, Zn2+ with zero magnetic moment occupy tetrahedral (A) site with spin
down and bipyramidal (C) site with spin up [145]. If Zn ions only occupy tetrahedral site then
saturation magnetization should increase but the results show that it decreases. It is possible
only if the Zn ions also occupy the sites with spin up. The third factor which also contributes
to decrease in saturation magnetization is the formation of non magnetic phase (α-Fe2O3). The
XRD patterns shown in figure 6.1 indicates that the compositions (X=0.2 and 0.4) have non
magnetic phase (α-Fe2O3). This non magnetic phase also contribute to the sharp decrease in
saturation magnetization of these compositions. A small increase in saturation magnetization
for X=0.6 is due to the absence of this phase. Figure 6.7 also shows that coercivity decreases
with the increase in Cr-Zn content. This decrease in coercivity (Hc) of the samples (X≤0.6) is
attributed to the increase in particle size shown in figure 6.3. The increase in the grain size
and hence decrease in the grain boundaries results in decrease in opposition to the domain-
wall motion and consequently in the coercivity. For X=0.8, it increases due to decrease in
particle size.
6.5 Conclusion
The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4,
0.6, 0.8) sintered at 940C show the successful substitution of Cr and Zn on interstitial sites of
strontium hexaferrite lattice. The particle size obtained from SEM micrographs indicates an
increasing trend with the increase in doping concentration. The dielectric constant and
dielectric loss tangent measured in the frequency range 10kHz-3MHz shows a decreasing
trend with the increase in Cr-Zn concentration. This is mainly due to the fact that Zn doping
reduces the concentration of Fe2+ ions in ferrites. The dielectric losses obtained with this
doping are lowest. So this doping in strontium hexaferrites is more useful for the devices
operating at high frequency. The temperature dependent dc resistivity measurements show a
small decreasing trend with the increase in doping content. This behavior is attributed to the
increase in particle size. As the particle size increases, grain boundary decreases and hence
resistivity decreases. The hysteresis loops of Cr-Zn doped samples indicate that both
saturation magnetization and coercivity decreases with the increase in doping concentration.
The decrease in saturation magnetization is due to the replacement of Fe3+ with Cr3+ having
less magnetic moment than that of Fe3+ ion. The decrease in coercivity is due to the increase
in particle size. Iqbal et al. [126] prepared strontium hexaferrites with the composition
SrZrxCuxFe12-2xO19 (X = 0.0–0.8) with co-precipitation method. The dielectric loss tangent
obtained is in the range 0.98-1.55 respectively. Ashiq et al. [62] synthesized SrAlxCrxFe12-
2xO19, (X=0.0–0.6) via co-precipitation route. The dielectric loss tangent obtained is in the
range 1.26-0.95 respectively. In the present work, the dielectric loss tangent obtained is in the
range 0.32-0.02 which is much lower than the already reported. So it is expected that Cr-Zn
substitution in strontium hexaferrites would also be more suitable for high frequency
application due to low loss.
Chapter 7
Structural and electrical properties of Cr-Zn doped strontium hexaferrites prepared by WOWS sol-gel method
7. Structural and electrical properties of Cr-Zn doped strontium hexaferrites prepared by WOWS sol-gel method
Cr-Zn doped strontium hexaferrite with composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 is synthesized by simplified sol-gel method. This new
method is developed in our lab and is named as WOWS (With Out Water and Surfactants)
sol-gel method [196]. M-type strontium hexaferrite is prepared for the first time with this
method.
7.1 Structural studies
The phase formation studies of the composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 , prepared by WOWS sol-gel method, are made by using
powder X-ray diffraction data. All the peaks of X-ray diffraction (XRD) patterns shown in
figure 7.1 are identified by using ICDD patterns with reference code 01-080-1197. The
indexed XRD patterns show that all the compositions have single phase of strontium
hexaferrite material. This confirmed the successful substitution of Cr and Zn cations on the
interstitial sites of strontium hexaferrite lattice as there was no separate peak of these
substituted cations detected. The parameters calculated from XRD patterns are given in table
7.1. Both lattice parameters a & c are increased with the increase in Cr-Zn concentration. This
change is attributed to Zn2+ whose ionic radius is greater than that of Fe3+ ion [197]. The
replacement of large cation on the interstitial sites of strontium hexaferrites causes the crystal
structure to expand and in other words lattice parameters increases and X-ray density
decreases.
25 30 35 40 45 50 55 60 65 70
Inte
nsi
ty (
a.u.
)
X=0.0
2 (Deg.)
X=0.2
X=0.4(0
2 1
2)
(1 1
12)
(0 0
14)
(0 1
10)
(0 2
5)
(0 2
3)
(0 2
1)(1
1 4
)
(0 1
7)
(1 1
2)
(0 0
8)
X=0.6
X=0.8
Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.
Table 7.1: Lattice parameters (a & c), crystallite size (D114), X-ray density (ρx), bulk density (ρm), % porosity, dielectric constant (ε'), dielectric loss tangent (tanδ) and ac conductivity (σac) of the prepared samples of SrFe12-2xCrxZnxO19 . , . , . , . , . by WOWS sol-gel method.
SrFe12-2xCrxZnxO19 X=0.0 X=0.2 X=0.4 X=0.6 X=0.8 Lattice Constant a(Å) c(Å) c/a
5.868(2) 23.008(1) 3.92
5.879(3) 23.009(2) 3.91
5.886(2) 23.014(4) 3.91
5.892(2) 23.032(3) 3.91
5.891(2) 23.059(6) 3.91
Crystallite Size D1 1 4 (nm)
114 106 139 138 104
X-ray Density ρx (g/cm3)
5.14 5.12 5.11 5.10 5.09
Bulk Density ρm (g/cm3)
2.82(2) 2.13(1) 2.16(2) 2.28(2) 2.32(3)
% Porosity 45 58 58 55 54 Dielectric constant (ε') at 3MHz
5.53(1) 5.35(1) 4.92(1) 4.72(1) 4.58(1)
Dielectric loss (tanδ) at 3MHz
0.037(1) 0.038(1) 0.035(1) 0.028(1) 0.023(1)
7.2 Dielectric properties
7.2.1 The dielectric constant ()
The figure 7.2 indicates that ε' have a decreasing trend with the increase in doping contents.
This is due to the similar reason as described in section 6.2 and 6.2.1. The values of dielectric
constant are much smaller than those obtained from similar composition synthesized by co-
precipitation method. This is because of better crystallinity.
9 10 11 12 13 14 154.4
4.8
5.2
5.6
6.0
6.4
6.8
7.2
7.6
8.0
X=0.0 X=0.2 X=0.4 X=0.6 X=0.8
ln f
Die
lect
ric C
on
sta
nt '
)
SrFe12-2x
CrxZn
xO
19
Fig 7.2: Plot of dielectric constant (ε') as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method
9 10 11 12 13 14 150.0
0.1
0.2
0.3
0.4
0.5
Die
lect
ric L
oss
Tan
gen
t (ta
n
ln f
X=0.0 X=0.2 X=0.4 X=0.6 X=0.8
SrFe12-2x
CrxZn
xO
19
Fig 7.3: Plot of dielectric loss tangent (tanδ) as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with WOWS sol-gel method
7.2.2 The dielectric loss tangent (tan)
Figure 7.3 shows that the dielectric loss tangent also decreases with the increase in
doping concentration. This may be due to the decrease in Fe2+ ion concentration, which is
responsible for conduction losses, because of increase in Zn2+ content as explained earlier.
This may also be due to the increase in lattice parameters. The values of dielectric loss
tangent obtained from this composition of strontium hexaferrite are much smaller than
reported in [24, 65]. So it is expected that this substitution (X=0.2 to 0.8) would also be more
suitable for the devices operating at high frequencies than already reported composition.
7.2.3 The ac conductivity (ac)
The ac conductivity (ac) is calculated by using the equation 2.13. Figure 7.4 show
that ac is fairly constant for a range of low frequencies and then starts increasing at higher
frequencies. The hoping frequency of electron is small at lower frequencies and it increases
with the increase in frequency. This is attributed to the fact that the required energy correlated
with forward-backward hoping is only a fraction of the energy necessary to activate long
range diffusive conduction [189]. The decrease in ac with the increase in Cr-Zn concentration
is due to decrease in Fe2+ ion concentration discussed earlier in this chapter.
9 10 11 12 13 14 15
5.0x10-6
1.0x10-5
1.5x10-5
2.0x10-5
2.5x10-5
3.0x10-5
3.5x10-5
X=0.0X=0.2X=0.4X=0.6X=0.8
SrFe12-2x
CrxZn
xO
19
ac (
S-m
-1)
ln f
Fig 7.4: Plot of ac conductivity (σac)as a function of ln of frequency for SrFe12-2xCrxZnxO19 (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared with sol-gel method.
2 4 6 8 10 12 14 16
0
10
20
30
40
50
60
70 Experimental Theoretical
Die
lect
ric
Co
nst
an
t
')
ln f
Fig 7.5: Plot of experimental and theoretically calculated dielectric constant () for the sample X=0.6 (SrFe12-2xCrxZnxO19) as a function of frequency.
The dielectric properties (dielectric constant and dielectric loss tangent) of strontium
hexaferrites follow Maxwell-Wagner two layer model, already discussed in section 1.8.6.
Figure 7.5 shows the plot of theoretically calculated values by using the equation 1.14 and
experimentally observed measurements of dielectric constant of sample X=0.6 prepared by
sol-gel method. The obtained results match well with the theoretical results. A small
difference in both curves may be due to the fact that grain boundaries do not act as perfectly
insulating regions. A leakage current takes place at grain boundaries. The leakage current is
more at lower frequencies and less at higher frequencies.
7.3 Conclusion
The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 with X=0.0, 0.2,
0.4, 0.6, 0.8 prepared with co-precipitation method show that there are some impurity phases
present in some samples and these are successfully eliminated by WOWS sol-gel method.
This may be due to different synthesis temperatures (70ºC for co-precipitation and 200ºC for
sol-gel) and methods. This indicates that WOWS sol-gel method provide much better control
on impurity phases than that prepared by co-precipitation method. Lattice parameters
calculated from XRD data show a random trend for the samples prepared by co-precipitation
method while a regular trend is observed in the samples prepared by WOWS sol-gel method
for the same composition. Lattice constant depends on composition and cations distribution
and these are dependent on synthesis method so WOWS sol-gel method provide much better
control on cations distribution than that of co-precipitation method. The dielectric constant
and dielectric loss obtained from the samples prepared by WOWS sol-gel method is much
smaller than that of prepared by co-precipitation method.
Ashiq et al. [62] prepared strontium hexaferrites with the composition SrAlxCrxFe12-
2xO19 (X=0.0–0.6) with co-precipitation method. The dielectric loss tangent, measured at
1MHz, is in the range 1.26-0.95 respectively. Hussain et al. [124] synthesized strontium
hexaferrites with the composition Sr0.5Pb2+0.5Fe12−xPbx
3+O19 (X=0.0-1.0) with solid state
reaction method. The dielectric loss tangent, measured at 1MHz, is in the range 0.55-0.06
respectively. Iqbal et al. [125] prepared strontium hexaferrites with the composition Sr0.5Ba0.5-
xCexFe12-Y NiYO19 (X=0.0-0.1 and Y=0.0-1.0) with co-precipitation method. The dielectric
loss tangent obtained is in the range 1.26-0.95 respectively. Iqbal et al. [64] synthesized
strontium hexaferrites with the composition SrZrxNixFe12-2xO19 (X=0.0–0.8) with co-
precipitation technique. The values of dielectric loss tangent varies from 0.35-0.2 measured at
1MHz. Hussain et al. [123] prepared strontium hexaferrites with the composition
SrFe12O19+SiO2 (0-0.2 wt%) by solid state reaction method. The dielectric loss tangent,
measured at 1MHz, is in the range 1.0-0.21 respectively. Iqbal et al. [65] synthesized
SrAlxGaxFe12–2xO19 (X=0.0–0.8) via co-precipitation route. The dielectric loss tangent
obtained is in the range 0.98-0.26 respectively.
In the present work, the values of the dielectric loss tangent, measured at 1MHz, is in
the range 0.04-0.02 respectively which is much smaller than already reported for different
compositions of strontium hexaferrites. So it is expected that Cr-Zn doped strontium
hexaferrites synthesized by WOWS sol-gel method would also be more useful for the devices
operating at higher frequencies due to very low dielectric loss than any of the already reported
compositions of strontium hexaferrites.
7.4 Comparison
In this section, a comparison between structural and dielectric properties of the composition SrFe12-2xCrxZnxO19 with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared by co-precipitation and sol-gel methods is presented.
7.4.1 Structural properties
The structural properties of the samples prepared by co-precipitation and sol-gel
method are compared by using indexed XRD patterns.
Figures 6.1 and 7.1 show that the crystallinity of the samples prepared with sol-gel
method is much better. In case of samples prepared with co-precipitation, there are some
impure phases (α-Fe2O3) present while in case of sol-gel method; no impurity peak in any
sample is present. Figure 7.1 indicates that the variation in both lattice parameters (a & c) is
composition dependent for samples prepared with sol-gel while this dependence is not
observed in case of co-precipitation. As lattice constant is composition dependent so WOWS
sol-gel method provide much better control on cation distribution and stresses in lattice which
results in phase purity of strontium hexaferrites.
25 30 35 40 45 50 55 60 65 70
Inte
nsity
(a.
u.)
X=0.0
2 (Deg.)
X=0.2
X=0.4
(0 2
12
)
(1 1
12
)(0
0 1
4)
(0 1
10
)
(0 2
5)
(0 2
3)
(0 2
1)(1
1 4
)
(0 1
7)
(1 1
2)
(0 0
8)
X=0.6
X=0.8
7.4.2 Dielectric properties
The dielectric properties (dielectric constant and dielectric loss) of the samples
prepared with co-precipitation and sol-gel method are compared by using the data obtained
from precision component analyzer.
A comparison of dielectric constant for the composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared by co-precipitation and sol-gel method is given in
figures 6.2 and 7.2. The plot shows that the dielectric constant of the composition synthesized
by sol-gel is much lower than that of the samples prepared by co-precipitation method.
25 30 35 40 45 50 55 60 65 70
Inte
nsi
ty (
a.u.
)
X=0.0
2 (Deg.)
X=0.2
X=0.4
X=0.6
X=0.8(1
0 4
)
*
(0 3
2)
(0 0
14)
(1 1
12)
(0 2
12)
(0 3
8)
(0 2
7)
(0 1
10)
(0 2
5)
(0 2
4)
(0 2
3)
(0 2
1)(1
1 4
)
(0 1
7)
(1 1
2)(0 0
8)
*
-Fe2O
3
Fig 6.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 with (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation
method. ( α-Fe2O3)
Fig 7.1: Indexed XRD patterns of SrFe12-2xCrxZnxO19 with (X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.
9 10 11 12 13 14 154.4
4.8
5.2
5.6
6.0
6.4
6.8
7.2
7.6
8.0
X=0.0 X=0.2 X=0.4 X=0.6 X=0.8
ln f
Die
lect
ric
Co
nst
ant
')
SrFe12-2x
CrxZn
xO
19
A comparison of dielectric loss tangent for the composition SrFe12-2xCrxZnxO19
with X 0.0, 0.2, 0.4, 0.6, 0.8 prepared with co-precipitation and sol-gel method is given in
figures 6.3 and 7.3. The plot shows that the dielectric loss tangent of the composition
synthesized by sol-gel is much smaller than that of the samples prepared by co-precipitation
method.
9 10 11 12 13 14 150.0
0.1
0.2
0.3
0.4
0.5
Die
lect
ric
Loss
Tan
gent
(ta
n
ln f
X=0.0 X=0.2 X=0.4 X=0.6 X=0.8
SrFe12-2x
CrxZn
xO
19
(WOWS sol-gel)
Figure 6.2: The dielectric constant for the composition SrFe12-2xCrxZnxO19 with
(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation.
Figure 7.2: The dielectric constant for the composition SrFe12-2xCrxZnxO19 with (X=0.0,
0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method
9 10 11 12 13 14 150
20
40
60
80
100 SrFe12-2x
CrxZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric
Con
sta
nt '
)
ln f
9 10 11 12 13 14 15
0.0
0.4
0.8
1.2
1.6
2.0SrFe
12-2xCr
xZn
xO
19
X=0.0X=0.2X=0.4X=0.6X=0.8
Die
lect
ric L
oss
Tan
gen
t (ta
n
ln f
(Co-precipitation)
Fig 6.3: The dielectric loss tangent for the composition SrFe12-2xCrxZnxO19 with
(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by co-precipitation.
Fig 7.3: The dielectric loss tangent for the composition SrFe12-2xCrxZnxO19 with
(X=0.0, 0.2, 0.4, 0.6, 0.8) prepared by WOWS sol-gel method.
7.4.3 Conclusion
The indexed XRD patterns of the composition SrFe12-2xCrxZnxO19 with X=0.0, 0.2,
0.4, 0.6, 0.8 prepared with co-precipitation method show that there are some impurity phases
present in some samples but are not present in the same composition prepared by WOWS sol-
gel method. This indicates that WOWS sol-gel method provide much better control on
impurity phases than that of co-precipitation method. Lattice parameters calculated from XRD
data show a random trend for the samples prepared by co-precipitation method while a regular
trend is observed in the samples prepared by WOWS sol-gel method for the same
composition. As lattice constant is composition dependent so WOWS sol-gel method provide
much better control on cations distribution than that with co-precipitation method. The
dielectric constant and dielectric loss obtained from the samples prepared by WOWS sol-gel
method is much lower than that of prepared by co-precipitation method. As these properties
are dependent on cations distribution of microstructures so WOWS sol-gel synthesis method
is better than that of co-precipitation.
Chapter 8
Oxygen reduced strontium hexaferrite for microwave absorbing coatings
8. Oxygen reduced strontium hexaferrite for microwave absorbing coatings
In ferrites, the major source of charge carriers is Fe2+ ion. By changing the grain
boundaries and surface defects of the grains and the concentration of Fe2+ ions inside the
grain, the electrical properties of the material could be tailored according to the desired
application. When sintered strontium hexaferrite is exposed to hydrogen at high temperature,
oxygen from the surface as well as from the bulk is removed. It results in the formation of
free iron atoms as well as Fe2+ ions which could result in the increase in dielectric properties
such as dielectric constant and dielectric loss [198].
In the present work, oxygen is intentionally reduced to increase the concentration of
Fe2+ ions and surface defects in order to increase the dielectric constant and making the
material useful for microwave absorber coatings.
8.1 Reduction procedure
Fig 8.1: Experimental setup for oxygen reduction
Annealed powder of strontium hexaferrite was placed in tube furnace. The
temperature of the tube furnace was raised to 800C at the rate of 5C/min. Then hydrogen
was passed through the tube with the flow rate of 50scc/min for one hour and then the furnace
was turned off. Experimental setup for oxygen reduction is shown in figure 8.1. The effect of
this oxygen reduction on structural and electrical properties of annealed strontium hexaferrite
was investigated by using X-ray powder diffractometer (XRD) and precision component
analyzer respectively.
8.2 Effect of oxygen reduction on structural properties of strontium
hexaferrites (SrM)
Effect of oxygen reduction on structural properties of strontium hexaferrites was
studied by using XRD. The indexed XRD patterns of annealed strontium hexaferrite powder
before and after oxygen reduction is shown in figure 8.2. It can clearly be seen from XRD
patterns that after oxygen reduction the major peaks, having miller indices (107) and (114), of
M-type strontium hexaferrites are vanished and other phases of strontium ferrites are formed.
It indicates that oxygen reduction disturbs the structure of M-type hexaferrites.
8.3 Frequency dependent ac electrical properties of oxygen reduced strontium hexaferrites
Frequency dependent electrical properties are mainly characterized by two parameters
named as dielectric constant and dielectric loss tangent. The dielectric constant and dielectric
loss tangent of reduced samples are shown in figures 8.3 and 8.4 respectively.
Figure 8.3 indicates that dielectric constant of reduced strontium hexaferrite sample is
very high as compared to non reduced sample. This is due to the reason that reduction of
oxygen causes the transformation of Fe3+ ions to Fe2+ ions or free iron atoms. This results in
the increase in the concentration of available charge carriers and hence increases the space
charge polarization due to which dielectric constant has increased to large extent.
8.4 The dielectric loss tangent (tanδ)
The plot of dielectric loss tangent (tanδ) of reduced and non reduced strontium
hexaferrite sample is shown in figure 8.4. The figure shows that dielectric loss tangent
Fig 8.2: Indexed XRD patterns of sintered strontium hexaferrite samples before and after reduction
9 10 11 12 13 14 15
0
1000
2000
3000
4000
5000
6000 After ReductionBefore Reduction
Die
lect
ric
Co
nst
an
t
')
ln f
Fig 8.3: Dielectric constant () of sintered SrFe12O19 before and after reduction
of reduced sample is much higher than that of non reduced sample. This is due to the increase
in carrier concentration because of the reduction of oxygen. As the carrier concentration
increases, the conduction losses increases and dielectric losses increases. A comparison of the
dielectric constant and dielectric loss tangent of reduced and non reduced sample is given in
table 8.1.
9 10 11 12 13 14 15
0
20
40
60
80
100
120
140
160After ReductionBefore Reduction
Die
lect
ric
Lo
ss T
an
ge
nt
(ta
n
ln f
Fig 8.4: Dielectric loss tangent (tanδ) of sintered SrFe12O19 before and after reduction
Table 8.1: Dielectric constant () and dielectric loss tangent (tanδ) of reduced and non reduced sample
Table 8.1 shows that % age increase in dielectric constant is much bigger than that of
loss tangent. The results show that the oxygen reduced sample is very suitable for microwave
absorbing coatings.
24 26 28 30 32 34 364
6
8
10
12
14
16
18
20
22
24
1/kBT (eV-1)
After reduction Befire reduction
ln
dc (
-cm
)
Fig 8.5: Plot of temperature dependent dc electrical resistivity of sintered SrFe12O19 before and after reduction
Composition SrFe12O19
(Before reduction)
SrFe12O19
(After Reduction)
Dielectric const. ε' at 3 MHz 23 1530
Loss tangent (tanδ) at 3 MHz 0.32 2.76
8.6 Temperature dependent dc electrical properties of sintered SrFe12O19 before and after reduction
Figure 8.5 show that dc electrical resistivity is dropped to very low values after
oxygen reductions. This is because the reduction of oxygen results in the increase in
concentration of Fe2+ ions and charge carriers.
8.7 Conclusion
Reduction of oxygen from sintered SrFe12O19 has been made in order to increase the
concentration of charge carriers. The effect of oxygen reduction on the structure of SrFe12O19
is analyzed by using X-ray diffractions patterns. Indexed XRD patterns show that the
characteristic peaks (1 0 7) and (1 1 4) are vanished. This shows that the reduction of oxygen
results in the formation of new phases. The temperature dependent dc electrical resistivity is
decreased due to increase in carrier concentration. The frequency dependent dielectric
constant and dielectric loss tangent are increased to large extent. The dielectric results show
that oxygen reduction makes the strontium hexaferrites useful for microwave absorption.
Further investigations are required to know the detailed phenomenon behind this behavior and
also for reflection losses.
9 Conclusions
In the present work, single phase strontium hexaferrites (SrFe12O19) are synthesized
by two different methods. These methods used are co-precipitation and sol-gel methods. In
co-precipitation technique, the properties of a synthesized material are strongly affected by
synthesis conditions. A systematic study of the major synthesis parameters which impart a
significant contribution on the properties of hexaferrites has been made. These major
synthesis parameters include molar ratio of cations (Fe/Sr), volume rate of addition of
precipitating agent and the pH of the solution. A number of samples has been prepared by
varying any one of the synthesis parameters such as molar ratio of cations (Fe/Sr), volume
rate of addition of precipitating agent and the pH of the solution and keeping all other
parameters same. After analyzing the results obtained, these parameters are optimized for
phase purity and particle size. To study the effect of molar ratio (Fe/Sr) on phase purity, the
samples were prepared with molar ratio MR = 08, 09, 10, 11, 12. The indexed XRD patterns
of molar ratio (Fe/Sr) varying samples show that almost all patterns are similar and contain no
impurity peak. It is concluded that molar ratio (MR) within the range (08 to 12) does not
affect the phase purity of strontium hexaferrites. To study the effect of volume rate of addition
of precipitating agent on phase purity, the samples are prepared with very slow (drop by
drop), intermediate and very fast addition of solution of the precipitating agent. X-ray
diffraction patterns are used to analyze the phase formation. It is observed that variation in
volume rate of addition of precipitating agent also plays very important role in the phase
purity of strontium hexaferrites. The results show that the increase in the volume rate of
addition of precipitating agent improves the phase purity. The effect of this parameter on
surface morphology is analyzed by using scanning electron micrographs (SEM). The SEM
micrographs indicate that the particle size decreases with the increase in volume rare of
addition of precipitating agent. This may be due to the reason that high volume rate of
addition of precipitating agent increases the number of nucleation sites. As the number of
nucleation sites increases, the possibility of agglomeration decreases. To analyze the effect of
one of the most important synthesis parameter i.e. pH of the solution, the samples with
different pH (08 to 13) are prepared. The obtained results of the samples prepared with
different pH of the solution indicate that pH of the solution imparts significant effects on
phase and microstructural properties of strontium hexaferrites. High pH of the solution
improves the phase purity and decreases the particle size. This decrease in particle size may
be due to the reason that high pH of the solution increases the number of nucleation sites. As
the number of nucleation sites increases, the possibility of agglomeration decreases. It is
observed that the dc electrical resistivity is increased in the samples which are prepared with
the increase in the pH of the solution. This is due to the increase in grain boundaries.
Ferrites are being commonly used as dielectric materials in the devices operating at
high frequency. To improve coercivity which results in the increase in the operating
frequency of ferrites, Cr is doped in strontium hexaferrite and the composition SrFe12-xCrxO19
with X=0.0, 0.2, 0.4, 0.6, 0.8 is prepared by co-precipitation method and its dielectric studies
are reported. The X-ray diffraction patterns indicate that Cr doping results in the formation of
secondary phase of α-Fe2O3. A peak of Cr2O3 is also appeared for X=0.8 indicating the
solubility limit of Cr in strontium hexaferrites. It is also observed that for X0.6, both
dielectric constant and coercivity are increased while saturation magnetization is decreased.
The increase in coercivity is mainly due to the impurity phase acting as pinning center while
the decrease in saturation magnetization is due to the replacement of Fe3+ ions (5µB) with Cr3+
ions, having less magnetic moment (3µB), on octahedral sites.
As Cr causes the increase in coercivity and it is reported that Zn causes the decrease
in Fe2+ ions concentration and dielectric losses. So keeping these observation in view, both Cr
and Zn are doped simultaneously in strontium hexaferrites and the resulting composition
SrFe12-2xCrxZnxO19 with X=0.0, 0.2, 0.4, 0.6, 0.8 is prepared with co-precipitation method.
The XRD patterns show that in two samples, impurity peak of α-Fe2O3 is appeared. SEM
micrographs indicate that the particle size shows an increasing trend with the increase in
doping concentration. As the particle size is increased, density is decreased and hence
porosity is increased. It is also observed that Cr-Zn doping causes the decrease in the
dielectric constant and dielectric loss tangent due to decrease in Fe2+ ions concentration
because of Zn doping. Frequency dependent ac conductivity is also decreased with the
increase in Cr-Zn concentration. This is due to the decrease in the carrier concentration (Fe2+
ions) because of the Zn doping. Temperature dependent dc electrical resistivity measurements
show that there is a small drop in dc resistivity. This is attributed to the fact that Cr-Zn doping
resulted in the increase in particle size. The increase in particle size caused the decrease in the
grain boundaries acting as highly resistive medium and as a consequence resistivity is
decreased. The hysteresis loops of the Cr-Zn doped samples reveal that both coercivity and
saturation magnetization are decreased with increase in doping concentration. The decrease in
coercivity is due to the increase in particle size.
As the properties of these materials are sensitive to synthesis methods and conditions
so the composition SrFe12-2xCrxZnxO19 with x=0.0, 0.2, 0.4, 0.6, 0.8 is also prepared with
WOWS sol-gel method (a much simplified method developed in our lab) in order to compare
the properties of this composition prepared by these two methods. The structural results
obtained from XRD patterns indicate that the variation in both the lattice parameters (a & c) is
composition dependent for samples prepared with sol-gel while this dependence is not
observed in case of the samples prepared with co-precipitation method. As lattice constant is
composition dependent so WOWS sol-gel method provides much better control on cation
distribution, stresses in lattice and hence phase purity of strontium hexaferrites. The frequency
dependent dielectric measurements are much better than that of the samples prepared with co-
precipitation method. The dielectric loss of the sample (X=0.0) synthesized by this method is
about 90% lower than that of prepared with co-precipitation method. It is due to the better
microstructural properties such as particle size and its distribution provided by WOWS sol-
gel method.
In some cases, high loss may be desired in applications such as heating and EM wave
absorption. To increase the dielectric losses, reduction of oxygen from sintered SrFe12O19 is
made. The indexed XRD pattern indicate that the characteristic peaks (1 0 7) and (1 1 4) of
strontium hexaferrites are missing. This shows that due to the reduction of oxygen, some
chemical bonds of SrFe12O19 are broken up and new phases are formed. The reduction of
oxygen resulted in the increase in the concentration of Fe2+ ions and free iron atoms. The
temperature dependent dc electrical resistivity is sharply decreased due increase in carrier
concentration. The frequency dependent dielectric constant and dielectric loss are increased to
large extent due increase in carrier concentration and hence making the material useful for
microwave absorption.
9.1 Future work
The dielectric properties of the samples prepared by WOWS sol-gel method should
be studied in higher GHz range. The difference in dielectric properties of the samples
prepared by co-precipitation and WOWS sol-gel method should be studied in terms of site
occupation of dopants in crystal lattice by using EXAFS. The synthesized material should be
sintered in oxygen environment in order to reduce the Fe2+ ion concentration and hence to
reduce the dielectric losses. The elements with low atomic number should also be tried. The
effect of oxygen reduction on dielectric properties of strontium hexaferrites should also be
studied systematically.
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