Structural and dielectric characterization on multiferroic xNi 0.9Zn0.1Fe2O4/(1 - X)PbZr 0.52Ti0.48O3 particulate composite

Article abstract of DOI:10.1016/j.jallcom.2014.01.007

We have carried out the powder X-ray diffraction and dielectric studies on multiferroic particulate composite xNi_0.9Zn_0.1Fe ₂O₄/(1 - x)PbZr_0.52Ti_0.48O ₃ with x = 0.15, 0.30, 0.45, 0.60, 0.75 and 0.90 to explore the structural and ferroelectric properties. A conventional double sintering method was used to prepare the xNi_0.9Zn_0.1Fe₂O ₄/(1 - x)PbZr_0.52Ti_0.48O₃ composites. The structure of one of the component Ni_0.9Zn _0.1Fe₂O₄ is spinel cubic with space group Fd3a€m, while the other component PbZr_0.52Ti _0.48O₃ is selected around the morphotropic phase boundary region in which the tetragonal and monoclinic phases with space group P4mm and Cm coexist respectively. We have carried out Rietveld refinement of the structure to check the formation of ideal composites with separate ferroelectric and ferrite phases. Even though the structural characterization does not reveal the formation of any new phase due to reaction between the two components of the composite during sintering, the tetragonality of the PbZr _0.52Ti_0.48O₃ continuously decreases with increasing the ferrite fraction while the lattice parameter of ferrite phase increases with increasing fraction of the ferroelectric phase. Similarly, the dielectric study reveals clear shift in the ferroelectric to paraelectric phase transition temperature of PbZr_0.52Ti_0.48O₃ during composite formation suggesting that part of Ni²⁺, Zn ²⁺/Fe³⁺ ions are diffusing at the B-site of PbZr _0.52Ti_0.48O₃ replacing Ti⁴⁺, which in turn decreases its transition temperature. Scanning electron micrograph of sintered pellet surface confirms the presence of two types of particle morphology in the particulate composite, corresponding to ferrite and ferroelectric phases.

Full text of DOI:10.1016/j.jallcom.2014.01.007

Journal of Alloys and Compounds 593 (2014) 224–229
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Journal of Alloys and Compounds
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Review
Structural and dielectric characterization on multiferroic
xNi0.9Zn0.1Fe2O4/(1 ꢀ x)PbZr0.52Ti0.48O3 particulate composite
⇑
Rishikesh Pandey, Braj Raj Meena, Akhilesh Kumar Singh
School of Materials Science & Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have carried out the powder X-ray diffraction and dielectric studies on multiferroic particulate com-
posite xNi0.9Zn0.1Fe2O4/(1 ꢀ x)PbZr0.52Ti0.48O3 with x = 0.15, 0.30, 0.45, 0.60, 0.75 and 0.90 to explore the
structural and ferroelectric properties. A conventional double sintering method was used to prepare the
xNi0.9Zn0.1Fe2O4/(1 ꢀ x)PbZr0.52Ti0.48O3 composites. The structure of one of the component Ni0.9Zn0.1Fe2O4
is spinel cubic with space group Fd3m, while the other component PbZr0.52Ti0.48O3 is selected around the
morphotropic phase boundary region in which the tetragonal and monoclinic phases with space group
P4mm and Cm coexist respectively. We have carried out Rietveld refinement of the structure to check
the formation of ideal composites with separate ferroelectric and ferrite phases. Even though the struc-
tural characterization does not reveal the formation of any new phase due to reaction between the two
components of the composite during sintering, the tetragonality of the PbZr0.52Ti0.48O3 continuously
decreases with increasing the ferrite fraction while the lattice parameter of ferrite phase increases with
increasing fraction of the ferroelectric phase. Similarly, the dielectric study reveals clear shift in the fer-
roelectric to paraelectric phase transition temperature of PbZr0.52Ti0.48O3 during composite formation
Received 17 November 2013
Received in revised form 25 December 2013
Accepted 1 January 2014
Available online 12 January 2014
Keywords:
Particulate composite
XRD
Rietveld refinement
Dielectric
Multiferroics
suggesting that part of Ni2+, Zn2+/Fe3+ ions are diffusing at the B-site of PbZr0.52Ti0.48O3 replacing Ti4+
,
which in turn decreases its transition temperature. Scanning electron micrograph of sintered pellet sur-
face confirms the presence of two types of particle morphology in the particulate composite, correspond-
ing to ferrite and ferroelectric phases.
Ó 2014 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
2. Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
3.1. Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
3.2. Microstructure studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
3.3. Dielectric studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
1. Introduction
and memories [1,2]. In multiferroics, more than one ferroic proper-
ties like ferroelectricity (FE), ferromagnetism (FM), etc. are present
simultaneously and are coupled. For example, presence of coupled
ferroelectric and ferromagnetic responses provides an extra degree
of freedom in which data could be written electrically and read
magnetically and vice versa for memory applications [3]. Usually
presence of ferroelectricity in perovskites requires the empty
d-orbitals at B-site cation which soften the orbital overlapping
between B-site cations and O anions in the ABO3 perovskite
In recent years, multifunctional materials such as multiferroics
have stimulated great interest of researchers due to the potential
applications for devices such as sensors, actuators, transducers
⇑
Corresponding author. Tel.: +91 9415619018; fax: +91 542 2368428.
E-mail addresses: akhilesh_bhu@yahoo.com, aksingh.mst@itbhu.ac.in (A.K.
Singh).
0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2014.01.007

R. Pandey et al. / Journal of Alloys and Compounds 593 (2014) 224–229
225
structure [4]. However, ferromagnetism requires unpaired elec-
trons in the d-orbitals of the ions at B-site. That is the reason
why these materials are rarely found in nature and difficult to syn-
thesize in laboratory [5]. Single phase multiferroics such as BiFeO3,
BiMnO3 etc. exhibit very low magnetoelectric (ME) coefficient and
low resistivity limiting their applications [6,7]. Most of them exhi-
bit magnetoelectric coupling much below room temperature and
therefore are not suitable for potential device applications [3,8,9].
In view of this, the composite multiferroics in which two phases
are coupled via strain may be a good substitute for single phase
multiferroics [2]. The composite multiferroics may exhibit high
ME-coefficient which have not been observed in the single phase
ME materials. The appearance of magnetoelectric coupling in com-
posites is relatively new concept introduced in 1972 by Suchtelen
[10]. Subsequently large number of papers appeared, reporting
significant ME-coefficient (ꢁ100 V/cm-Oe) in ferroelectric/ferro-
magnetic composites [11]. In the recent years, large numbers of
particulate composites and piezoelectric–magnetostrictive hetero-
particulate composites, the sintering temperature needs to be
brought down by using nanocrystalline powders [25] or by reduc-
ing the sintering time by adopting the methods like hot pressing
[26] or spark plasma sintering [20].
2. Experimental details
Samples used in the present work were prepared by conventional double sinter-
ing solid state route. To prepare Ni0.9Zn0.1Fe2O4, stoichiometric amounts of AR grade
NiO (obtained by thermal decomposition of NiCO3ꢂ2Ni(OH)2 (QUALIGENS) at
550 °C), ZnO (QUALIGENS, 99%) and Fe2O3 (HIMEDIA, 99%) were mixed in agate
mortar and then ball milled for 4 h. The powder mixture was calcined at 800 °C
for 6 h. Similarly, PbZr0.52Ti0.48O3 (PZT) was prepared by conventional solid state
route using AR grade PbCO3 (HIMEDIA, 99.9%), ZrO2 (HIMEDIA, 99%) and TiO2
(HIMEDIA, 99%). Stoichiometric amounts of these ingredients were ball milled in
acetone (as mixing media) for 6 h. The mixed powder was dried and then calcined
at 800 °C for 6 h. Powder X-ray diffraction (XRD) patterns were recorded using an
18 kW rotating Cu-target based RIGAKU (Japan) powder X-ray diffractometer fitted
with a graphite monochromator in the diffracted beam. To prepare different com-
positions of NZFO/PZT composites, the mixture of calcined powders of PZT and
NZFO were first ball-milled in acetone for 6 h and then dried. 2% polyvinyl alcohol
(PVA) solution in water, which acts as binder was mixed with the powders. The
powder mixture was pressed in the form of pellets of diameter 12 mm and thick-
ness ꢁ(1.5–2.0) mm using a stainless-steel die and uniaxial hydraulic press at an
optimized load of 65 kN. Before sintering, the green pellets were kept at 500 °C
for 10 h to burn out the binder. The pellets were finally sintered at 1150 °C for
6 h in PbO atmosphere in sealed Alumina crucible. For dielectric measurements,
structures have been explored such as Ni0.9Co0.1Fe2O4/Pb(ZrxTi1ꢀx
)
O3 [12], BiFe0.5Cr0.5O3/NiFe2O4 [13], Ni0.93Co0.02Mn0.05Fe1.95O4/
Pb(ZrxTi1ꢀx)O3 [14], Pb(Zr0.52Ti0.48)O3/NiFe2O4 [15], BaTiO3/
(Ni0.3Zn0.7)Fe2.1O4 [16], Pb(Zr0.52Ti0.48)O3/NiFe1.9Mn0.1O4 [17],
BaTiO3/NiFe1.98O4 [18] and CoFe2O4/PbZr0.52Ti0.48O3 [19]. Signifi-
cant improvement in ME-coefficient (ꢁ25 mV/cm-Oe) is reported
in the composites prepared by NiFe2O4 (NFO) and Pb(ZrxTi1ꢀx)O3
(PZT) using spark plasma sintering method [20]. Particularly the
composite heterostructure of Terfenol-D with PVDF prepared in
the form of multilayer thin film which exhibit very high ME-coef-
ficient (>1 V/cm-Oe) termed as giant magnetoelectric coefficient
(GME) [21]. There are several papers in the literature which report
the magnetoelectric investigations on particulate composites using
various types of ferroelectric phases such as BaTiO3, PZT etc. and
magnetic phases such as nickel ferrite, cobalt ferrite, nickel zinc
ferrite etc. assuming that the two components form ideal compos-
ite without reacting/modifying each other. However, at high sin-
tering temperature during composite formation it is very unlikely
that the magnetic and ferroelectric phases remain intact without
diffusion of ions from one component to other. To the best of our
knowledge the possible modifications in the ferroelectric and mag-
netic components during particulate composite formation has not
been investigated into detail which may have very crucial impact
on the ME response of the composite. In the present work we have
looked into this aspect by investigating several compositions of
xNi0.9Zn0.1Fe2O4/(1 ꢀ x)PbZr0.52Ti0.48O3 (NZFO/PZT) composite.
Multiferroic composite of NZFO and PZT has been prepared
choosing the suitable composition of perovskite PZT in the MPB
region and NZFO using conventional double sintering route. PZT is
a well-known piezoelectric material and the composition Pb(Zr0.52-
Ti0.48)O3 chosen is in the MPB region in which both tetragonal
(P4mm) and monoclinic (Cm) phases coexists showing maximum
piezoelectric response [22]. The transition temperature for PZT
with x = 0.52 is ꢁ380 °C [23]. Nickel zinc ferrite (Ni0.9Zn0.1Fe2O4)
shows ferrimagnetic to paramagnetic phase transition at
TN ꢁ 530 °C [24]. Our study reveals that both the ferrite and ferro-
electric phases get modified during composite formation at high
sintering temperature which in turn may deteriorate the ME re-
sponse of the composite. We observed a systematic variation in
the lattice parameter of the ferrite and ferroelectric phases with
changing their phase fraction in the composite which suggest that
the ions from the two components are diffusing into each other
thereby modifying the lattice parameters. The transition tempera-
ture (TC) of PZT also shows a systematic shift to lower temperatures
for various compositions of the composite and the ferroelectric to
paraelectric transition becomes more diffused with increasing the
ferrite fraction in the composite. To prevent the reaction between
two components and to improve the ME response of such
the flat surface of sintered pellets was gently polished with 0.25 lm diamond paste
for about 2 min and then washed with acetone. Isopropyl alcohol was then applied
for removing the moisture, if any, on the pellet surfaces. Fired on silver paste was
subsequently applied on both the surfaces of the pellets. It was first dried around
120 °C in an oven and then cured by firing at 500 °C for about 5 min. Dielectric mea-
surements were carried out using a Novocontrol, Alpha-A high performance fre-
quency analyzer. For the high temperature dielectric measurements, the
temperature of the sample was controlled by using a Eurotherm programmable
temperature controller with an accuracy of 1 °C. The measurements were carried
out during heating the sample at a rate of 1 °C per min. FULLPROF program [27] was
used for Rietveld refinement of the crystal structure. Pseudo-Voight function was
used to define the peak profiles and six-coefficients polynomial was used to fit
the background. In the spinel cubic phase of space group Fd3m (Space group #
3þ
227), Zn2+ and FeI3þ ions occupy the tetrahedral 8(a) sites at (1/8, 1/8, 1/8), FeII
and Ni2+ ions occupy octahedral sites 16(d) at (1/2, 1/2, 1/2) and O2ꢀ ions occupy
32(e) sites at (ꢀx + 1/4, ꢀx + 1/4, x) as listed in the ‘‘International Table for Crystal-
lography, vol. A (2005)’’. In the tetragonal phase with P4mm space group, the Pb2+
ion occupies 1(a) sites at (0, 0, z), Ti4+/Zr4+ and O2I ꢀ occupy 1(b) sites at (1/2, 1/2, z),
2ꢀ
and O occupy 2(c) sites at (1/2, 0, z). In the monoclinic phase with space group
II
Cm, Pb2+, Ti4+/Zr4+ and O2I ꢀ occupy 2(a) sites at (x, 0, z) and O2ꢀ occupy 4(b) sites
II
at (x, y, z). The microstructure of the sintered pellets was studied using scanning
electron microscope (SEM) (ZEISS SUPRA 40). Before the microstructural study, sin-
tered pellets were sputter coated with Pd/Au alloy.
3. Results and discussion
3.1. Crystal structure
The XRD patterns of calcined powders of NZFO and PZT are
shown in Fig. 1(a) and (b), respectively. All the peaks shown in
Fig. 1(a) are indexed with the cubic spinel structure with space
group Fd3m and no impurity phase is present. Similarly, for PZT
all the peaks shown in Fig. 1(b) correspond to perovskite structure.
The indices shown on the peaks in Fig. 1(b) correspond to pseudoc-
ubic cell. For this composition of PZT, both the tetragonal (P4mm)
and monoclinic (Cm) phases are reported to coexist [22] which is
evident from the broad triplet character of the (200) pseudocubic
profile. The Rietveld refinement of the structure by us confirms the
coexistence of the monoclinic and tetragonal structures. Fig. 2
shows the XRD profile for different compositions of NZFO/PZT
composites prepared by sintering the mixture of the calcined pow-
ders of NZFO and PZT in different proportions. Peaks marked with
‘‘p’’ and ‘‘n’’ denote the reflections corresponding to PZT and NZFO
phases, respectively. We can see that as we increase the fraction of
NZFO, the intensity of peaks corresponding to PZT decreases while
the peak intensity corresponding to NZFO increases. For all the

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R. Pandey et al. / Journal of Alloys and Compounds 593 (2014) 224–229
(a)
(b)
Fig. 1. Powder XRD pattern of (a) NZFO, and (b) PZT ceramics calcined at 800 °C for
6 h.
patterns shown in Fig. 2, except the peaks corresponding to the PZT
and NZFO, there is no additional peak. This suggests that during
composite formation any new phase due to possible reactions be-
tween the two components is not present. Phase fractions of the
NZFO and PZT were calculated using the intensity of the most in-
tense peak of the XRD pattern of the two phases using Eqs. (1)
and (2),
Fig. 3. Variation of phase fractions (%) for (a) NZFO (PNZFO), and (b) PZT (PPZT) in
xNZFO/(1 ꢀ x) PZT particulate composite.
Fig. 4. observed (dots), calculated (continuous line), and difference (bottom line)
profiles obtained after the Rietveld refinement of the structure of 0.60NZFO/0.40PZT
particulate composite sintered at 1150 °C using coexisting [Cubic (Fd3m) + Tetrag-
onal (P4mm) + Monoclinic (Cm)] phases. The vertical tick marks above the
difference plot show the positions of the Bragg-peaks.
I
ð311ÞNZFO
Phase fraction of NZFO ¼
Phase fraction of PZT ¼
ð1Þ
ð2Þ
ðIð311ÞNZFO þ Ið110ÞPZTÞ
I
ð110ÞPZT
ðIð311ÞNZFO þ Ið110ÞPZTÞ
As shown in Fig. 3(a) and (b), we obtained good agreement be-
tween this calculated phase fractions (%) with weight fraction of
the NZFO and PZT used to prepare various compositions. This again
suggests that the particulate composite formation during sintering
of the mixture is not leading to any new phase.
We could successfully refine the structure of the various com-
positions of the composites using Rietveld method. The Rietveld
fit for the XRD pattern of NZFO/PZT composite with x = 0.60 is
shown in Fig. 4. Using the cubic space group Fd3m for NZFO, and
coexisting tetragonal (P4mm) and monoclinic (Cm) space groups
for PZT, very good fit between observed and calculated profiles is
obtained. The refined structural parameters for pure NZFO, PZT
Fig. 2. Powder XRD profile of xNZFO/(1 ꢀ x)PZT particulate composites for the
composition with x = 0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.90 and 1.0 sintered at 1150 °C
for 6 h.

R. Pandey et al. / Journal of Alloys and Compounds 593 (2014) 224–229
227
Table 1
Refined structural parameters and agreement factors for pure NZFO, PZT and 0.60NZFO/0.40PZT composite sintered at 1150 °C using Cubic (Fd3m), Tetragonal (P4mm) and
Monoclinic (Cm) phases.
PbZr0.52Ti0.48O3 (P4mm)
& (Cm)
Ni0.9Zn0.1Fe2O4 (Fd3m)
Pure
Zn/FeI: x = y = z = 0.125, Biso = 0.59(7) Å2
FeII/Ni: x = y = z = 0.50, Biso = 0.54(6) Å2
O: x = y = z = 0.2546(3), Biso = 1.0(1) Å2
a = b = c = 8.3462(2) Å, v2=2.12
Pb: x = y = z = 0, b11 = b22 = 0.0310(3) Å2, b33 = 0.027(3) Å2
Zr/Ti: x = y = 0.50, z = 0.447(2), Biso = 0.005(2) Å2
OI: x = y = 0, z = ꢀ0.109(6), Biso = 0.029(1) Å2
Pb:x = y = z = 0, b11 = b22 = 0.0310(3) Å2
Zr/Ti:x = 0.578(3), y = 0, z = 0.473(3), Biso = 0.0147(3) Å2
OI:x = 0.50(1), y = 0, z = ꢀ0.10(1), Biso = 0.01(1) Å2
OII:x = 0.36(1), y = 0.219(8), z = 0.404(8), Biso = 0.04(1) Å2
a = 5.752(1) Å, b = 5.743(2) Å, c = 4.091(4) Å, b = 90.48(1)°
OII: x = 0.5, y = 0.0, z = 0.389(3), Biso = 0.029(1) Å2
a = b = 4.0429(2) Å, c = 4.1318(3) Å, v2 = 1.32
Composite
Zn/FeI: x = y = z = 0.125, Biso = 2.2 (2)Å2
FeII/Ni: x = y = z = 0.50, Biso = 2.6(2) Å2
O: x = y = z = 0.2563(6), Biso = 3.0(3)Å2
a = b = c = 8.3552(2) Å
Pb:x = y = z = 0, b11 = b22 = 0.1204(2)Å2, b33 = 0 Å2
Zr/Ti:x = y = 0.50, z = 0.5039(2), Biso = 1.23(1) Å2
OI:x = y = 0, z = 0.1144(7), Biso = 0.02 Å2
OII:x = 0.5, y = 0, z = 0.3933(1), Biso = 0.03 Å2
a = b = 4.0663(4) Å, c = 4.1001(3) Å
Pb:x = y = z = 0, b11 = b22 = 0.0310(3) Å2, b33 = 0.027(3) Å2
Zr/Ti:x = 0.4515(1), y = 0, z = 0.5281(7), Biso = 0.3413(1) Å2
OI: x = 0.5924(3), y = 0, z = 0.0523(7), Biso = 0.02 Å2
OII:x = 0.312(5), y =0.2137(1), z = 0.3649(2), Biso = 0.03 Å2
a = 5.657(3) Å, b = 5.744(4) Å, c = 4.112(3) Å, b = 90.92(3)°
v
2 = 4.67
(a)
(b)
Fig. 7. Room temperature permittivity (e0) of xNZFO/(1 ꢀ x)PZT particulate com-
posite for the compositions with x = 0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.90 and 1.0
measured in the frequency range 103–106 Hz.
Fig. 5. variation of the (a) lattice parameter of NZFO in the term of doped PZT (%) in
NZFO, and (b) lattice parameters and tetragonality of PZT (tetragonal phase) in the
term of doped NZFO (%) in PZT, in xNZFO/(1 ꢀ x)PZT particulate composite.
Fig. 8. Temperature variation of permittivity (e0) of xNZFO/(1 ꢀ x)PZT particulate
composite for the composition with x = 0.60 measured in the frequency range 5–
102 kHz .
thermal parameters (b) for these ions in tetragonal and monoclinic
structures. For the cubic phase, Biso were used for all the ions. The
refined structural parameters for pure PZT are in good agreement
with that reported by Ragini et al. [22].
Fig. 6. SEM image of the sintered pellet surface of xNZFO/(1 ꢀ x)PZT particulate
composite for the composition with x = 0.60 sintered at 1150 °C for 6 h.
Variation of lattice parameter of NZFO and tetragonal phase of
PZT obtained after Rietveld refinement of the structures for various
compositions of composite is shown in Fig. 5(a) and (b) respec-
tively. As can be seen from Fig. 5(a) the lattice parameter of NZFO
and NZFO/PZT composite with x = 0.60 are listed in Table 1. The
isotropic thermal parameters (Biso) for the Pb2+ ion was found to
be very high (>3 Å2). In view of this, we considered anisotropic

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R. Pandey et al. / Journal of Alloys and Compounds 593 (2014) 224–229
systematically increases with increasing the fraction of PZT in the
composite. Similarly the ‘a’ and ‘c’ parameters of the tetragonal
phase of PZT come closer to each other with increasing the fraction
of the NZFO in the composite. The tetragonality (c/a) continuously
decreases with increasing the fraction of NZFO and structure be-
comes pseudocubic for x = 0.85. These modifications in the lattice
parameters of NZFO and PZT are not possible unless the ions from
the two components diffuse to each other. Thus the solid state sin-
tering route of the composite formation is not leading to the ideal
composite where the two components should not react with each
other [28]. Most important impact of the modification in the lattice
parameter will correspond to the ferroelectric phase where re-
duced tetragonality will decrease the magnitude of the polariza-
tion of the unit cell which in turn will reduce the piezoelectric
response of the ferroelectric phase. Since the ME effect in particu-
late composite results from the coupling between polarization and
magnetization mediated by strain, the reduced piezoelectric re-
sponse will decrease the value of ME-coefficient also.
Fig. 10. Variation of full width at half maximum (FWHM) of the permittivity (e0
)
peaks at 10 kHz for the compositions with x = 0, 0.15, 0.30, 0.45 and 0.60 of xNZFO/
(1 ꢀ x)PZT particulate composites.
3.2. Microstructure studies
The SEM image for NZFO/PZT composite with x = 0.60 is shown
in Fig. 6. Appearance of two types of morphology could be seen
clearly in the SEM image confirming the formation of composite.
The smaller grains with elongated oval shape are identified as
NZFO. The rock shaped bigger grains correspond to PZT. The aver-
age grain size for NZFO and PZT is calculated to be ꢁ0.09
lm and
1.0 m, respectively. As can be seen from the micrograph, there
l
is uniform distribution of the two components.
3.3. Dielectric studies
The room temperature permittivity (e0) of the NZFO/PZT com-
posite in the frequency range of 103–106 Hz for different composite
compositions is shown in Fig. 7. It is evident from Fig. 7 that with
increasing the amount of NZFO, the permittivity decreases. This is
due to decreasing the fraction of the ferroelectric phase PZT. An-
other reason for decrease in the dielectric constant with increasing
ferrite content may be attributed to the Verwey type electron ex-
change polarization [29].
Fig. 8 shows the temperature variation of permittivity for NZFO/
PZT composite with x = 0.60 at various frequencies. With
increasing frequency, the peak in the temperature variation of
the permittivity shifts to the higher temperature side along with
the significant frequency dispersion. This suggests the relaxor
nature of the ferroelectric phase transition in PZT after composite
Fig. 11. Variation of the transition temperature (TC) for the different compositions
of xNZFO/(1 ꢀ x)PZT particulate composite with x = 0, 0.15, 0.30, 0.45 and 0.60 at
1 kHz.
formation. However, the pure PZT behaves like a normal ferroelec-
tric and does not exhibit relaxor features [30]. The appearance of
relaxor features must be linked with the modification of the PZT
phase by diffusion of ions from NZFO during composite formation.
It is well known that the relaxor nature of ferroelectric phase tran-
sition is associated with the local ionic size/charge disorder in
perovskite solid solutions [31]. When disorder increases above a
certain level the average crystal symmetry becomes cubic as long
range ferroelectric order could not be established, which will
otherwise be a non-centrosymmetric structure. This is evident in
Fig. 10 where increasing the NZFO concentration in PZT increases
the relaxor behaviour while the tetragonality continuously de-
creases in favour of pseudocubic structure (see Figs. 2 and 5(b)).
Relaxor nature of ferroelectric phase transition in the NZFO/PZT
composite must be resulting from the introduction of more disor-
der at the B-site cations in the ABO3 perovskite structure due to
diffusion of Ni2+/Zn2+/Fe3+ ions from the NZFO to the Zr4+/Ti4+ site.
This may lead to the Maxwell–Wagner type interfacial polarization
[32] as discussed in Koop’s phenomenological theory [33]. Dielec-
tric relaxation time (s) was modeled by using Arrhenius and
Vogel–Fulcher equations. However, the linear fit was observed
for Vogel–Fulcher type of freezing. The activation energy and relax-
Fig. 9. Temperature variation of permittivity (e0) of xNZFO/(1 ꢀ x)PZT particulate
composite at 10 kHz for the compositions with x = 0, 0.15, 0.30, 0.45 and 0.60.
ation time were obtained to be Ea ꢁ 0.19 eV and
s
ꢁ 1.38 ꢃ 10ꢀ11 s,

R. Pandey et al. / Journal of Alloys and Compounds 593 (2014) 224–229
229
respectively with corresponding Vogel–Fulcher freezing tempera-
ture Tvf ꢁ 220 °C for x = 0.60.
ferroelectric phase and consequently better ME-coefficient in com-
posite, a composition with higher tetragonality may be chosen, the
structure of which latter may get modified to MPB phase. The tran-
sition temperature of the ferroelectric phase (PZT) also decreases
significantly after composites formation and the nature of the
phase transition becomes relaxor like. The SEM studies reveal the
two types of particle morphology in the particulate composite, cor-
responding to ferrite and ferroelectric phases.
Fig. 9 shows the temperature variation of permittivity at a fre-
quency of 10 kHz for the NZFO/PZT composite with x = 0, 0.15,
0.30, 0.45 and 0.60. For pure PZT with x = 0 the permittivity shows
a peak at 380 °C which is in well agreement with the value of TC
reported in literature [23] for Pb(Zr0.52Ti0.48)O3. With increasing
the value of ‘x’ i.e. the NZFO fraction the permittivity peak system-
atically shifts to the lower temperature side and the nature of
phase transition becomes very diffused. This suggests that the
relaxor feature of the phase transition become more pronounced
with increasing NZFO concentration in the composite. As discussed
earlier, the appearance of relaxor features is linked with the diffu-
sion of Ni2+/Zn2+/Fe3+ ions from NZFO to the Zr4+/Ti4+ site of the
PZT. It is expected that at higher ‘x’ more Ni2+/Zn2+/Fe3+ ions are
available to diffuse to the Zr4+/Ti4+ site of the PZT which in turn
strengthen the relaxor nature of the ferroelectric phase transition.
To elaborate the strengthening of the relaxor character with
increasing NZFO concentration in the composite, we have plotted
the full width at half maximum (FWHM) of the permittivity peaks
at 10 kHz for various compositions in Fig. 10. It is evident from
Fig. 10 that the FWHM increases continuously with increasing
NZFO concentration, indicating enhanced diffuseness of the ferro-
electric phase transition. Fig. 11 shows the composition depen-
dence of TC determined at 1 kHz frequency for NZFO/PZT
composite. As compared to pure PZT with TC = 380 °C, the TC
decreases drastically for x = 0.15 (ꢁ350 °C) and then decreases
linearly with increasing NZFO concentration. The significant
change in TC of PZT after composite formation clearly suggests that
the ferrite and ferroelectric phases react with each other. This is in
contrast to the proposition of the Chougule and Chougule [34] that
the two phases do not react.
Acknowledgements
RP acknowledges University Grant Commission (UGC), India for
financial support. Authors are thankful to Professor Dhananjai Pan-
dey, School of Materials Science and Technology, IIT (BHU) Varana-
si, India for extending laboratory facilities and support.
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4. Conclusions
The dielectric and structural characterization of NZFO/PZT
composites by us reveals that at the XRD level no secondary phases
appear after composites formation. However, the lattice parame-
ters of both the ferrite and ferroelectric phases are modified
suggesting that the ions from the two phases diffuse to each other
during sintering. The tetragonality of ferroelectric phase
continuously decreases with increasing the ferrite fraction in the
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