ser

bia

Available online 7 February 2015

Keywords:NanostructuresCo-precipitationX-ray diffraction

xC

ferrite with single cubic spinel phase. The lattice constant was found to decrease with increasing Cuions content. Infrared spectral analysis conrmed formation of the spinel structure for the respective

aving

Much attention has been focused onMg-ferrite due to its high elec-trical resistivity and porosity [35]. Moreover, Mgmay increase thesequences of the magnetic hysteresis, so it can be used as a dia-magnetic dopant in ferrites. In sensors technology, Mg ferritewas used as humidity sensor as reported by Shimizu et al. [6].

used in order to]. Copper ferritel family mis a well

materials for high frequency applications in the telecommtion eld due to their high resistivity and low eddy currentThey are used in radio frequency circuits, high quality ltersformer cores and read/write heads for high-speed digital tapes[13]. Recently there is a growing interest in using MgZn ferritesin electronic devices instead of NiZn ferrites because of thecarcinogenic effects of nickel and their potential and environmen-tal hazards, although they posses high permeability and high resis-tivity at higher frequencies [14]. MgCuZn ferrite has beenconsidered as an alternate system due to its high resistivity, high

Corresponding author at: Department of Physics, Faculty of Science, KingAbdulaziz University, Jeddah, Saudi Arabia; Department of Physics, Faculty ofScience, Zagazig University, Zagazig, Egypt.

E-mail addresses: dakdik2001@yahoo.com, hmzaki@kau.edu.sa (H.M. Zaki).

Journal of Alloys and Compounds 633 (2015) 104114

Contents lists availab

Journal of Alloys a

.e ltetrahedral (A-site) and octahedral (B-site), depending on the coor-dination of the nearest neighboring oxygen. Ferrite materials areused in many applications such as electronics, magnetic storageand as contrast agents in magnetic resonance imaging (MRI) [2].

ferrite, containing range of compositions is oftenobtain highly dense and resistive ferrites [9,10doped with magnesium ions, is important spine[11,12]. Un-doped and substituted MgCuFe2O4http://dx.doi.org/10.1016/j.jallcom.2015.01.3040925-8388/ 2015 Elsevier B.V. All rights reserved.ember-suitedunica-losses., trans-ture (where M is a divalent metal cation) are promising materialsdue to their interesting magnetic and electric properties withchemical and thermal stabilities [1]. The structure of spinel ferritesconsists of a closed-packed oxygen arrangement in which 32 oxy-gen ions are packed in a unit cell and between the layers of oxygenions exist interstices which can be classied into two categories,

ing divalent ions such as Zn2+ have conrmed to be among the bestalternatives to conventional materials used in particular applica-tions such as switches, tiny inductors, antenna rods and recordingheads owing to their distinctive properties [8]. It has been con-cluded that addition of Cu2+ ions may improve the resistivity,dielectric and magnetic properties of MgZn ferrites. MagnesiumInfrared spectroscopyMagnetic properties

1. Introduction

Nano-sized spinel-type ferrites hferrite system. Magnetic data showed that the saturation magnetization (Ms) increases with Cu2+ concen-tration up to x = 0.2 and then decreases with further increase of Cu2+ ions in this ferrite system. Theproposed cation distribution deduced from X-ray diffraction, infrared spectra and magnetization dataindicated mixed ferrite type. Dielectric constants e0 , dielectric loss e0 0 , dielectric loss tangent tand andac conductivity, rac, were investigated as a function of frequency and temperature. Inuence of Cu2+ sub-station on the ac conductivity exhibited signicant behavior at low frequencies and low temperatures,T 6 100 C. Both dielectric constants (e0 , e00) found to increase with the increase of the temperature. Atlow temperatures, dielectric loss tand indicated a decrease with frequency with slight change at hightemperatures.

2015 Elsevier B.V. All rights reserved.

formula MFe2O4 struc-

For example, one of the main factors which candidate Mg ferritein microwave applications is its very high specic resistance(1081010X cm) [7]. Doped Mg-ferrites particularly that contain-Received in revised form 27 January 2015Accepted 30 January 2015

using co-precipitation method. The inuence of Cu ions substitution on the structural and magneticproperties was investigated. X-ray diffraction measurements revealed the formation of nano-crystalline

2+Structural, magnetic and dielectric studienano-crystalline spinel magnesium zinc f

H.M. Zaki a,b,, S.H. Al-Heniti a, T.A. Elmosalami baDepartment of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi ArabDepartment of Physics, Faculty of Science, Zagazig University, Zagazig, Egypt

a r t i c l e i n f o

Article history:Received 30 November 2014

a b s t r a c t

Nano-crystalline Mg0.5Zn0.5

journal homepage: wwwof copper substitutedrite

uxFe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) ferrite powders were synthesized2+

le at ScienceDirect

nd Compounds

sevier .com/locate / ja lcom

Curie temperature and environmental stability [15]. In addition,multilayer chip inductors (MLCIs) based on this ferrite system havebeen developed for promising electronic applications. Such devicesare fabricated by laminating alternate layers of ferrite and conduc-tors which then co-ring to form the monolithic structure appro-priate for multilayer chip inductor applications (MLCIs) [16,17].Metallic silver is usually used as the internal electrode materialof MLCIs due to its high conductivity and low cost. Silver as aninternal conductor has a melting point of 961 C. Consequently,the ferrite powder is sintered at T < 961 C in order to co-re withsilver electrodes. Since the magneto-striction constant of MgCuZn ferrite is lower than that of NiCuZn ferrite, MLCI usingMgCuZn ferrite can provide better magnetic properties. Techno-logically, NiCuZn ferrite is preferred as magnetic material forMLCIs because of its better properties at high frequencies andlower densication temperatures [18,19]. In comparison, MgCuZn ferrites have similar magnetic properties as NiCuZn ferriteswith the advantage that the former is more economical [20] andeasy to synthesize. Murbe et al. [21] investigated MgCuZn fer-rites for obtaining low sintering temperature, high permeabilityand high thermal stability as multilayer chip inductor materialsalternative to the NiCuZn ferrite system. The study found thatMgCuZn ferrite is promising material for MLCIs applicationswith high-performance and low cost.

This study describes the inuence of Cu2+ ions substitution inplace of Zn ions on the structural, dielectric andmagnetic properties

2. Experimental details

nano-powder was washed several times with distilled water to remove impuritiesand then dried at 85 C for 5 h. After drying, the powders were calcined at 900 C ina programmable thermolyne furnace for 5 h to obtain a single spinel cubic phase. X-ray powder diffraction patterns were scanned using Philips Xpert diffractometerequipped with a graphite crystal monochromator and Copper target(kka1 = 1.54056 ). The working conditions for the X-ray tube were 40 kV and25 mA. Each X-ray scan was performed between 15 and 70 (2h). Fourier trans-form infrared spectra were measured by means of PerkinElmer spectrophotometermodel 1430 within the wave number range of 3501000 cm1 using KBr pressedpellet technique. Magnetization measurements were performed at room tempera-ture using vibrating sample magnetometer (Lake Shore 7400 series) with sensitivityof 107 emu at the magnetic eld range 20 to 20 kG.

In the present study, ac conductivity and dielectric parameters were carried outin the frequency range from 100 Hz to 1 MHz at the temperature range between300 K and 673 K using LCR HI-Tester (HIOKI 3532-50). The pelletized samples withdiameter 12 mm and thickness of 4 mmwere coated on both faces with silver pastein order to form parallel plate capacitor.

The value of the real dielectric constant (e0) was calculated using the followingrelation:

e0 Cd=Aeo; 1

where C, d and A refer to capacitance, thickness and cross sectional area of the pellet.The symbol eo is the permittivity of free space.

The complex dielectric constant was calculated using the relation:

e00 e0 tan d; 2

where tand is the dielectric loss. Using the dielectric constant and dielectric loss, theac conductivity rac of the prepared samples was calculated using the followingformula:

rac e0e0 tan d; 3

H.M. Zaki et al. / Journal of Alloys and Compounds 633 (2015) 104114 105Nano-crystalline powders of Mg0.5Zn0.5xCuxFe2O4 (x = 0.0, 0.1, 0.2, 0.3, 0.4 and0.5) were synthesized using co-precipitation method. Stoichiometric quantities ofanalytical grade MgCl26H2O, ZnCl2, CuCl2, FeCl36H2O and NaOH were mixed thor-oughly in 150 ml distilled water using magnetic stirrer at 80 C. The precipitatedof Mg0.5Zn0.5xCuxFe2O4 ferrites synthesized by co-precipitationmethod. In addition, the inuence of cation redistribution betweenthe tetrahedral and octahedral sites on the structural and magneticproperties of Cu-doped MgZn ferrites is also investigated.Fig. 1. X-ray diffraction patterns of nanocrystalline Mg0.5CuxZn0.5xFwhere x is the angular frequency.

3. Results and discussion

3.1. X-ray diffraction

Fig. 1 shows typical X-ray diffraction patterns for thesynthesized Cu2+ substituted MgZn ferrites with some traces ofa secondary phase of Fe2O3 (JCPDS le No. 87-1166) for the samplewith concentration x = 0.3. The appearance of such secondarye2O4 ferrites powders synthesized by co-precipitation method.

The average cation radius at the tetrahedral and octahedralsites, rA and rB, are calculated for all samples using the above cationdistributions and given as [30]:

rA 0:05 y rMg 0:5 x rZn 0:85 x rFe t rCu;0:0 6 x 6 0:2

0:05 y rMg 0:5 x rZn 0:4 x rFe t rCu;0:3 6 x 6 0:5

rB 1=20:45 y rMg x t rCu 1:15 x rFe;0:0 6 x 6 0:2

(rA,stal

0

410088210054

andphase has been reported for some ferrite compounds in literature[22]. The indexed peaks with diffraction planes (111), (220),(311), (222), (400), (422), (511), (440), (620) and (533) indi-cated the cubic crystal structure of the spinel phase with Fd-3mspace group (JCPDS 10-0325) for all concentrations of the investi-gated ferrites. Values of the lattice constant were determined forMg0.5Zn0.5xCuxFe2O4 ferrites and are listed in Table 1. The deducedlattice constant decreases with increasing Cu2+ ion substitution, asimilar behavior was reported by Bhosale et al. using oxalate pre-cursors [23] and Sujatha et al. Studied Mg0.5Cu0.05Zn0.45Fe2O4nanoparticles through solgel method got a nearly lattice constant[24]. This behavior is attributed to the difference of the ionic radiusbetween Cu2+ ion (0.72 ) and that of Zn2+ ion (0.74 ) [25]. Conse-quentially, the substitution of Cu2+ ions in place of Zn2+ ions isexpected to decrease the lattice parameter of the unit cell. The X-ray density (dx) of the compounds was determined using therelation [26]:

dx ZMNAa3 4

where M is the molecular weight, Z is the number of molecules perunit cell (Z = 8), NA is Avogadros number and a is the lattice con-stant of the unit cell. The X-ray density (dx) found to increase withincreasing Cu2+ concentration due to the decrease of the lattice con-stant as shown in Table 1. The porosity (P) of the samples was cal-culated using the following relation [27]:

P 1 dbulkdx

5

where dbulk is the bulk density, which was measured using the

Table 1Bulk density (dbulk), percentage porosity (p%), lattice constants (aexp, ath), ionic radiitetrahedral (mT) and octahedral (mO) infrared bands for Mg0.5Zn0.5xCuxFe2O4 nano-cry

Concentration (x) 0.0 0.1

dbulk (g/cm3) 4.8108 4.838P (%) 2.1 1.7rA () 0.9572 0.8336rB () 0.5264 0.5872ath () 8.4297 8.4016aexp () 8.4167 8.4097RA 2.278 2.154RB 1.883 1.927U 0.406 0.398d 0.031 0.023mT (cm1) 572.2 584.1mO (cm1) 396.2 437.8

106 H.M. Zaki et al. / Journal of AlloysArchimedes principle. The percentage porosity P (%) of the samplesdecreases gradually with concentration up to x 6 0.3 indicatingdense packing. Samples with concentration x = 0.4 and x = 0.5 exhi-bit higher percentage porosity around 6% which may attributed tothe increase of their particle sizes as indicated in Table 2. Thisdependence between porosity and particles size has been reportedfor some ferrites [28].

In Mg0.5Zn0.5xCuxFe2O4 ferrites the Zn2+ ions exclusivelyoccupy the tetrahedral (A) sites, while most of the Mg2+ and Cu2+

ions prefer to occupy the octahedral sites (B). The Fe3+ ions are dis-tributed among the tetrahedral and octahedral sites. Furthermore,substitution of sufcient amount of Cu2+ ions may cause migrationof Fe3+ ions from (B) site to (A) site because the preferency of cop-per ions to be in the octahedral sites (B-sites) [29]. Considering theX-ray, infrared and magnetization data the most reliable cationdistribution for the studied ferrites are given as:

Mg20:05yZn20:5xFe30:85xCu2t Mg20:45yCu2xtFe31:15xO4;0:0 6 x 6 0:2Mg20:05yZn20:5xFe30:4xCu2t Mg20:45yCu2xtFe31:6xzFe2z O4;0:3 6 x 6 0:5

rB), bond lengths (RA, RB), oxygen parameter (U), deviation of oxygen parameter (d),line ferrites.

.2 0.3 0.4 0.5

.8704 4.9114 4.6374 4.6649

.06 0.79 6.3 6.0

.7100 0.5314 0.5318 0.5550

.6480 0.6633 0.6601 0.6569

.3734 8.3717 8.3699 8.3682

.4059 8.3896 8.3887 8.3790

.031 1.856 1.856 1.870

.976 2.068 2.068 2.059

.390 0.378 0.378 0.379

.015 0.003 0.003 0.00485.5 576 586.4 588.136.5 435.1 433.1 430.2

Table 2Particle size (D), surface area (S), magnetic moments (lth, lexp), saturation magne-tization (Ms), coercivity (Hc) and remanence magnetization (Mr) for Mg0.5Zn0.5xCux-Fe2O4 nano-crystalline ferrites.

Composition (x) 0.0 0.1 0.2 0.3 0.4 0.5

D (nm) 51 62 45 25 76 67S (m2/g) 24.46 20.00 27.38 48.87 17.02 19.20lth (lB) 1.50 2.58 3.66 2.95 1.95 0.95lexp (lB) 1.794 2.075 2.197 1.925 1.747 1.194Ms (emu/g) 45.424 52.589 55.722 48.867 44.398 30.361Hc (G) 33.612 27.305 22.153 41.056 32.273 45.989Mr (G) 3.2178 2.7598 2.0322 3.6258 2.8799 3.7240Ms/Mr 0.071 0.053 0.037 0.074 0.065 0.123

Compounds 633 (2015) 104114 1=20:45 y rMg x t rCu 1:6 x z rFe zrFe2;0:3 6 x 6 0:5

where rMg, rZn, rFe and rCu are the ionic radii of Mg, Zn, Fe and Cuions, respectively. Table 1 shows the calculated structural parame-ters ath, rA and rB based on the proposed cation distributions. Thetheoretical lattice parameter (ath) is calculated using the relationgiven below, which relates the cation radius of the constituent ele-ments to the lattice parameter at different lattice sites [31]:

ath 83

3

p rA Ro 3

prB Ro 6

where rA and rB are the cations radii at the tetrahedral and octahe-dral sites, respectively and Ro is the oxygen ion radius (Ro = 1.32 )[17]. The calculated and experimental values of the lattice parame-ters (ath, aexp) are given in Table 1. The value of ath vary linearly withincreasing Cu2+ concentration for all the samples. The data in Table 1shows that the value of ath is slightly different from the experimentalvalue aexp. This small difference is attributed to the approximation

andH.M. Zaki et al. / Journal of Alloysin the calculations of ath which assumes ideal unit cell of closepacked spinel structure where the anions and cations are regardedas rigid spheres in perfect manner [32]. Using the values of bothexperimental and theoretical lattice parameters, oxygen positionalparameter is calculated using the following relation [33]:

U 1a

3

p rA RO 14 7

where the symbols have their usual meaning. The calculated valuesof the oxygen positional parameter are listed in Table 1. In most oxi-dic spinels, the oxygen ionic radius is apparently larger than that of

Fig. 2. Room temperature FTIR spectra of nanocrysta

-20000 -10000 0 10000 20000

-60

0

60

X=0.0X=0.1X=0.2X=0.3X=0.4X=0.5

Mom

ent/M

ass

(em

u/g)

Field (G)

Fig. 3. Room temperature magnetic hysteresis loops for Mg0.5CuxZn0.5-xFe2O4ferrites.the metallic ions, and in spinel like structures. The oxygen posi-

lline Mg0.5CuxZn0.5xFe2O4 (0 6 x 6 1.0) ferrites.

Compounds 633 (2015) 104114 107tional parameter has a value in the neighborhood of 0.375 for whichthe arrangement of O2 ions equals exactly a cubic closed packing.However, in actual spinel lattice this ideal pattern is slightlydeformed. The higher values of oxygen parameter obtained in thisstudy may be attributed to the small displacement of anions dueto the expansion of the tetrahedral interstices and the differencein the chemical composition [34,35]. It is obvious from the data inTable 1 that the oxygen position parameter decreases with increas-ing Cu2+ concentration up to x = 0.3 which causes decrease in latticeparameter in this range as obtained from X-ray results.

The cationanion distances (the bond lengths) at the A-sites, RA,and B-sites, RB, are calculated using the following relations [35]:

RA a3

pd 1

8

RB a 116d2 3d2

12

8

d U Uidealwhere d represents the deviation from the ideal oxygen parameterUideal (=0.375). Values of (U) for Mg1xZnxFe2O4 samples were foundby interpolation between those reported earlier for pure magne-sium and zinc ferrites, which are 0.381 and 0.385 A, respectively[36].

Table 1 also shows the variation of bond lengths RA and RB withCu2+ content. RA is the shortest distance between A-site cation andoxygen ion and RB is the shortest distance between B-site cationand oxygen ion. It is well known that there is a correlation betweenionic radius and lattice parameter. The data in Table 1 indicate thatwith increasing Cu2+ concentration, radius of tetrahedral site isdecreased. The decrease in bond length RA can be attributed tothe decrease in lattice parameter (a) with increasing of Cu2+

andcontent due to the fact that the ionic radius of Cu2+ is smaller thanZn constituent ions that it replaces.

3.2. Infrared spectroscopy

The formation of Mg0.5CuxZn0.5xFe2O4 (0 6 x 6 1.0) ferriteswere conrmed by Fourier transforms infrared spectroscopy (FTIR)through the existence of characteristic vibrational bands of the tet-rahedral (A) and octahedral (B) sites. According to Waldron [37]there are four bands characterizing ferrites which can be easilyobserved in the range of 200700 cm1. The difference in positionsof the bands for the various compositions was expected because ofthe difference in the distances for the octahedral and tetrahedralions [38]. The four bands can be classied into two groups astwo high-frequency bands and two low-frequency bands. Waldronattributed the higher absorption band, mT, around 600 cm1 to theintrinsic vibrations of tetrahedral complexes and the band, mO,around 400 cm1 to octahedral complexes. The low-frequencyband around 350 cm1 is related to the divalent octahedralmetaloxygen ion complexes while the band around 200 cm1 isassigned to the divalent tetrahedral vibrations [39,40]. In our case

1

Fig. 4. Variation of the saturation magnetization (Ms) and X-ray diffractionintensity ratios.

108 H.M. Zaki et al. / Journal of Alloysthe low band around 200 cm is not observed due to the limitedrange of the available spectrometer. It is well known that in fer-rites, metal cations are situated according to the geometric cong-uration of the oxygen ion nearest neighbors in two different sublattices such as tetrahedral (A-sites) and octahedral sites (B-sites)[41]. The band mT is assigned to the vibration of the bond betweenthe oxygen ion and the tetrahedral metal ion OMtetra and the bandmO is assigned to the vibration of the bond between the oxygen ionand the octahedral metal ion OMocta [42]. The difference in bandpositions is due to change in bond length of Fe3+O2 at tetrahedral(0.189 nm) and octahedral (0.199 nm) sites respectively [43].Generally, vibrational frequencies depend on cation mass, cation-oxygen distance and the bonding force [44]. The frequencies ofthe IR absorption bands and the assignment of these bands corre-sponding to the studied ferrite samples are shown in Fig. 2.

Fig. 2 exhibits two strong frequency bands (mT, mO) in all thesamples within the measured range 580 and 430 cm1 as thecharacteristics bands for the spinel structure conrming the ferritestructure, where Kamble et al. [45] reported the synthesis ofMgCuZn ferrite (Mg0.48Cu0.12Zn0.40Fe2O4) by ceramic and self-sustaining auto-combustion (sucrose) methods and IR absorptionbands were found to be in the expected high frequency range(574.95 cm1 and 582.39 cm1) and low frequency range(431 cm1) which forties the spinel phase and coincide with ourresults. The positions of these bands are presented in Table 1 andfound to change slightly from one ferrite composition to another.This suggests that the preparation method, particle size, substitu-tion with different ions and porosity play an important role ininuencing the band positions. For example, in the present studyit is observed that there is a small shift in the frequency band mTand mO for the ferrite samples after substituting with copper ions.

The frequency is inversely proportional to reduced mass andbond length. According to the IR spectra, it is found that thefrequency band mo shifts towards higher wave number as Cu2+ ionssubstituted at concentration (x = 0.1). This may be due to the largeradius of Cu2+ ions (0.72 ) occupying octahedral sites to that ofFe3+ ions with radius (0.64 ). As a result, Fe3+ ions are pushedtowards oxygen ion resulting in reduced Fe3+O2 bond length.There is an inverse relation between wave number and bondlength which produces slight change in the size of octahedronand eventually a shift in mo band position is observed. In addition,the migration of some Cu2+ ions to tetrahedral sites as observedfrom IR spectra would increase the values of mT with increase ofCu2+ concentration. The slight increase of both mT and mo withincreasing Cu2+ could also be due to the difference in the ionicradius between Cu2+ and Zn2+ and the difference of their massesin both sites. This increase in frequency bands (mo and mT) are con-sistent with the reduction in unit cell dimensions [46]. A similarshift in peak positions was found with the increasing Cu2+ concen-tration has also been reported [47]. This behavior was attributed topreparation conditions which affect the grain size and density.

3.3. Magnetization

Room temperature magnetization hysteresis curves of the sam-ples calcined at 900 C (Fig. 3) show that the saturation magnetiza-tion is achieved at a eld of 20 kG. From the gure it is also clearthat, all the curves of different compositions produced narrowloops, with a behavior characteristic of soft magnetic materials(easy magnetization and demagnetization with low coercivityimplied reduced hysteresis losses in the present samples whichis an important requirement for a good electromagnet). The valuesof saturation magnetization (Ms), coercivity (Hc), remanent magne-tization (Mr) and magnetic moment (lB) are listed in Table 2. Fig. 3shows plots of the saturation magnetization (Ms) as a function ofcopper content. It is clear that, the saturation magnetizationincreases linearly with increasing copper concentration up tox = 0.2 followed by a remarkable decrease. The observed variationin saturation magnetization can be explained on the basis of cationdistribution and the exchange interactions between A and B-sites,respectively. The AB interaction is known to be the strongest anddominates over the BB and AA interactions. According to Neelstwo sub lattice model of ferri-magnetism [48] saturation magneti-zation is given byMS =MBMA in whichMB andMA are the octahe-dral and tetrahedral sub lattice magnetizations, respectively. Theinitial increase in the MS value is attributed to the fact that Cu2+

ions with magnetic moment of 1 lB preferentially tend to migrateto the B-sites by replacing non magnetic Zn2+ ions with zero lBwhich have a stronger preference for the tetrahedral (A) site. Thus,Zn2+ displaces Fe3+ from A to B site and the increase of Cu2+ ions inB-sites would increase the magnetization of these sub lattice sites.As the magnetic spin of neighboring A and B-sites are anti-ferro-magnetically coupled (due to the super exchange interaction in fer-rite lattice), the net result is an increase in magnetic moment onthe B sub-lattice, and thus the net magnetic moment of the spinelferrite increases with Cu2+ concentration up to x = 0.2 as shownfrom Fig. 4. The infrared spectrum of samples with composition

Compounds 633 (2015) 104114x = 0.3 to x = 0.5 clearly indicates the presence of Fe ions in its diva-lent state (Fe2+) beside the trivalent state (Fe3+). So the existence ofFe2+ ions in B-sites will reduce the magnetization of these samples.

and-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

x=0.0

RTolo

g (

-1m

-1)

H.M. Zaki et al. / Journal of AlloysThe continuous decrease in magnetization for these samples wouldreect the increase of the occupation of Fe2+ ions of B-sites for sam-ples with x = 0.3 up to x = 0.5. Further, the migration of Mg2+ to Asites, accompanied with migration of Fe3+ ions to B-sites will leadto an increase in magnetization in B-sites up to x = 0.2 and hence tothe increase of the saturation magnetization of the MgCuZnferrite. Moreover, the rate of increase in magnetization with con-centration (x) shows a decline which indicates an increase of Fe3+

in A-site with increasing Cu2+ ions conrming the validity of theproposed cation distribution for x > 0.2. The decrease in Ms forxP 0.30 may be also explained assuming the cation redistributionin which Cu2+ ion can force some of Fe3+ ions to migrate from B-site

-7.0

-6.5

-6.0

-5.5

log (Hz)

log (Hz)

log (Hz)

100 C 200oC 300oC 400oC

-6

-4

-2

x=0.1

log

(-1

m-1

)lo

g (

-1m

-1)

RT 100oC 200oC 300oC 400oC

-7

-6

-5

-4

-3

-2

x=0.2

RT 100oC 200oC 300oC 400oC

2 3 4 5 6 7

2 3 4 5 6 7

2 3 4 5 6 7

Fig. 5a. The frequency dependence of the ac conductivity at slog

(-1

m-1

) -2

0

x=0.3

Compounds 633 (2015) 104114 109to A-site. Moreover, the creation of some Fe2+ with lower magneticmoment (4 lB) will result in reduced magnetization of B sites andconsequently the net magnetization of the samples. However, Mg2+

can also migrate from B-sites to A-sites depending upon heat treat-ment as discussed in the cation distribution of the studied ferritesin section one. One can conclude that the saturation magnetizationof the investigated samples by this technique is better than thatreported by other ceramic technique [45,49] and auto-combustionroute [50]. That mean the used method (co-precipitation) has abil-ity of improving the ferrite with higher saturation magnetizationwith low sintering time which make this ferrite important over awide range of frequency.

log (Hz)

log (Hz)

log (Hz)

log

(-1

m-1

)lo

g (

-1m

-1)

-6

-4 RT 100oC 200oC 300oC 400oC

-8

-6

-4

-2

0x=0.4

RT 100oC 200oC 300oC 400oC

2 3 4 5 6 7

2 3 4 5 6 7

2 3 4 5 6 7-7

-6

-5

-4

-3

-2

-1

0

x=0.5

RT 100oC 200oC 300oC 400oC

elected temperature for the Mg0.5CuxZn0.5xFe2O4 ferrites.

andAccording to Ohnishi and Teranishi [51], the intensity ratios ofplanes I(220)/I(440), I(400)/I(422) and I(220)/I(400) are consid-ered to be sensitive to any change in cation distribution. The inten-sities of (220) and (422) planes are mostly sensitive to cations onthe tetrahedral sites [52,53], while the intensity of (400) planedepends on the existence of cations in both sites. In the presentinvestigation (220), (222), (400) and (422) planes were used tocalculate the intensity ratio using the integrated intensity of theX-ray diffraction patterns. The trends of the intensity ratiosI(220)/I(222), I(400)/I(422) and I(220)/I(422) match the behav-ior of saturation magnetization. This trend conrms the sensitivityof this ratios to the distribution of cations between A and B siteswhich found to be in good agreement with the proposed cation dis-tribution in this study. The comparison of X-ray diffraction lineintensity ratios for various possible cation distributions for thestudied compositions is shown in Fig 4. I suggest to be in a separateTable to avoid the problem of the huge error bars.

It has been reported that nanosized materials possess signi-cantly distinguished physical and chemical properties from thoseof bulk materials due to the their extremely small particle size orlarge surface area [54,55]. For example, it has been found thatthe surface area of the nanoparticles play an important role inthe eld of nanomagnetism [56]. Consequently, the surface area

250 300 350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

s

T (K)

x=0.0 x=0.1 x=0.2 x=0.3 x=0.4 x=0.5

Fig. 5b. Effect of temperature on the variation of the exponent factor (s).

110 H.M. Zaki et al. / Journal of Alloysis calculated for all compositions using the following relation [57]:

S 6000Ddbulk

9

where D is the particle size in nm and dbulk is the bulk density of theparticle in g/cm3. The calculated surface area of the nanoparticlesincreases with the reduction of the particle size as listed in Table 2.The saturation magnetizations (Ms) is sensitive to the particle sizeor surface area. It is obvious from Table 2, that as the particle sizedecreases the surface area increases and the Ms value is increasedtoo. This behavior is observed for samples with compositionsx = 0.1, 0.2 and 0.3 where the particle size decreases from 62 to25 nm. Other samples with concentration x = 0.4 and 0.5 exhibitsignicant decrease of the saturation magnetization due to theincrease of their particles sizes or decrease in surface area. This cor-relation between the particle size and magnetic properties has beenreported for some ferrites compounds [58,59].

3.4. AC conductivity

Fig. 5a indicates the variation of conductivity rac as a function offrequency at particular temperatures. The plots of log rac againstlogx show straight lines with different slopes at high-range of fre-quency and low temperature (i.e., for T 6 100 C) whereas forT > 100 C the behavior of the conductivity becomes less frequencydependent and almost have a constant value. In other words, theelevated temperatures enhance the conductivity in which the rateof enhancement is noticed to be higher in the region of relativelylow temperature. Generally, the behavior of the electrical conduc-tivity can obey the relation:

r r0 expE=kT 10where k is Boltzmann constant, E is the activation energy requiredfor hopping of charge carriers, T is the absolute temperature andr0 represents specic conductivity. Fig. 5 shows that the studiedsamples have semiconducting behavior with temperatures as indi-cated from the increase of rac with increasing in temperature atselected frequencies. An obvious dispersion behavior is observedat T 6 100 C whereas at higher temperature range the conductivityseems to be less temperature-dependent. The increase of conductiv-ity rac with increasing temperature may be ascribed to the increasein the drift mobility and charge carriers hopping frequency. Thisbehavior is consistent with the relation; r = nel; in which e is thecharge of the electron, l is the drift mobility of charge carriersand n is the concentration of charge carriers (n = ne + nh, wherethe subscripts e and h refer to electron and hole respectively, ascharge carriers responsible for the conduction process). Ferritesare considered as semiconductors with low-mobility, so the activa-tion energy is often associated with concentration rather than theirmobility of charge carriers.

It is pertinent to mention that the type of the conduction mech-anism basically based on the variation of the exponent s with tem-perature. The values of the exponent factor s as a function oftemperature were derived from the slope of the curves shown inFig. 5a. The variation of swith temperature for the present samplesis plotted in Fig. 5b. Recently, number of theoretical models as thesmall polaron (SP) model, the overlapping large polaron-tunneling(OLPT), the correlated barrier-hopping (CBH) and quantummechanical tunneling (QMT) model have been assumed as applica-ble models for explaining the conduction mechanism in ferritematerials [60,61]. In the present investigation, the observeddecrease of s values with the increasing of temperature agree wellwith the correlated barrier-hopping (CBH) model. Similar behaviorhas been reported by Zaki [62], Farea et al. [63] and Ghatak et al.[64]

3.5. Dielectric constant

Different parameters can vary the dielectric properties of solidssuch as temperature, frequency of applied electric eld, crystalstructure, humidity and some other external factors [65]. Thedielectric response of a solid can be described by expressing therelative dielectric constant as a complex quantity consists of a realcomponent (e0) and an imaginary component (e00) i.e.,

e e0 ie00 11The real part of dielectric constant represents the amount of energystored in a dielectric material as polarization while the imaginarypart, represents the energy loss. The effect of frequency on bothparts of the relative dielectric constant (e0, e00) at different tempera-tures are shown in Figs. 6 and 7, respectively. It is found that all thesamples show similar behavior with frequency. As a general trend,the dielectric constant decreases with increasing frequency. Fur-thermore, the dispersion in dielectric constant is more signicant

Compounds 633 (2015) 104114at low frequencies in comparison to high values. The decrease ine00 is more obvious as compared to the e0 as the frequency increasesfrom 100 Hz to 1 MHz.

and103

104

'x=0.0

RT 100oC 200oC 300oC

o

H.M. Zaki et al. / Journal of AlloysThe observed behavior of the dielectric constant can beexplained on the basis of MaxwellWagner [66,67] and Koops the-ory [68]. According to these two models, ferrites in general are con-sidered to be consist of grains and grain boundaries. The grainboundary is more resistive than that of the grain itself. Moreover,at higher frequencies the grains are found to be more effective,on contrast with the grain boundaries, which are predominant atlower frequencies. As the frequency increases, the charge carriersresponsible for the polarization lack behind the applied eld. Thisbehavior will appear as dielectric relaxation in the obtained results.

101

102

103

x=0.1

'

RT 100oC 200oC 300oC 400oC

100

101

102

103

104

'

x=0.2 RT 100oC 200oC 300oC 400oC

4

1 2 3 4 5 6

1 2 3 4 5 6

62101

102

logf (Hz)

logf (Hz)

logf (Hz)

400 C

Fig. 6. The frequency dependence of dielect102

103

104

'

x=0.3 RT 100oC 200oC 300oC 400oC

Compounds 633 (2015) 104114 111The observed peaks in the graphs of the dielectric constant (e0)with frequency at different selected temperatures may beexplained utilizing the Rezlescu model [69]. According to thismodel, the inverse peaks observed in Fig. 6 can be ascribed tothe contribution of the two different types of charge carriers tothe polarization. For the studied samples, the presence of two dif-ferent types of charge carriers, electron transfer between Fe2+ andFe3+as n-type, and hole exchange between Cu+ and Cu2+ at theoctahedral sites as p-type may be enhance the conduction process[70,71]. In terms of the polarity, the direction of the holes transfer

10-1

100

101

logf (Hz)

100

102

104x=0.4

'

RT 100oC 200oC 300oC 400oC

100

101

102

103

104

105

RT 100oC 200oC 300oC 400oC

x=0.5

'

1 2 3 4 5 6

1 2 3 4 5 6

1 2 3 4 5 6

logf (Hz)

logf (Hz)

ric constant e0 at selected temperature.

and105

"x=0.0 RT

100oC 200oC 300oC 400oC

112 H.M. Zaki et al. / Journal of Alloysis opposite to that of the electrons under the effect of the appliedexternal electric eld beside the mobility of holes is relatively verysmall with respect to that of electrons. As a result of that, the resul-tant polarization of both types of charge carriers will give raise tothe appearance of peaks as shown in Fig. 6. Such peaking behaviorhas been also observed for CuNi, CuMn and CuZn ferrite [69]and NiZn ferrite [72].

The high values of the dielectric constant may be correlated tothe space charge polarization mechanism. In such mechanismthe conductivity of grain boundaries having high resistance differsfrom that of grains with low resistance, hence the charge carriersface different resistances causing an accumulation of charges atseparating boundaries which in turn produces an increase in thedielectric constant.

100

logf (Hz)

101

102

103

104

105

106

"

x=0.1 RT 100oC 200oC 300oC 400oC

100

105

"

x=0.2 RT 100oC 200oC 300oC 400oC

logf (Hz)

logf (Hz)

1 2 3 4 5 6

1 2 3 4 5 6

1 2 3 4 5 6

Fig. 7. The frequency dependence of dielectric loss (e00) at sel102

103

104

105

106

107

108

"

x=0.3 RT 100oC 200oC 300oC 400oC

Compounds 633 (2015) 104114The variations of the two parts of dielectric constants (e0; e00) asa function of temperature for Mg0.5Zn0.5xCuxFe2O4 ferrites atselected frequencies were found to increase with increasing inthe temperature conrming the semiconducting nature of thesamples. For more details, at relatively low temperatures thecharge carriers can not align themselves in the direction ofapplied eld which result as an increase in the polarization andhence in the dielectric constant. As the temperature increases,the thermal energy increases and more bound charges get liber-ated which result as an increase of the mobility of charges thatwould enable them to orient easily in the direction of the appliedeld. This, in turn, enhances their contribution to the spacecharge polarization causing a pronouncing increase in the valueof dielectric constant.

101

logf (Hz)

logf (Hz)

logf (Hz)

101

102

103

104

105

106

107

108

"

x=0.4 RT 100oC 200oC 300oC 400oC

642

1 2 3 4 5 6

1 2 3 4 5 6101

102

103

104

105

106

107

108

109

"

RT 100oC 200oC 300oC 400oC

x=0.5

ected temperature for the Mg0.5CuxZn0.5xFe2O4 ferrites.

and101

102

103

tan

x=0.0

RT 100oC 200oC 300oC 400oC

H.M. Zaki et al. / Journal of AlloysThere are different types of polarization which contribute to thedielectric constant; dipolar, electronic, ionic, and interfacialpolarizations. Interfacial and dipolar polarizations depend on tem-perature and found to play main role at low frequency region. Theinterfacial polarization increases as the temperature increaseowing to creation of crystal defects, consequently, dipolar polariza-tion decreases. The effect of temperature is more distinct on theinterfacial than on dipolar polarization, so the overall effect is theobserved rapid increase in the dielectric constant with tempera-ture at low frequencies. At high frequencies, electronic and ionicpolarizations are the main contributors to the dielectric constantand their temperature dependence is irrelevant [73,74].

100

101

102

103

104

tan

x=0.1 RT 100oC 200oC 300oC 400oC

100

101

102

103

tan

x=0.2 RT 100oC 200oC 300oC 400oC

1 2 3 4 5 6

1 2 3 4 5 6

1 2 3 4 5 6

100

logf (Hz)

logf (Hz)

logf (Hz)

Fig. 8. The frequency dependence of dielectric loss tangent (tand)2

103

104

105

106x=0.3

RT 100oC 200oC 300oC 400oC

tan

Compounds 633 (2015) 104114 1133.6. Dielectric loss tangent (tand)

The dielectric loss factor (tand) characterizes the energy loss.The dielectric loss arises when the applied ac eld leads the polar-ization and can be expressed according to Eq. (2).

Many factors can affect the dielectric losses of the samples asimpurities, imperfections in the crystal lattice and the density ofthe samples. Fig. 8 shows the variation of loss tangent tand withfrequency at selected temperatures. It was found that tanddecreases with increasing frequency for all samples. A decreasingbehavior was reported by Penchal Reddy et al. [75] forPolycrystalline MgCuZn ferrites with chemical formula

100

101

10

logf (Hz)

100

101

102

103

104 x=0.4 RT 100oC 200oC 300oC 400oC

100

101

102

103

104

105x=0.5

RT 100oC 200oC 300oC 400oC

1 2 3 4 5 6

1 2 3 4 5 6

2 4 6

tan

tan

logf (Hz)

logf (Hz)

at selected temperatures for the Mg0.5CuxZn0.5xFe2O4 ferrites.

114 H.M. Zaki et al. / Journal of Alloys andMg0.50xCuxZn0.50Fe2O4 prepared by microwave sintering method.The frequency response of tand shows small peaks or relaxationprocesses in some samples as for x = 0.1, 0.3, 0.4 and 0.5. The differ-ence in environment of different ions in a condensed material leadsto the distribution of relaxation processes. For observing a maximain the dielectric loss the following condition must be veried:

xmax:s 1 12where xmax 2pfmax. The relaxation time s is related to the jump-ing probability per unit time p as:

s 1=2p or xmax: 2p 13According to Eq. (12), the Debye relaxation process occurs when thejumping rate becomes approximately equals to applied eldfrequency which is known as ferrimagnetic resonance [76].

The disappear of the resonance peaks at the compositions,x = 0.0 and 0.2 may be ascribed to the relatively high resistivityof these samples or due to shifting of their positions to higherfrequencies beyond the applied range of frequency.

4. Conclusion

Nano-crystalline Mg0.5CuxZn0.5xFe2O4 (0 6 x 6 1.0) ferritepowders were synthesized by co-precipitation technique. Latticeconstant showed decreasing trend with increasing of Cu2+ concen-tration due to smaller ionic radius of Cu2+ ions in comparison tozinc ions. As the lattice constant decreases the X-ray densityincreases and the porosity decreases indicating dense packing ofthe samples. The magnetic behavior of the nano-crystalline ferritesexhibited some sensitivity to the particle size and surface area.Variation of saturation magnetization with Cu content wasexplained in terms of the cations redistribution between the sublattices A and B and X-ray intensity ratios of the diffraction linessensitive to the tetrahedral and octahedral sites. The decrease ofthe dielectric constant at low frequency and the appearance ofanomalous dispersion behavior was explained in the light ofMaxwellWagner and Koops models of polarization. Contributionof two types of charge carries as electrons and holes in the conduc-tion mechanism was attributed to the appearance of two oppositeresonance peaks in plots of the dielectric constant with frequency.Dielectric loss tand found to decrease with frequency at low tem-peratures without signicant change at high temperatures.

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Compounds 633 (2015) 104114

Structural, magnetic and dielectric studies of copper substituted nano-crystalline spinel magnesium zinc ferrite1 Introduction2 Experimental details3 Results and discussion3.1 X-ray diffraction3.2 Infrared spectroscopy3.3 Magnetization3.4 AC conductivity3.5 Dielectric constant3.6 Dielectric loss tangent (tan)

4 ConclusionReferences