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Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite
Accepted Manuscript Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite Rubiya Samad, Mehraj ud Din Rather, Basharat Want PII:
S0925-8388(17)31456-1
DOI:
10.1016/j.jallcom.2017.04.246
Reference:
JALCOM 41650
To appear in:
Journal of Alloys and Compounds
Received Date: 13 February 2017 Revised Date:
19 April 2017
Accepted Date: 22 April 2017
Please cite this article as: R. Samad, M.u.D. Rather, B. Want, Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.246. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite Rubiya Samad, Mehraj ud Din Rather, Basharat Want1. Solid State Research Lab, Department of Physics, University of Kashmir, Srinagar-190006, India
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ABSTRACT
We investigate the structural, dielectric, ferroelectric and magnetic properties of a ferroelectric-ferrite material: praseodymium doped lead zirconium titanate (PZT) and cobalt ferrite (CFO) composite. The praseodymium doped PZT and CFO powders were synthesized
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separately by sol-gel auto combustion technique. The ferroelectric-ferrite composites were then synthesized by mixing the appropriate amount of individual phases using conventional
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sintering method.The morphological studies revealed a uniform distribution of grains of parent phases in the composite. Powder X-ray diffraction studies showed that the Pr doped PZT possess tetragonal perovskite structure while as Pr doped CFO possesses cubic spinel structure. The parent phases retain their crystal structures in the composite, without formation of any new secondary phase.The dielectric studies of composite, in the temperature range 100-750 K at a selected frequency of 100 kHz, revealed a diffused ferroelectric phase
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transition. Variation of dielectric constant with frequency in the range of 20 Hz- 3 MHz was carried out at room temperature. The reduction in the leakage current due to Pr doping leads to typical saturated P-E hysteresis loops for the composite, confirming their ferroelectric nature. The low temperature ac conductivity follows Motts law, confirming the hopping
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conduction mechanism in the composites. At room temperature, the composite exhibit typical magnetic hysteresis loop, indicating the presence of a long range magnetic order. The
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calculated values of squareness ratio for the composites confirmed the presence of multidomain structure.
Keywords: Ferroelectrics, sol gel processes, conduction behaviour, phase transition, Motts law.
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Corresponding author. Tel: +91-194-2420078; fax: +91-194-2421357 E-mail address: [email protected]
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ACCEPTED MANUSCRIPT 1. Introduction A material is said to be functional when its one or more properties are sensitive to external environmental variables such as temperature, electric or magnetic fields [1]. Among the functional materials, multiferroics became attractive from technological point of view due to their magneto-electric (ME) effects [2-3]. ME effect makes these materials best candidate for
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sensors, electrical field tunable devices, voltage converters etc. [4-6]. Keeping in view these technological applications, the prime focus of the scientific community has been on the multiferroic materials with large magneto-electric effects. However, the difficulty in synthesis of single-phase multiferroics under ambient conditions with strong room
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temperature polarization and magnetoelectric effects have motivated researchers to switch for ME composite materials [7]. In composite materials the ME effect is activated by the mechanical strain transfer at the interfaces between the two ferroic phases. In addition, each
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ferroic phase retains its unique property separately in the ME composite [8]. The most widely studied composites are made with PZT, BaTiO3 and PVDF as ferroelectric phase and NiFe2O4, CoFe2O4, MnFe2O4, ZnFe2O4, terfenol as ferrite phase [9-15]. Keeping in view the excellent ferroelectric and piezoelectric properties of PZT and magneto-crystalline anisotropy, high coercivity, moderate saturation magnetization of CFO, the current research
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on (PZT–CFO) composites is going on at an increasing rate [16–20].One of the main issue with a PZT–ferrite composite material is its unsaturated ferroelectric loop with low polarization values, generally due to leakage currents [21-23]. Therefore, in order to enhance the dielectric and polarization nature and to minimize leakage currents in (PZT–CFO)
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composites, praseodymium (Pr) was doped both in the ferroelectric PZT and ferromagnetic CFO parent phases in the present work. Further, a literature survey also reveals that
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praseodymium (Pr) doped PZT-CoFe2O4 multiferroic composite prepared by sol gel method has not been reported so far. In this paper, we will present important results of enhancing ferroelectric and dielectric character of praseodymium doped PZT-CFO composites.
2. Experimental procedure: Raw materials such as lead nitrate, zirconium isopropoxide, titanium isopropoxide, and praseodymium nitrate with ethylene glycol, nitric acid and acetylacetone (as solvent) respectively, were used for the synthesis of Pb0.95Pr0.05Zr0.52Ti0.48O3 (Pr-PZT) via autocombustion route. In a similar way ferric nitrate, cobalt nitrate, praseodymium nitrate and glycine were used as raw materials in appropriate molar ratios for the synthesis of CoPr0.1Fe1.9O4 (CPrFO) by auto combustion method. Both the precursors obtained from their 2
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ACCEPTED MANUSCRIPT respective solutions were kept at 80 °C under continuous stirring until a viscous gel was formed. This viscous gel was preceded to self-ignition, followed by combustion. The residue left after combustion was collected and grounded in an agate-mortar to form as-synthesized individual powders of Pr-PZT and CPrFO. The synthesized Pr-PZT and CPrFO powders were calcinated at 650 °C for 1h and 750 °C for 5 h, respectively. The calcinated Pr-PZT and
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CPrFO powders were then sintered at 1250°C for 2h and 950 °C for 7 hrs respectively. The composites of (1-x) Pb0.95Pr0.05Zr0.52Ti0.48O3 – (x) CoPr0.1Fe1.9O4 (where x=0.02, 0.05, 0.1, denoted asPPP2, PPP5, PPP10 respectively) were prepared by mixing the two phases i.e. ferroelectric and ferrite phases by proportion of their weights. These composites were then pressed into circular pellets using (3-5 wt %) polyvinyl alcohol as a binder. The pellets were
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sintered at 800oC for 6 hrs with constant heating and cooling rate of 5oC/min. After sintering the XRD data of these composites were collected on
D8 Advance Bruker X-ray
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diffractometer with CuKα (λ=1.5406 Å) radiation. A Scanning electron microscope ( JEOLJSM-6490 LV) was used to study the in-depth external morphology of the prepared samples. The dielectric measurements were carried out in the frequency range of 20 Hz - 3 MHz and over a temperature range 100K– 750K using Agilent 4284A precision LCR meter. Electrodes were made out of the high grade silver paint, which was applied on both sides of the pellet to
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make it a disc shaped parallel plate capacitor with the material as a dielectric medium. The
LCR meter used in the present investigation directly provides the values of capacitance C and dielectric loss. Other parameters such as dielectric constant ε´ and ac conductivity, σ are computed using the relations:
and = 2
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=
--------------------------------------- (1)
where C is capacitance (in Farad), t is thickness (in metres), A is area (in m2), εo = 8.854 × 10−12
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Fm−1, f is the frequency (in Hz) of the applied electric field and the ac conductivity (in Ω-1m-1).
The P-E hysteresis loops were traced by using PE Loop Tracer, Radiant Technologies–Inc. Silver paint was used as electrode and was deposited on both sides of pellets for the dielectric and ferroelectric measurements. The magnetic measurements were carried out on a vibrating sample magnetometer (Micro Sense EZ9 VSM, USA).
3. Results and discussion 3.1. SEM and EDS Analysis: Typical SEM micrographs of (1-x) Pr PZT- (x) CPrFO (x= 0, 0.02, 0.05 and 0.10) composites are shown in Fig. 1 at a magnification of 10,000X. It is observed from the micrographs that 3
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ACCEPTED MANUSCRIPT the ceramics possess fine crystalline structures. The grains in the pure Pr-PZT are compact as compared to composite samples due to its higher density (8.7g/cm3). It is observed from Fig. 1 that ferrite grains are almost uniformly distributed in the ferroelectric matrix. The Pr-PZT and cobalt ferrite grains retain tetragonality and cubic structure respectively in the composite. The EDS micrographs (Fig. 2) show that all the elements (Pr, Pb, Zr, Ti, Fe, Co, O) are
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present in the composite. In addition, the intensity value of Pr peaks increase from PPP2 to
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PP10, which is in accordance with the molar percentage taken during synthesis.
Fig 1. Typical SEM micrographs of (a) Pr-PZT (b) PPP-2 (c) PPP-5 (d) PPP-10
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Fig 2. EDS spectra of samples sintered at 950 oC (a) Pr-PZT (b) PPP-2 (c) PPP-5 (d) PPP-10
3.2 Powder X-ray diffraction results
Fig.3 shows the XRD patterns of (1-x) PrPZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1 and 1)
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composites. The peaks corresponding to Pr-PZT and CPrFO were indexed with the tetragonal structure of PZT(JCPDS Card no. 33-0784) and cubic spinel structure of CFO (JCPDS Card no. 77-0426) respectively. For composites, two well-defined sets of diffraction peaks characterize the
constituent phases. No reflections other than those belonging to a cubic spinel structure of the
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ferrite phase and tetragonal perovskite structure of the ferroelectric phase were observed, suggesting the absence of any chemical reaction between the constituent phases.No structural
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changes have occurred with varying molar percentages of the constituent phases. Only the intensity of the ferrite-phase peaks increases with its content in the composites. The intensity of the spinel ferrite peak at (2θ=35o) increases while the intensity of the perovskite ferroelectric peak at (2θ=30o) decreases with increasing ferrite content. This confirms that intensity of XRD peaks in composites is highly dependent on the quantity of the individual ferroic phases [24]. The approximate amount of ferrite phase present in the composites, after final sintering, was calculated by using the following formula [25].
% ℎ = × 100 -------------------------------------------(2)
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ACCEPTED MANUSCRIPT where IF and IFE corresponds to high intensity peaks of ferrite and ferroelectric phases in the composite. The percentage of the ferrite content calculated was in good agreement with the molar percentage used for the synthesis as depicted in Table 1.The densities of all the samples are also given in Table 1. Table 1.Percentage of ferrite content in PPP-2, PPP-5 and PPP-10 and their densities. Density (g/cm3)
High intensity
High intensity
peak of Pr-PZT
peak of CPrFO
phase (A.U)
phase (A.U)
% of ferrite
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Sample
8.74
-
-
PPP-2
6.49
91
3297
PPP-5
6.2
200
4519
4.42
PPP-10
6.64
210
1995
10.52
-
2.76
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Pr PZT
Fig.3: X-Ray diffraction patterns of (1-x) Pr PZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1 and 1). 3.3. Dielectric studies
The variation of real part of the complex permittivity (ε′) with temperature (100 K – 750K) for Pr-PZT, PPP-2, PPP-5 and PPP-10 at 100kHz is shown in Fig.4. It is observed that the dielectric constant increases with temperature and attains a maximum value at around 585 K followed by the decrease in the value of dielectric constant with the further increase in temperature. The mobility of charge carriers increases with increasing temperature, which leads to an increase in the conductivity and polarization of the sample and hence an increase 6
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ACCEPTED MANUSCRIPT in the dielectric constant. The increase in ε′ with increasing temperature can also be explained due to electron/hole exchange in CPrFO, acting as one of the ferroic phases of the composite [26]. The electron/hole hopping is a temperature dependent process, which results in an increase of ε' with temperature. It is clear from Fig.4 that a dielectric peak is observed at temperature 585 K for all the samples, therefore, confirming a transition from ferroelectric to
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paraelectric phase. At transition temperature Tc, the value of dielectric constant is observed to be high for all the samples. At this temperature the domain motion is a dominant factor in the dielectric response of the material as compared to that at lower temperatures. Above Tc, the electrons/holes have random vibrational motion and the ferroelectric phase is transformed to
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the paraelectric phase, resulting in decrease of dielectric constant [27]. The values of transition temperatures and their corresponding maximum dielectric constant are given in Table 2. It is observed from Table 2 that Tc shifts to higher temperatures (590 K) for PPP-10
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which is attributed to maximum electron/hole exchange.
Fig.4: Variation of Dielectric constant with temperature for Pr-PZT, PPP-2, PPP-5 and PPP10. It is also observed that the dielectric peak is broadened in all the samples, which shows that the ferroelectric phase transition is a diffused one. This diffuseness in the composites was successfully examined by the following type of modified Curie-Weiss law.
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′
−
!
′ #
%$= &'( − () *+ ---------------------------------------------------------------(3)
′ is the peak value of the dielectric constant and / is a critical exponent which lies in here ,-.
the range 1< / ≤ 2; / = 1 represents the ideal Curie-Weiss behaviour while a value of /
between 1and 2 indicates diffuse behaviour [28]. The values of /, given in Table 2, were !
#
%$4versus 2'( − () * plots as shown in Fig. 5. The
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obtained from the slope of 2 3′ − ′
results confirm that ferroelectric phase transition observed in the present composites is of diffused type. This diffuseness may be due to the compositional fluctuations and structural disordering in the composite [29-30]. A microscopic in-homogeneity in the material under
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study with different local Curie points may also be the reason for this diffuseness. Such a
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type of behavior has also been found in many ceramic materials [31-32].
!
Fig.5. Plot of 2 3′ − ′
!
#
%$4 versus 2'( − () *for Pr-PZT, PPP-2, PPP-5 and PPP-10
The variation of dielectric loss (tan δ) with temperature for Pr-PZT, PPP-2, PPP-5 and PPP10 composites at 100 kHz is shown in Fig 6. The behaviour of tanδ with temperature is similar to that of the variation of the ε' with temperature as reported by other researchers [33]. 8
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increase in the ferrite content which is attributed to high conducting nature of CPrFO grains.
Fig.6: Temperature dependence of dielectric loss (tan δ) for Pr-PZT, PPP-2, PPP-5 and PPP-
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10. The inset shows variation of tan δ with temperature for CPrFO
Table.2: Dielectric parameters of (1-x) Pr PZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1, 1) composites.
,-. '() *
n
Pr PZT 587.6
2614.85
0.778
1.46
PPP 2
575.2
1155.8
0.831
1.55
PPP 5
580.6
1414.44
0.734
1.16
PPP 10 587.4
1168.92
0.547
1.44
CPrFO
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0.216
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Sample Tc(K)
-
/
The variation of dielectric constant with frequency in the range of 20 Hz to 3 MHz at room temperature for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO is shown in Fig 7.As observed from Fig.7 the value of dielectric constant decreases with the increase in frequency. Variation of dielectric constant with frequency can be explained on the basis of polarization 9
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ACCEPTED MANUSCRIPT mechanisms. Polarization may be separated into three parts: dipolar, ionic and electronic. For the case of multiferroic ceramic compounds, there is another type of polarization: interfacial or space charge polarization that may originate in the materials from the point defects during sintering. At low frequency, all the types of polarization are present, but major contribution to ε ′ comes from space charge polarization [34]. The space charges are able to follow the
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variation of the applied electric field at low frequency; however, at high frequencies they cannot follow the variation of field frequency. Thus contribution of space charges at high frequency towards dielectric constant decreases. Moreover, the dipolar and the ionic contributions are also small at higher frequency due to the inertia of the molecules and ions
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[35]. In the case of the composites, the high value of the dielectric constant can also be explained on the basis of the fact that the ferroelectric regions (Pr PZT) are surrounded by non- ferroelectric regions (CPrFO); a situation that is similar to relaxor ferroelectrics [36-37].
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to the low resistivity of ferrite phase.
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The decrease of dielectric constant with increase in ferrite phase (shown in Fig.8) is attributed
Fig.7: Variation of dielectric constant (ε') with frequency for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO The variation of dielectric loss with frequency in the range of 20 Hz to 3 MHz at room temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10 is shown in Fig 8.The dielectric loss 10
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ACCEPTED MANUSCRIPT arises mainly due to impurities and imperfections in the crystal lattice. It is observed from Fig. 8 that the dielectric loss decreases rapidly in the low frequency region, while the rate of decrease is slow in the middle region and then at high frequency it becomes almost frequency independent. Such behaviour for ferrites and composites in low frequency region is due to low conductivity of grain boundaries. Due to the lower conductivity, more energy is required
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for electron to exchange between Fe2+ and Fe3+ ions; resulting in higher values of tan δ. In high frequency region, which corresponds to high conductivity of grains, a small energy is required for electron transfer between the Fe2+/Fe3+ ions at the octahedral site [38-39]. In comparison to CPrFO, the losses observed for Pr-PZT and composites are negligible and
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hence are shown in inset of Fig 8. It is observed from Fig.8 that pure CPrFO and composites exhibit a loss peak in accordance with Debye type of relaxation. The electric dipoles cannot follow the alternating field instantaneously; some time is needed for the alignment of dipoles
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with the field [35]. This peak is observed in ferrite materials when the jumping/hopping frequency of electrons/holes among ions becomes equal to the frequency of the applied field, i.e., maximum electric energy is transferred to the electrons and the loss shoots up at resonance [40]. Further the loss peaks in composites becomes more prominent as the ferrite
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content increases as depicted in inset of Fig.8.
Fig.8: Frequency dependence of dielectric loss for CPrFO. The inset shows variation of tan δ with frequency for Pr-PZT, PPP-2, PPP-5 and PPP-10
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observed that conductivity remains constant up to a certain frequency. This frequencyindependent conductivity is attributed to the band conduction [41]. The increase in conductivity with increasing frequency is attributed to the small-range polaron hopping mechanism [42]. As the frequency increases the conductivity increases abruptly. The
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frequency independent conductivity is shifted to frequency-dependent conductivity beyond the hopping frequency, indicating the translation from band conduction to polaron hopping conduction thereby obeying Motts law [42-44].The frequency dependent ac conductivity can
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be expressed by the following empirical formula[45]
σ'ω* = Aω6 ----------------------------------------------------------------------(4)
where ω '= 2πf* is the angular frequency, A is a constant and has units of σac and n is dimensionless. The value of n can be determined from the slope of lnσ versus lnω plots as shown in Fig 10. The value of ‘n’ lies between 0 and 1. If n = 0, the electrical conductivity is
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frequency-independent or dc conductivity (σdc), however for n >0, the conductivity is frequency-dependent [46]. The estimated values of n for Pr-PZT, PPP-2,PPP-5, PPP-10 and CPrFO are almost zero below hopping frequency, however, beyond this frequency a finite values are obtained given in Table 2.These results confirm the hopping conduction
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mechanism.
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shows variation of σac with frequency for CPrFO, PPP-2, PPP-5 and PPP-10.
Fig. 10: Plot of log: versus log for Pr-PZT,PPP-2,PPP-5, PPP-10 and CPrFO. The ac conductivity is thermally activated process, showing weak temperature dependence at
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lower temperatures, while as stronger dependence on temperature at higher temperatures as shown in Fig 11. At higher temperatures, the ac conductivity shows the activated temperature dependence as <=>
σ = A e ?@ ----------------------------------------------------------------------------(5)
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where A is constant, Ea is the activation energy, which is the energy needed to release an electron from the ion for a jump to neighbouring ion, so giving rise to the electrical
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conductivity, k is Boltzmann’s constant, and T is the absolute temperature.
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Fig.11: Variation of σac with temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10. The activation energy as calculated from the slope of ln(σ) versus 1/T (Fig. 12) shows an increasing trend with temperature. The activation energy in ferrites, acting as one of the ferroic phase of the composite, is associated with the variation of mobility of the charge carriers between Fe2+ and Fe3+ rather than with their concentration. Since the charge carriers are localized in the ions or vacant sites, the conduction occurs via a hopping process which is
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temperature dependent. For the ferrites having high conductivity, the activation energy is of the order of 0.1 eV, however for insulating ferrites is of the order of 0.4 eV [47]. In addition to Pr-PZT, the values of activation energies in composites for higher temperature region (275- 700 K) are of the order of 0.1eV, however a value of 0.01eV is observed in lower
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temperature region(100-275 K) given in fig.12. Further, from the plots of ln (σ) versus (1/T)1/4 in the lower temperature region (100-220 K) depicted in Fig.13, it is clear that Pr-
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PZT as well as composite obeys Motts law [43] of the form ( !/F = A B− C D G (
where T0 is a constant proportional to the density of states at the Fermi level N(EF), kB is the Boltzmann constant. Both the studies confirm that the composites are conductive and the conduction is due to hopping process, which in turn is temperature dependent.
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Fig.12: Plot of log versus 1000/T for Pr-PZT, PPP-2, PPP-5 and PPP-10.
Fig.13: Plot of log versus T-1/4for Pr-PZT, PPP-2,PPP-5 and PPP-10. 3.5.Ferroelectric Studies: The plots of polarization (P) versus electric field (E) for Pr-PZT, PPP-2, PPP-5 and PPP-10 are shown in Fig 14. The loops indicate ferroelectric behavior of Pr-PZT as well as that of the 15
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ACCEPTED MANUSCRIPT composites. It is observed that the values of saturation polarization, remanence polarization and coercive field for pure Pr-PZT are higher as compared to that of composites ( see Table 3). It is because the ferrite phase, mixed into ferroelectric phase, acts as pores in the presence of applied electric field and breaks the electric circuit, resulting in the decrease of various ferroelectric parameters [48]. Also the decrease in electrical parameters in the composite is
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believed to be due to the conducting nature of CPrFO. Due to high dielectric constant and large polarization values of Pr-PZT, the PrPZT-CPrFO composite prepared in the present work shows completely saturated ferroelectric loops with higher polarization values as compared to already reported elliptical/banana loops for PZT-ferrite composites [21-23]. It is
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further observed that a 10% Pr doping in CFO (ferrite phase) increases its electrical resistivity as compared to pure CFO, thereby, minimizing leakage currents and preventing the composites from electric breakdown during poling of their ferroelectric phase. This shows
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that the presence of Pr in CFO and PZT has a considerable effect on the ferroelectric properties of the composites. Further, the PPP-10 has significant saturation polarization, remenant polarization as well as high dielectric constant due to higher density, which makes
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this kind of a material a suitable candidate for technological applications.
Fig.14: The P-E loops at room temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10.
Table 3: Ferroelectric and magnetic parameters for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO 16
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Ps
Pr
Ec(kV/cm) Ms
(µC/cm2
(µC/
Hc
Mr
(emu/gm) (Oe)
(emu/gm)
Mr/Ms
2
cm )
Pr PZT
15.12
11.29
24.56
-
-
-
-
PPP2
4.046
2.27
18.54
0.309
235.85
0.0935
0.30
PPP5
2.903
2.23
15.35
2.54
375.146 0.712
0.28
PPP10
8.005
6.08
29.03
6.9509
787.049 2.833
0.41
CPrFO
-
-
-
71.228
960
0.50
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)
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35.965
3.6. Magnetic properties
To investigate the magnetic nature of (1-x) PrPZT- (x) CPrFO (x= 0.02, 0.05, 0.1 and 1), composites, M-H hysteresis loops were studied.Fig.15 shows the M-H hysteresis loops of PPP-2, PPP-5 and PPP-10 traced at room temperature. The inset of Fig.15 shows M-H loop
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of CPrFO. The various magnetic parameters calculated from the loops are tabulated in Table 3. All the composites exhibit typical magnetic hysteresis loop and get saturated at a field of about 10 kOe. This indicates the presence of an ordered magnetic structure in the mixed cubic
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spinel structure of ferrite.
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Fig.15: Room temperature M-H loops for PPP-2, PPP-5 and PPP-10.The inset shows M-H loop for CPrFO.
The magnetic behaviour of the composites is due to the presence of ferrite phase [49].It is observed from Table 3 that the saturation magnetization of all composites is lower than that
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of the pure CPrFO phase (Ms=71.228 emu/gm). The decrease in the various magnetic parameters is due to presence of non-magnetic Pr-PZT phase, which suppresses the magnetic behaviour. The saturation magnetization (Ms) and remenant magnetization (Mr) show an increasing trend with the increase of ferrite concentration. The increase in saturation
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magnetization with increasing CPrFO content provides an indication that the spontaneous magnetization of the composites, which originates from unbalanced antiparallel spins, leads
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to net spins other than those introduced by the structural distortion [50].The squareness ratio (Mr/Ms) was also calculated for CPrFO as well as for PPP-2, PPP-5 and PPP-10. From these values it is clear that CPrFO has single domain structure, while as the composites exhibit multi-domain structures [51].
Conclusion Synthesis of praseodymium doped PZT and CFO composite phase has been achieved through sol-gel auto combustion technique. The XRD and morphological studies reveal composite formation of parent ferro-ferrite phases with uniform distribution of grains of each phase. The individual phases retain their structure in the composite, without formation of any new
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loops and saturated magnetic hysteresis loop has been demonstrated for ferrite-ferroelectric composite. Our results thus demonstrate multiferroic character with improvement in dielectric and ferroelectric properties and deduction in leak current with Pr doping. The squareness ratio confirmed the presence of multidomain structure of the composite. It may be
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concluded that the 10% ferrite content is a substantial amount to maintain its high dielectric constant, high saturation polarization and high saturation magnetization in Pr-PZT–CPrFO
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composites.
Acknowledgement
One of the authors Rubiya Samad highly acknowledges Department of Science and Technology, Government of India for financial support vide reference no. SR/WOS-A/PM1006/2015 under Women Scientist Scheme to carry out this work. The authors are thankful
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to the authorities of University of Kashmir for providing the vibrating sample magnetometer facility (MicroSense EZ9 VSM) to the Department of Physics under DST Govt. of India special package for sophisticated instrumentation. Authors would also like to thank Centre Director, UGC DAE CSR Indore for XRD and PE-Loop measurements.
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[51] P. N. Dhruv, , N. Solanki, , S. Kulkarni, , and R. B. Jotania,,, Effect of sintering temperature and vinca petals extract on structural and magnetic properties of delafossite
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CuFeO2, 1728,(2016) 020074; doi: 10.1063/1.4946125.
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ACCEPTED MANUSCRIPT Highlights
The composite exhibits saturated electric hysteresis loops without leakage current. At low temperatures, the electrical conductivity is due to hopping mechanism.
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There are ordered magnetic domains and multi-domains in the composite.
S0925-8388(17)31456-1
DOI:
10.1016/j.jallcom.2017.04.246
Reference:
JALCOM 41650
To appear in:
Journal of Alloys and Compounds
Received Date: 13 February 2017 Revised Date:
19 April 2017
Accepted Date: 22 April 2017
Please cite this article as: R. Samad, M.u.D. Rather, B. Want, Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.246. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT Dielectric, ferroelectric and magnetic properties of Pb0.95Pr0.05Zr0.52Ti0.48O3 – CoPr0.1Fe1.9O4 ceramic composite Rubiya Samad, Mehraj ud Din Rather, Basharat Want1. Solid State Research Lab, Department of Physics, University of Kashmir, Srinagar-190006, India
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ABSTRACT
We investigate the structural, dielectric, ferroelectric and magnetic properties of a ferroelectric-ferrite material: praseodymium doped lead zirconium titanate (PZT) and cobalt ferrite (CFO) composite. The praseodymium doped PZT and CFO powders were synthesized
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separately by sol-gel auto combustion technique. The ferroelectric-ferrite composites were then synthesized by mixing the appropriate amount of individual phases using conventional
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sintering method.The morphological studies revealed a uniform distribution of grains of parent phases in the composite. Powder X-ray diffraction studies showed that the Pr doped PZT possess tetragonal perovskite structure while as Pr doped CFO possesses cubic spinel structure. The parent phases retain their crystal structures in the composite, without formation of any new secondary phase.The dielectric studies of composite, in the temperature range 100-750 K at a selected frequency of 100 kHz, revealed a diffused ferroelectric phase
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transition. Variation of dielectric constant with frequency in the range of 20 Hz- 3 MHz was carried out at room temperature. The reduction in the leakage current due to Pr doping leads to typical saturated P-E hysteresis loops for the composite, confirming their ferroelectric nature. The low temperature ac conductivity follows Motts law, confirming the hopping
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conduction mechanism in the composites. At room temperature, the composite exhibit typical magnetic hysteresis loop, indicating the presence of a long range magnetic order. The
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calculated values of squareness ratio for the composites confirmed the presence of multidomain structure.
Keywords: Ferroelectrics, sol gel processes, conduction behaviour, phase transition, Motts law.
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Corresponding author. Tel: +91-194-2420078; fax: +91-194-2421357 E-mail address: [email protected]
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ACCEPTED MANUSCRIPT 1. Introduction A material is said to be functional when its one or more properties are sensitive to external environmental variables such as temperature, electric or magnetic fields [1]. Among the functional materials, multiferroics became attractive from technological point of view due to their magneto-electric (ME) effects [2-3]. ME effect makes these materials best candidate for
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sensors, electrical field tunable devices, voltage converters etc. [4-6]. Keeping in view these technological applications, the prime focus of the scientific community has been on the multiferroic materials with large magneto-electric effects. However, the difficulty in synthesis of single-phase multiferroics under ambient conditions with strong room
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temperature polarization and magnetoelectric effects have motivated researchers to switch for ME composite materials [7]. In composite materials the ME effect is activated by the mechanical strain transfer at the interfaces between the two ferroic phases. In addition, each
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ferroic phase retains its unique property separately in the ME composite [8]. The most widely studied composites are made with PZT, BaTiO3 and PVDF as ferroelectric phase and NiFe2O4, CoFe2O4, MnFe2O4, ZnFe2O4, terfenol as ferrite phase [9-15]. Keeping in view the excellent ferroelectric and piezoelectric properties of PZT and magneto-crystalline anisotropy, high coercivity, moderate saturation magnetization of CFO, the current research
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on (PZT–CFO) composites is going on at an increasing rate [16–20].One of the main issue with a PZT–ferrite composite material is its unsaturated ferroelectric loop with low polarization values, generally due to leakage currents [21-23]. Therefore, in order to enhance the dielectric and polarization nature and to minimize leakage currents in (PZT–CFO)
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composites, praseodymium (Pr) was doped both in the ferroelectric PZT and ferromagnetic CFO parent phases in the present work. Further, a literature survey also reveals that
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praseodymium (Pr) doped PZT-CoFe2O4 multiferroic composite prepared by sol gel method has not been reported so far. In this paper, we will present important results of enhancing ferroelectric and dielectric character of praseodymium doped PZT-CFO composites.
2. Experimental procedure: Raw materials such as lead nitrate, zirconium isopropoxide, titanium isopropoxide, and praseodymium nitrate with ethylene glycol, nitric acid and acetylacetone (as solvent) respectively, were used for the synthesis of Pb0.95Pr0.05Zr0.52Ti0.48O3 (Pr-PZT) via autocombustion route. In a similar way ferric nitrate, cobalt nitrate, praseodymium nitrate and glycine were used as raw materials in appropriate molar ratios for the synthesis of CoPr0.1Fe1.9O4 (CPrFO) by auto combustion method. Both the precursors obtained from their 2
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ACCEPTED MANUSCRIPT respective solutions were kept at 80 °C under continuous stirring until a viscous gel was formed. This viscous gel was preceded to self-ignition, followed by combustion. The residue left after combustion was collected and grounded in an agate-mortar to form as-synthesized individual powders of Pr-PZT and CPrFO. The synthesized Pr-PZT and CPrFO powders were calcinated at 650 °C for 1h and 750 °C for 5 h, respectively. The calcinated Pr-PZT and
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CPrFO powders were then sintered at 1250°C for 2h and 950 °C for 7 hrs respectively. The composites of (1-x) Pb0.95Pr0.05Zr0.52Ti0.48O3 – (x) CoPr0.1Fe1.9O4 (where x=0.02, 0.05, 0.1, denoted asPPP2, PPP5, PPP10 respectively) were prepared by mixing the two phases i.e. ferroelectric and ferrite phases by proportion of their weights. These composites were then pressed into circular pellets using (3-5 wt %) polyvinyl alcohol as a binder. The pellets were
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sintered at 800oC for 6 hrs with constant heating and cooling rate of 5oC/min. After sintering the XRD data of these composites were collected on
D8 Advance Bruker X-ray
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diffractometer with CuKα (λ=1.5406 Å) radiation. A Scanning electron microscope ( JEOLJSM-6490 LV) was used to study the in-depth external morphology of the prepared samples. The dielectric measurements were carried out in the frequency range of 20 Hz - 3 MHz and over a temperature range 100K– 750K using Agilent 4284A precision LCR meter. Electrodes were made out of the high grade silver paint, which was applied on both sides of the pellet to
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make it a disc shaped parallel plate capacitor with the material as a dielectric medium. The
LCR meter used in the present investigation directly provides the values of capacitance C and dielectric loss. Other parameters such as dielectric constant ε´ and ac conductivity, σ are computed using the relations:
and = 2
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=
--------------------------------------- (1)
where C is capacitance (in Farad), t is thickness (in metres), A is area (in m2), εo = 8.854 × 10−12
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Fm−1, f is the frequency (in Hz) of the applied electric field and the ac conductivity (in Ω-1m-1).
The P-E hysteresis loops were traced by using PE Loop Tracer, Radiant Technologies–Inc. Silver paint was used as electrode and was deposited on both sides of pellets for the dielectric and ferroelectric measurements. The magnetic measurements were carried out on a vibrating sample magnetometer (Micro Sense EZ9 VSM, USA).
3. Results and discussion 3.1. SEM and EDS Analysis: Typical SEM micrographs of (1-x) Pr PZT- (x) CPrFO (x= 0, 0.02, 0.05 and 0.10) composites are shown in Fig. 1 at a magnification of 10,000X. It is observed from the micrographs that 3
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ACCEPTED MANUSCRIPT the ceramics possess fine crystalline structures. The grains in the pure Pr-PZT are compact as compared to composite samples due to its higher density (8.7g/cm3). It is observed from Fig. 1 that ferrite grains are almost uniformly distributed in the ferroelectric matrix. The Pr-PZT and cobalt ferrite grains retain tetragonality and cubic structure respectively in the composite. The EDS micrographs (Fig. 2) show that all the elements (Pr, Pb, Zr, Ti, Fe, Co, O) are
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present in the composite. In addition, the intensity value of Pr peaks increase from PPP2 to
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PP10, which is in accordance with the molar percentage taken during synthesis.
Fig 1. Typical SEM micrographs of (a) Pr-PZT (b) PPP-2 (c) PPP-5 (d) PPP-10
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Fig 2. EDS spectra of samples sintered at 950 oC (a) Pr-PZT (b) PPP-2 (c) PPP-5 (d) PPP-10
3.2 Powder X-ray diffraction results
Fig.3 shows the XRD patterns of (1-x) PrPZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1 and 1)
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composites. The peaks corresponding to Pr-PZT and CPrFO were indexed with the tetragonal structure of PZT(JCPDS Card no. 33-0784) and cubic spinel structure of CFO (JCPDS Card no. 77-0426) respectively. For composites, two well-defined sets of diffraction peaks characterize the
constituent phases. No reflections other than those belonging to a cubic spinel structure of the
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ferrite phase and tetragonal perovskite structure of the ferroelectric phase were observed, suggesting the absence of any chemical reaction between the constituent phases.No structural
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changes have occurred with varying molar percentages of the constituent phases. Only the intensity of the ferrite-phase peaks increases with its content in the composites. The intensity of the spinel ferrite peak at (2θ=35o) increases while the intensity of the perovskite ferroelectric peak at (2θ=30o) decreases with increasing ferrite content. This confirms that intensity of XRD peaks in composites is highly dependent on the quantity of the individual ferroic phases [24]. The approximate amount of ferrite phase present in the composites, after final sintering, was calculated by using the following formula [25].
% ℎ = × 100 -------------------------------------------(2)
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ACCEPTED MANUSCRIPT where IF and IFE corresponds to high intensity peaks of ferrite and ferroelectric phases in the composite. The percentage of the ferrite content calculated was in good agreement with the molar percentage used for the synthesis as depicted in Table 1.The densities of all the samples are also given in Table 1. Table 1.Percentage of ferrite content in PPP-2, PPP-5 and PPP-10 and their densities. Density (g/cm3)
High intensity
High intensity
peak of Pr-PZT
peak of CPrFO
phase (A.U)
phase (A.U)
% of ferrite
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Sample
8.74
-
-
PPP-2
6.49
91
3297
PPP-5
6.2
200
4519
4.42
PPP-10
6.64
210
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10.52
-
2.76
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Pr PZT
Fig.3: X-Ray diffraction patterns of (1-x) Pr PZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1 and 1). 3.3. Dielectric studies
The variation of real part of the complex permittivity (ε′) with temperature (100 K – 750K) for Pr-PZT, PPP-2, PPP-5 and PPP-10 at 100kHz is shown in Fig.4. It is observed that the dielectric constant increases with temperature and attains a maximum value at around 585 K followed by the decrease in the value of dielectric constant with the further increase in temperature. The mobility of charge carriers increases with increasing temperature, which leads to an increase in the conductivity and polarization of the sample and hence an increase 6
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paraelectric phase. At transition temperature Tc, the value of dielectric constant is observed to be high for all the samples. At this temperature the domain motion is a dominant factor in the dielectric response of the material as compared to that at lower temperatures. Above Tc, the electrons/holes have random vibrational motion and the ferroelectric phase is transformed to
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the paraelectric phase, resulting in decrease of dielectric constant [27]. The values of transition temperatures and their corresponding maximum dielectric constant are given in Table 2. It is observed from Table 2 that Tc shifts to higher temperatures (590 K) for PPP-10
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which is attributed to maximum electron/hole exchange.
Fig.4: Variation of Dielectric constant with temperature for Pr-PZT, PPP-2, PPP-5 and PPP10. It is also observed that the dielectric peak is broadened in all the samples, which shows that the ferroelectric phase transition is a diffused one. This diffuseness in the composites was successfully examined by the following type of modified Curie-Weiss law.
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′
−
!
′ #
%$= &'( − () *+ ---------------------------------------------------------------(3)
′ is the peak value of the dielectric constant and / is a critical exponent which lies in here ,-.
the range 1< / ≤ 2; / = 1 represents the ideal Curie-Weiss behaviour while a value of /
between 1and 2 indicates diffuse behaviour [28]. The values of /, given in Table 2, were !
#
%$4versus 2'( − () * plots as shown in Fig. 5. The
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!
obtained from the slope of 2 3′ − ′
results confirm that ferroelectric phase transition observed in the present composites is of diffused type. This diffuseness may be due to the compositional fluctuations and structural disordering in the composite [29-30]. A microscopic in-homogeneity in the material under
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study with different local Curie points may also be the reason for this diffuseness. Such a
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type of behavior has also been found in many ceramic materials [31-32].
!
Fig.5. Plot of 2 3′ − ′
!
#
%$4 versus 2'( − () *for Pr-PZT, PPP-2, PPP-5 and PPP-10
The variation of dielectric loss (tan δ) with temperature for Pr-PZT, PPP-2, PPP-5 and PPP10 composites at 100 kHz is shown in Fig 6. The behaviour of tanδ with temperature is similar to that of the variation of the ε' with temperature as reported by other researchers [33]. 8
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increase in the ferrite content which is attributed to high conducting nature of CPrFO grains.
Fig.6: Temperature dependence of dielectric loss (tan δ) for Pr-PZT, PPP-2, PPP-5 and PPP-
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10. The inset shows variation of tan δ with temperature for CPrFO
Table.2: Dielectric parameters of (1-x) Pr PZT- (x) CPrFO (x=0, 0.02, 0.05, 0.1, 1) composites.
,-. '() *
n
Pr PZT 587.6
2614.85
0.778
1.46
PPP 2
575.2
1155.8
0.831
1.55
PPP 5
580.6
1414.44
0.734
1.16
PPP 10 587.4
1168.92
0.547
1.44
CPrFO
-
0.216
-
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Sample Tc(K)
-
/
The variation of dielectric constant with frequency in the range of 20 Hz to 3 MHz at room temperature for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO is shown in Fig 7.As observed from Fig.7 the value of dielectric constant decreases with the increase in frequency. Variation of dielectric constant with frequency can be explained on the basis of polarization 9
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ACCEPTED MANUSCRIPT mechanisms. Polarization may be separated into three parts: dipolar, ionic and electronic. For the case of multiferroic ceramic compounds, there is another type of polarization: interfacial or space charge polarization that may originate in the materials from the point defects during sintering. At low frequency, all the types of polarization are present, but major contribution to ε ′ comes from space charge polarization [34]. The space charges are able to follow the
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variation of the applied electric field at low frequency; however, at high frequencies they cannot follow the variation of field frequency. Thus contribution of space charges at high frequency towards dielectric constant decreases. Moreover, the dipolar and the ionic contributions are also small at higher frequency due to the inertia of the molecules and ions
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[35]. In the case of the composites, the high value of the dielectric constant can also be explained on the basis of the fact that the ferroelectric regions (Pr PZT) are surrounded by non- ferroelectric regions (CPrFO); a situation that is similar to relaxor ferroelectrics [36-37].
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to the low resistivity of ferrite phase.
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The decrease of dielectric constant with increase in ferrite phase (shown in Fig.8) is attributed
Fig.7: Variation of dielectric constant (ε') with frequency for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO The variation of dielectric loss with frequency in the range of 20 Hz to 3 MHz at room temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10 is shown in Fig 8.The dielectric loss 10
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ACCEPTED MANUSCRIPT arises mainly due to impurities and imperfections in the crystal lattice. It is observed from Fig. 8 that the dielectric loss decreases rapidly in the low frequency region, while the rate of decrease is slow in the middle region and then at high frequency it becomes almost frequency independent. Such behaviour for ferrites and composites in low frequency region is due to low conductivity of grain boundaries. Due to the lower conductivity, more energy is required
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for electron to exchange between Fe2+ and Fe3+ ions; resulting in higher values of tan δ. In high frequency region, which corresponds to high conductivity of grains, a small energy is required for electron transfer between the Fe2+/Fe3+ ions at the octahedral site [38-39]. In comparison to CPrFO, the losses observed for Pr-PZT and composites are negligible and
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hence are shown in inset of Fig 8. It is observed from Fig.8 that pure CPrFO and composites exhibit a loss peak in accordance with Debye type of relaxation. The electric dipoles cannot follow the alternating field instantaneously; some time is needed for the alignment of dipoles
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with the field [35]. This peak is observed in ferrite materials when the jumping/hopping frequency of electrons/holes among ions becomes equal to the frequency of the applied field, i.e., maximum electric energy is transferred to the electrons and the loss shoots up at resonance [40]. Further the loss peaks in composites becomes more prominent as the ferrite
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content increases as depicted in inset of Fig.8.
Fig.8: Frequency dependence of dielectric loss for CPrFO. The inset shows variation of tan δ with frequency for Pr-PZT, PPP-2, PPP-5 and PPP-10
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observed that conductivity remains constant up to a certain frequency. This frequencyindependent conductivity is attributed to the band conduction [41]. The increase in conductivity with increasing frequency is attributed to the small-range polaron hopping mechanism [42]. As the frequency increases the conductivity increases abruptly. The
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frequency independent conductivity is shifted to frequency-dependent conductivity beyond the hopping frequency, indicating the translation from band conduction to polaron hopping conduction thereby obeying Motts law [42-44].The frequency dependent ac conductivity can
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be expressed by the following empirical formula[45]
σ'ω* = Aω6 ----------------------------------------------------------------------(4)
where ω '= 2πf* is the angular frequency, A is a constant and has units of σac and n is dimensionless. The value of n can be determined from the slope of lnσ versus lnω plots as shown in Fig 10. The value of ‘n’ lies between 0 and 1. If n = 0, the electrical conductivity is
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frequency-independent or dc conductivity (σdc), however for n >0, the conductivity is frequency-dependent [46]. The estimated values of n for Pr-PZT, PPP-2,PPP-5, PPP-10 and CPrFO are almost zero below hopping frequency, however, beyond this frequency a finite values are obtained given in Table 2.These results confirm the hopping conduction
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mechanism.
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shows variation of σac with frequency for CPrFO, PPP-2, PPP-5 and PPP-10.
Fig. 10: Plot of log: versus log for Pr-PZT,PPP-2,PPP-5, PPP-10 and CPrFO. The ac conductivity is thermally activated process, showing weak temperature dependence at
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lower temperatures, while as stronger dependence on temperature at higher temperatures as shown in Fig 11. At higher temperatures, the ac conductivity shows the activated temperature dependence as <=>
σ = A e ?@ ----------------------------------------------------------------------------(5)
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where A is constant, Ea is the activation energy, which is the energy needed to release an electron from the ion for a jump to neighbouring ion, so giving rise to the electrical
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conductivity, k is Boltzmann’s constant, and T is the absolute temperature.
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Fig.11: Variation of σac with temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10. The activation energy as calculated from the slope of ln(σ) versus 1/T (Fig. 12) shows an increasing trend with temperature. The activation energy in ferrites, acting as one of the ferroic phase of the composite, is associated with the variation of mobility of the charge carriers between Fe2+ and Fe3+ rather than with their concentration. Since the charge carriers are localized in the ions or vacant sites, the conduction occurs via a hopping process which is
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temperature dependent. For the ferrites having high conductivity, the activation energy is of the order of 0.1 eV, however for insulating ferrites is of the order of 0.4 eV [47]. In addition to Pr-PZT, the values of activation energies in composites for higher temperature region (275- 700 K) are of the order of 0.1eV, however a value of 0.01eV is observed in lower
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temperature region(100-275 K) given in fig.12. Further, from the plots of ln (σ) versus (1/T)1/4 in the lower temperature region (100-220 K) depicted in Fig.13, it is clear that Pr-
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PZT as well as composite obeys Motts law [43] of the form ( !/F = A B− C D G (
where T0 is a constant proportional to the density of states at the Fermi level N(EF), kB is the Boltzmann constant. Both the studies confirm that the composites are conductive and the conduction is due to hopping process, which in turn is temperature dependent.
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Fig.12: Plot of log versus 1000/T for Pr-PZT, PPP-2, PPP-5 and PPP-10.
Fig.13: Plot of log versus T-1/4for Pr-PZT, PPP-2,PPP-5 and PPP-10. 3.5.Ferroelectric Studies: The plots of polarization (P) versus electric field (E) for Pr-PZT, PPP-2, PPP-5 and PPP-10 are shown in Fig 14. The loops indicate ferroelectric behavior of Pr-PZT as well as that of the 15
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ACCEPTED MANUSCRIPT composites. It is observed that the values of saturation polarization, remanence polarization and coercive field for pure Pr-PZT are higher as compared to that of composites ( see Table 3). It is because the ferrite phase, mixed into ferroelectric phase, acts as pores in the presence of applied electric field and breaks the electric circuit, resulting in the decrease of various ferroelectric parameters [48]. Also the decrease in electrical parameters in the composite is
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believed to be due to the conducting nature of CPrFO. Due to high dielectric constant and large polarization values of Pr-PZT, the PrPZT-CPrFO composite prepared in the present work shows completely saturated ferroelectric loops with higher polarization values as compared to already reported elliptical/banana loops for PZT-ferrite composites [21-23]. It is
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further observed that a 10% Pr doping in CFO (ferrite phase) increases its electrical resistivity as compared to pure CFO, thereby, minimizing leakage currents and preventing the composites from electric breakdown during poling of their ferroelectric phase. This shows
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that the presence of Pr in CFO and PZT has a considerable effect on the ferroelectric properties of the composites. Further, the PPP-10 has significant saturation polarization, remenant polarization as well as high dielectric constant due to higher density, which makes
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this kind of a material a suitable candidate for technological applications.
Fig.14: The P-E loops at room temperature for Pr-PZT, PPP-2, PPP-5 and PPP-10.
Table 3: Ferroelectric and magnetic parameters for Pr-PZT, PPP-2, PPP-5, PPP-10 and CPrFO 16
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Ps
Pr
Ec(kV/cm) Ms
(µC/cm2
(µC/
Hc
Mr
(emu/gm) (Oe)
(emu/gm)
Mr/Ms
2
cm )
Pr PZT
15.12
11.29
24.56
-
-
-
-
PPP2
4.046
2.27
18.54
0.309
235.85
0.0935
0.30
PPP5
2.903
2.23
15.35
2.54
375.146 0.712
0.28
PPP10
8.005
6.08
29.03
6.9509
787.049 2.833
0.41
CPrFO
-
-
-
71.228
960
0.50
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)
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35.965
3.6. Magnetic properties
To investigate the magnetic nature of (1-x) PrPZT- (x) CPrFO (x= 0.02, 0.05, 0.1 and 1), composites, M-H hysteresis loops were studied.Fig.15 shows the M-H hysteresis loops of PPP-2, PPP-5 and PPP-10 traced at room temperature. The inset of Fig.15 shows M-H loop
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of CPrFO. The various magnetic parameters calculated from the loops are tabulated in Table 3. All the composites exhibit typical magnetic hysteresis loop and get saturated at a field of about 10 kOe. This indicates the presence of an ordered magnetic structure in the mixed cubic
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spinel structure of ferrite.
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Fig.15: Room temperature M-H loops for PPP-2, PPP-5 and PPP-10.The inset shows M-H loop for CPrFO.
The magnetic behaviour of the composites is due to the presence of ferrite phase [49].It is observed from Table 3 that the saturation magnetization of all composites is lower than that
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of the pure CPrFO phase (Ms=71.228 emu/gm). The decrease in the various magnetic parameters is due to presence of non-magnetic Pr-PZT phase, which suppresses the magnetic behaviour. The saturation magnetization (Ms) and remenant magnetization (Mr) show an increasing trend with the increase of ferrite concentration. The increase in saturation
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magnetization with increasing CPrFO content provides an indication that the spontaneous magnetization of the composites, which originates from unbalanced antiparallel spins, leads
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to net spins other than those introduced by the structural distortion [50].The squareness ratio (Mr/Ms) was also calculated for CPrFO as well as for PPP-2, PPP-5 and PPP-10. From these values it is clear that CPrFO has single domain structure, while as the composites exhibit multi-domain structures [51].
Conclusion Synthesis of praseodymium doped PZT and CFO composite phase has been achieved through sol-gel auto combustion technique. The XRD and morphological studies reveal composite formation of parent ferro-ferrite phases with uniform distribution of grains of each phase. The individual phases retain their structure in the composite, without formation of any new
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ACCEPTED MANUSCRIPT secondary phase .The dielectric studies of composite in the temperature range of 100-750 K at a selected frequency of 100 kHz reveal a diffused ferroelectric phase transition. The loss peak with frequency was observed to be in accordance with Debye relaxation theory for pure CPrFO and composites.The low temperature ac conductivity follows Motts law, confirming the hopping mechanism in the composites. At room temperature, saturated P-E hysteresis
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loops and saturated magnetic hysteresis loop has been demonstrated for ferrite-ferroelectric composite. Our results thus demonstrate multiferroic character with improvement in dielectric and ferroelectric properties and deduction in leak current with Pr doping. The squareness ratio confirmed the presence of multidomain structure of the composite. It may be
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concluded that the 10% ferrite content is a substantial amount to maintain its high dielectric constant, high saturation polarization and high saturation magnetization in Pr-PZT–CPrFO
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composites.
Acknowledgement
One of the authors Rubiya Samad highly acknowledges Department of Science and Technology, Government of India for financial support vide reference no. SR/WOS-A/PM1006/2015 under Women Scientist Scheme to carry out this work. The authors are thankful
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to the authorities of University of Kashmir for providing the vibrating sample magnetometer facility (MicroSense EZ9 VSM) to the Department of Physics under DST Govt. of India special package for sophisticated instrumentation. Authors would also like to thank Centre Director, UGC DAE CSR Indore for XRD and PE-Loop measurements.
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The composite exhibits saturated electric hysteresis loops without leakage current. At low temperatures, the electrical conductivity is due to hopping mechanism.
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There are ordered magnetic domains and multi-domains in the composite.