Journal of Magnetism and Magnetic Materials 325 (2013) 47–51

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Journal of Magnetism and Magnetic Materials

0304-88

http://d

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E-m

cprakas1 Te

journal homepage: www.elsevier.com/locate/jmmm

Study of 0.1Ni0.8Zn0.2Fe2O4�0.9Pb1�3x/2LaxZr0.65Ti0.35O3

magnetoelectric composites

Rekha Rani a,d, J.K. Juneja b,n, Sangeeta Singh c, K.K. Raina d, Chandra Prakash e,1

a Electroceramics Research Laboratory, GVM Girls College, Sonepat 131001, Indiab Department of Physics, Hindu College, Sonepat 131001, Indiac Department of Physics, GVM Girls College, Sonepat 131001, Indiad School of Physics and Material Sciences, Thapar University, Patiala 147004, Indiae Directorate of ER&IPR, DRDO, DRDO Bhawan, New Delhi 110105, India

a r t i c l e i n f o

Article history:

Received 5 July 2012

Received in revised form

21 July 2012Available online 15 August 2012

Keywords:

Dielectric properties

Electric polarization

Magnetization

Poling

Magnetoelectric composite

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.08.011

esponding author. Tel.: þ919416260242.

ail addresses: jk_juneja@yahoo.com (J.K. June

h@hqr.drdo.in (C. Prakash).

l.: þ91 11 2300 7350; fax: þ91 11 2301 758

a b s t r a c t

Magnetoelectric composites of nickel zinc ferrite (NZF) and La substituted lead zirconate titanate (PLZT)

having representative formula 0.1Ni0.8Zn0.2Fe2O4�0.9Pb1�3x/2LaxZr0.65Ti0.35O3 (x¼0, 0.01, 0.02 and

0.03) were synthesized by a conventional solid state route. X-ray diffraction analysis was carried out to

confirm the coexistence of individual phases. Scanning electron microscope micrographs were taken for

microstructural study of the samples. Dielectric properties were studied as a function of temperature

and frequency. To study ferroelectric and magnetic ordering in composite samples, P–E and M–H

hysteresis loops were recorded respectively. M–H hysteresis loops were taken for electrically poled and

unpoled samples to confirm magnetoelectric coupling between the two phases (NZF and PLZT). La

substitution results in significant improvement in dielectric, ferroelectric and piezoelectric properties

of composite samples.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The area of magnetoelectric (ME) materials has attracted manyresearchers from both groups (ferroelectricity and magnetism)because these materials exhibit ferroelectric and ferromagnetic/ferrimagnetic properties of matter. These materials have potentialapplications in many multifunctional devices such as magnetic fieldsensors, multiple state memory elements, transducers, electro-opticdevices, filters, oscillators and phase shifters [1–6]. The ME effect inthese materials is defined as an induced electric polarization inexternal magnetic field or an induced magnetization in externalelectric field. These materials are classified into two groups: singlephase and two phase (composites). Single phase ME materials havecoexistence of ferroelectric and magnetic orders in single phase andexhibit direct ME coupling. Though the experimental evidences of MEeffect in single phase systems were observed during the first half of20th century the materials showing ME coupling were found to berare due to some limiting factors which restrict the coexistence offerroelectricity and magnetism in a single phase [7,8]. Two phasesystems exhibit indirect coupling between ferroelectricity and mag-netism. This indirect coupling takes place via stress and results inmagnetostriction induced deformation and the generation of piezo-electric charge [9–12] i.e. one phase should be magnetostrictive or

ll rights reserved.

ja),

2.

piezomagnetic and other should be piezoelectric or electrostrictive.The most widely studied systems that have been reported correspondto substituted NiFe2O4, CoFe2O4, MnFe2O4, ZnFe2O4, Terfenol, etc.with substituted PZT, PMN-PT, PVDF and BaTiO3 [13].

For the present work, bulk composites of Ni–Zn ferrite and Lasubstituted PZT with general formula 0.1Ni0.8Zn0.2Fe2O4�0.9Pb1–3x/

2LaxZr0.65Ti0.35O3 (x¼0, 0.01, 0.02 and 0.03) were synthesized by aconventional solid state reaction route. The presented composites areferroelectric rich ferrite–ferroelectric composites with small content(10%) of the ferrite phase. The reason is that composites with higherferrite content show lossy P–E loops due to high conductivity offerrite phase as compared to that for ferroelectric phase and are alsodifficult to pole. Higher value of ferrite content also results in lowerME output due to the leakage charges developed in the ferroelectricgrains which reduces the charges generated during the ME effect[14,15]. Further, La content (x) in ferroelectric phase was selectedfrom 0 to 0.03 which can be attributed to the fact that for higher Lacontent in PZT, the disordering effect of La becomes important,resulting in considerable decrease in values of remanant polarization(Pr) and saturation polarization (Ps) [16,17].

2. Experimental work

2.1. Material synthesis

The ferrite phase was prepared using AR grade NiO, ZnO andFe2O3. The powder mixture was ball milled, dried and then

Table 1Structural parameters for all composite samples

Parameters X

0 0.01 0.02 0.03

Lattice constant ‘a’ (ferroelectric phase) (A) 4.104 4.097 4.095 4.092

Lattice constant ‘a’ (ferrite phase) (A) 8.353 8.342 8.346 8.348

Experimental density (g/cm3) 6.88 6.79 6.87 6.81

X-ray density (g/cm3) 7.75 7.72 7.73 7.71

Relative density (%) 88.8 88.0 88.9 88.3

R. Rani et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 47–5148

calcined at 1000 1C for 4 h. A small amount of MnO2 (0.5% byweight) was added to the calcined powder to increase theresistivity of the ferrite. After ball milling and drying, the powdermixture was re-calcined at 1100 1C for 4 h. AR grade PbO, ZrO2,TiO2 and La2O3 for ferroelectric phase (PLZT) were weighed inrequired molar proportions and mixed. An excess of 2% PbO wasadded to compensate lead loss during sintering. The mixture ofraw materials was milled in distilled water using zirconia balls.The dried powder was calcined at 800 1C for 4 h. The reactedpowder mixture was again ball milled, dried and then re-calcinedat 1000 1C for 4 h. NZF–PLZT composites with general formula0.1Ni0.8Zn0.2Fe2O4�0.9Pb1�3x/2LaxZr0.65Ti0.35O3 were prepared bymixing the two phases. The mixing process was carried out byball milling in distilled water. After drying the mixture, smallamount of diluted polyvinyl alcohol (2–3 drops) was added to thepowder mixture as a binder and then the powder mixture waspressed into circular disks of �1 mm thickness and 15 mmdiameter using a hydraulic press. The pellets were finally sinteredat 1200 1C for 4 h in a programmable furnace.

2.2. Characterization

Experimental density of sintered pellets was determined usingArchimedes principle. Theoretical density of the samples was calcu-lated using the lattice parameters. The X-ray diffraction (XRD) datawas recorded using a Philips XPERT-PRO diffractogram with Cu-Ka

(l¼1.5406 A) in the range of 2y¼20�701 with step size of 0.011.Scanning electron microscope (SEM) micrographs of the freshlybroken pieces of sintered samples were obtained using a JEOL JSM6510LV, Japan. For measuring electrical properties, the sinteredpellets were ground and then electroded properly using silver epoxyon flat surfaces and subsequent heating at 400 1C for 30 min. Thedielectric properties were measured as a function of frequency(100 Hz–1 MHz) at room temperature and as a function of tempera-ture (35–500 1C) using an Agilent 4263B LCR meter. P–E hysteresisloops were recorded at 20 Hz using an automated P–E loop tracerbased on the Sawyer–Tower circuit. For electric poling, samples wereheated to 150 1C and a dc electric field (�15 kV/cm) was applied for1 h. Then the samples were cooled to room temperature in thepresence of field. M–H loops were recorded using a Lake Shore735 VSM Controller, Model 662, interfaced with a computer.

3. Results and discussion

3.1. Structural properties

Fig. 1 shows the XRD patterns for composite samples withx¼0.01 and 0.03 and confirms the coexistence of both phases

20 30 40 50 60 70

(312

)

(220

)(4

40)

(511

)(018

)

(116

)(024

)

(202

)(3

11)

(110

)

x = 0.01

x = 0.03 * * *

+

+

+ ++

++ +

2θ (degrees)

Inte

nsity

(a.u

.)

+ Ferroelectric (PLZT)* Ferrite (NZF)

(012

)

Fig. 1. XRD patterns for composite samples with x¼0.01 and 0.03.

(NZF and PLZT). These XRD patterns show well defined peaks withspecific indices characteristics of cubic spinel structure of ferritephase and rhombohedral perovskite structure of ferroelectricphase [16,18]. No additional peaks were observed. Intensity andnumber of peaks corresponding to NZF are very small due tolower concentration of ferrite phase. The values of lattice constant‘a’ corresponding to both phases were calculated for all thesamples and are given in Table 1 for comparison. There is a slightdecrease in the values of lattice constant of ferroelectric phasewhich may be due to substitution of less ionic size La at Pb site(r(Pb2þ)¼163 pm and r(La3þ)¼150 pm). The values of latticeconstant of ferrite phase are random which may be due tostresses induced by ferroelectric phase surrounding the ferritephase [19]. Experimental, theoretical and relative density for allthe samples were determined and are given in Table 1. Suchvariation in density values is already observed in many ferro-electric ceramics [20–22]. Fig. 2 shows the SEM micrographs forall composite samples. It can be easily observed that the averagegrain size decreases with increase in La substitution. Individualphases cannot be distinguished in SEM micrographs because ofthe small concentration of ferrite phase.

3.2. Dielectric properties

Fig. 3 shows the variation of dielectric constant (e) anddielectric loss (tan d) with temperature for all samples at100 kHz. It can be observed easily that initially dielectric constantincreases with increase in temperature and reaches a maximumvalue at a particular temperature followed by decrease withfurther increase in temperature for all composite samples. Thistemperature corresponds to ferroelectric Curie temperature (Tc).A decrease in Tc was observed with increase in x (La substitution)which may be due to substitution of less ionic size substituentand shrinkage in PZT lattice [23]. Higher values of dielectric lossat higher temperatures were observed. This may be due to thethermal conductivity losses which occur as a result of thermallyactivated electron hopping between Fe2þ/Fe3þ and Ni2þ/Ni3þ

ions present in NZF phase [24].Frequency dependence of dielectric properties at room tem-

perature was also studied for all samples and is shown in Fig. 4.Higher values of dielectric constants were observed at lowerfrequencies which may be because of space charge polarizationdue to the presence of inhomogeneities in the structure [25]. Thedielectric constant decreases with increase in frequency becauseionic and orientation polarizations decrease with increase infrequency for all the samples. For samples with x¼0 and 0.01,dielectric constant (e) and loss (tan d) rise steadily toward lowerfrequencies. This low-frequency behavior can be explained on thebasis of electronic charge carriers generated due to Pb2þ vacan-cies [26]. With further increase in La substitution, this low-frequency dispersion decreases which may be due to decreasein concentration of Pb2þ vacancies (La3þ ions substitute Pb2þ

ions) and the donor effect of La substitution that counteracts the

Fig. 2. SEM micrographs for all composite samples.

100 200 300 400 5000.0

2.0k

4.0k

6.0k

8.0k

x = 0.01

x = 0.03

x = 0.02

ε

T (°C) T (°C)

x = 0

100 200 300 400 500

0.0

0.3

0.6

0.9

1.2 x = 0 x = 0.01 x = 0.02 x = 0.03

tan δ

Fig. 3. Temperature dependence of dielectric properties for all composite samples.

102 103 104 105 106

500

600

700

800

900

x = 0.01

x = 0

x = 0.02

ε

frequency (Hz)

x = 0.03

102 103 104 105 106

0.0

0.1

0.2

0.3x = 0

x = 0.01

x = 0.02

tan δ

frequency (Hz)

x = 0.03

Fig. 4. Frequency dependence of dielectric properties for all composite samples.

R. Rani et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 47–51 49

R. Rani et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 47–5150

p-type conduction [18]. At higher frequencies, room temperaturedielectric constant increases with increase in La substitutionexcept for x¼0.01. Increase in permittivity values for donorsubstituents in PZT is well reported in literature [27]. But forx¼0.01, dielectric constant is low which may be due to decreasein experimental density (Table 1). Similar behavior was observedfor the frequency dependence of dielectric loss. The investigatedsamples with x¼0.02 and 0.03 are suitable for low loss applica-tions in the above given frequency range.

3.3. Ferroelectric properties

To study ferroelectric ordering in composites, room temperatureP–E hysteresis loops were recorded and are shown in Fig. 5.All samples show well defined ferroelectric behavior. A comparison

-20 -15 -10 -5 0 5 10 15 20

-6

-4

-2

0

2

4

E (kV/cm)

P ( μ

C/c

m2 )

x=0x=0.01x=0.02x=0.03

Fig. 5. Ferroelectric (P–E) hysteresis loops for all composite samples.

Table 2Ferroelectric and piezoelectric parameters for all composite samples

Parameters X

0 0.01 0.02 0.03

Pr (mC/cm2) 1.1 1.5 1.6 1.8

Ps (mC/cm2) 3.4 4.2 5.1 5.4

Ec (kV/cm) 4.6 4.9 4.2 4.5

d33 (pC/N) 72 76 80 89

-6000 -4000 -2000 0 2000 4000 6000

-6

-4

-2

0

2

4

6x = 0.01

unpoled

Electrically poled

Mom

ent/M

ass (

emu/

g)

Field (Gauss)

Fig. 6. Magnetic hysteresis loops for electrically poled

of P–E loops shows that there is increase in remanant polarization(Pr) and saturation polarization (Ps) as the substitution of Laincreases. It is due to the softening effect of donor substituent(La3þ) which results in higher multi-domain polarizations. Theseresults are consistent with the literature [28–30]. An increase inpiezoelectric charge coefficient (d33) with increase in La substa-tion was also observed. The values of remanant polarization (Pr),saturation polarization (Ps), coercivity (Ec) and piezoelectriccharge coefficient (d33) for all composite samples are given inTable 2. Observed values of d33 for these composites are smalleras compared to that for pure ferroelectric materials which may bedue to the presence of ferrite grains (non-ferroelectric) surround-ing the ferroelectric grains [31–33].

3.4. Magnetoelectric properties

To study magnetoelectric behavior, two pieces from a singlepellet were taken. One piece was electrically poled at 15 kV/cmbefore doing M–H measurements and M–H hysteresis curves weremeasured for both electrically poled and unpoled samples. M–H

hysteresis loops at room temperature for both samples arecompared in Fig. 6 for composite samples with x¼0.01 and0.03. For both composites, an enhancement in magnetization forelectrically poled samples was observed, which confirms thepresence of magnetoelectric coupling between the individualphases (NZF and PLZT) [34].

4. Conclusion

NZF–PLZT composites synthesized by the conventional solidstate reaction method were characterized by XRD and SEM forstructural and microstructural analysis respectively. Decrease inferroelectric Curie temperature (Tc) and increase in room tem-perature dielectric constant with increase in La substitution wereobserved for all composite samples. La substitution results inimproved dielectric, ferroelectric and piezoelectric properties.Observation of both P–E hysteresis loops and M–H hysteresisloops in composite samples indicates the presence of orderedferroelectric and magnetic behavior. Enhancement in magnetiza-tion of approximately 60% for electrically poled samples wasobserved which shows that there is a good electromagneticcoupling between the two phases. The investigated compositesseem to be very attractive for sensor applications and in multiplestate memory devices where data can be stored both as polariza-tion (P) and magnetization (M).

-6000 -4000 -2000 0 2000 4000 6000

-6

-4

-2

0

2

4

6

unpoled

Electrically poled

Mom

ent/M

ass (

emu/

g)

Field (Gauss)

x = 0.03

sample and unpoled sample for x¼0.01 and 0.03.

R. Rani et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 47–51 51

Acknowledgment

One of the authors (Rekha Rani) would like to thank Depart-ment of Science and Technology, Government of India, forawarding INSPIRE fellowship to her. Authors are thankful toDr. R.K. Kotnala of National Physical Laboratory, New Delhi, forhelping in magnetization measurements.

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