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Magneto-electric properties of in-situ prepared xCoFe2O4-(1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites
Author’s Accepted Manuscript Magneto-electric properties of in-situ prepared xCoFe2O4-(1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites Vinitha Reddy Monaji, J Praveen Paul, N. Shara Sowmya, A. Srinivas, Dibakar Das www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31419-5 http://dx.doi.org/10.1016/j.ceramint.2016.08.112 CERI13564
To appear in: Ceramics International Received date: 20 June 2016 Revised date: 4 August 2016 Accepted date: 18 August 2016 Cite this article as: Vinitha Reddy Monaji, J Praveen Paul, N. Shara Sowmya, A. Srinivas and Dibakar Das, Magneto-electric properties of in-situ prepared xCoFe2O4-(1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.112 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 galley proof before it is published in its final citable 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.
Magneto-electric properties of in-situ prepared xCoFe2O4-(1x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites Vinitha Reddy Monajia, Paul Praveen Ja, N. Shara Sowmyab, A. Srinivasb, Dibakar Dasa* a
b
School of Engineering Sciences & Technology, University of Hyderabad, Hyderabad, 500046, India
Defence Metallurgical Research Laboratory, Advanced Magnetics Group, Hyderabad 500 058, India *
Corresponding author: Tel:- +91 4066794454. E-mail address : [email protected]
Abstract Magneto-electric particulate composites, [xCoFe2O4 – (1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3] (x = 30, 40, 50 wt.%), were synthesized in-situ by a modified sol-gel method. This novel synthesis method has led to uniform distribution of the magnetostrictive and piezoelectric phases in the composite samples and improved magnetoelectric properties. Using autocombustion method CFO nanoparticles were synthesized and were added to the precursor gel of BCZT, further the composite powders were calcined at 800°C and sintered at 1300°C. Composite containing 50wt% CFO showed a maximum magnetostriction (λmax) of ~ 88ppm and strain sensitivity (dλ/dH) of ~ 41 x 10-9 Oe-1. 40CFO-60BCZT composite showed a high piezoelectric voltage constant (g33) of 8x103
V.m/N. All the composites showed magnetoelectric effect and the maximum
magnetoelectric coupling coefficient (dE/dH = αME) was measured ~ 161mV/cm.Oe for 40CFO-60BCZT sample at its resonance frequency. The effect of microstructure on the magnetoelectric properties of [(x)CFO-(1-x)BCZT] composites has been studied and reported in this investigation as a function of its piezoelectric (BCZT)/ferrite (CoFe2O4) content. Keywords: A. Sintering; C. Dielectric properties; D. Ferrites; magnetostriction; magnetoelectric 1
1. Introduction Multiferroics are a class of materials comprising of more than one ferroic order. The coupling between different order parameters in these materials results in a new type of effect, such as magnetoelectric (ME) effect. Magnetoelectric response from a material is the appearance of an electric polarization (P) under an applied magnetic field (H) and vice versa [1,2,3] Various materials have been investigated for magnetoelectric response which includes single phase multiferroics and composites. Composites with different connectivity schemes such as, particulate composites (0-3), laminated composites (2-2), nano pillars (1-3) and multilayer composites containing thin films were studied [4,5]. The ME response of single phase multiferroic materials such as BiFeO3 and various manganites is either relatively week or occurs at low temperatures. In contrast, multiferroic composites, which consist of ferroelectric and ferri-/ferromagnetic phases, exhibit high magnetoelectric effect, which makes them potential candidate for multifunctional device applications [4,5,6]. Magnetoelectric response of a material is expressed by the magnetoelectric coupling coefficient, αME = dE/dH, where E and H are strength of the electric and magnetic fields respectively. In order to achieve a high magnetoelectric coupling coefficient (αME) in a composite, it is important to select a magnetic phase with large magnetostriction and high strain sensitivity and a piezoelectric phase with large piezoelectric response [7,8]. Cobalt ferrite (CoFe2O4, CFO) is a soft magnetic material with cubic spinel structure and shows very high magnetostriction and strain sensitivity compared to other cubic ferrites [9,10]. (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3, [BCZT] is an 2
efficient lead free piezoelectric material, which shows excellent piezoelectric properties [11]. Hence, in this study CFO and BCZT have been used as the magnetic and piezoelectric phase in the composite, respectively. Agglomeration of the magnetic phase is a major issue during synthesis of particulate composites. To avoid it suitable dispersion technique can be adopted to synthesize particulate composites which should lead to enhancement in the ME output. In this study, therefore, an in-situ synthesis technique is being employed to synthesis CFO-BCZT multiferroic ceramic particulate composites with varying CFO content (30 to 50 wt%) to avoid agglomeration of the CFO phase. Microstructural, magnetic, dielectric, piezoelectric properties and the magnetoelectric coupling behavior of the ME particulate composites have been systematically studied and correlated with variation in the magnetic/piezoelectric phase content. 2. Experimental procedure CoFe2O4–(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 composites with varying CoFe2O4 (CFO) content (30 to 50 wt%) were synthesized in-situ by a modified sol-gel method. Cobalt ferrite (CFO) was prepared by auto-combustion method [12, 13]. Co(NO3)2.6H2O and Fe(NO3)3.9H2O were used as source of Co+2 and Fe+3 ions respectively and glycine was used as fuel in the combustion reaction. For the stoichiometric reaction of cobalt ferrite the glycine (which acts as fuel) to nitrate (which act as oxidizer) ratio has to be 0.3, but in this experiment glycine to nitrate ratio has been taken as 0.5, which can be considered as fuel rich ratio [14]. Required amount of nitrates and glycine have been weighed and taken into a beaker. 50-100ml of water was added to the mixture and was stirred continuously to obtain homogeneous aqueous solution. The temperature of the solution was maintained at 80°C. The complete evaporation of water led to the formation of a 3
viscous gel. Auto-ignition of this viscous gel was initiated by increasing the temperature to 200⁰C. The ignition process was accompanied by the evolution of large volume of gases along with cobalt ferrite powder in the form of black ash. Since the time for the ignition process is very small, traces of decomposition products were removed by calcining the powder at 800°C for 2hrs to obtain desired single phase. (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 was prepared by sol-gel synthesis method [15]. Barium acetate (Ba(CH3COO)2), zirconium oxychloride (ZrOCl2·8H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and titanium isopropoxide (C12H28O4Ti) were used as starting materials. Stoichiometric amounts of (ZrOCl2·8H2O) and (Ca(NO3)2·4H2O) were added to ethanol and (Ba(CH3COO)2) was taken in acetic acid and stirred separately until they got dissolved completely. After dissolution, these three solutions were mixed together followed by the addition of stoichiometric amount of titanium isopropoxide to this solution. Calcined CFO powder in appropriate weight ratios was added to this precursor solution of BCZT and the resultant solution so obtained was continuously stirred and subjected to heating at 90°C until the viscous gel was formed. The resultant gel was dried and was calcined at 800°C for 2hrs for complete phase formation of BCZT. BCZT-CFO ME composite thus prepared by the modified sol-gel method is different than that prepared by simple mechanical mixing of calcined BCZT and CFO powder. Particulate composite prepared by mechanical mixing might contain aggregates of magnetic phase because of the magnetic interaction between the CFO particles. These aggregates form a conducting path, which reduces the effect of poling and thus the piezoelectric properties. In situ mixing of CFO particles in BCZT gel will certainly help in avoiding direct contact of the ferrite particles between them and is expected to eliminate the low conducting path there by reducing the leakage current 4
problem. The calcined ME composite powders were compacted into circular pellets and were subjected to sintering at 1300°C for 4 hrs. The calcined powders and sintered pellets were subjected to x-ray diffraction studies using a Bruker D8 Advance X-ray diffractometer with θ–2θ geometry using Cu Kα radiation in the 2θ range in between 20°-80°. The microstructural analyses of the sintered composites were performed by Carl Zeiss Ultra 55 Field Emission Scanning Electron Microscope (FESEM). The sintered composite pellets were polished and electroded using silver paste to make electrical contact for the dielectric and piezoelectric properties measurements. To ensure the proper electrical contacts, silver paste coated composite pellets were heat treated at 500°C for 15 min. The magnetic hysteresis measurement (M vs. H) was carried out using a Lakeshore (Model 7407) Vibrating Sample Magnetometer (VSM) up to 15kOe at room temperature (~ 298 K). Magnetostriction curves were obtained in a dc magnetic field setup with standard strain gauges (350W) at room temperature. The polarization versus electric field (P-E) hysteresis loops were measured at 1Hz using a Radiant Precision Premier II P-E loop tracer. The dielectric constants as a function of frequency and temperature were measured using Agilent E4980A precession LCR meter in the frequency range 20Hz to 2MHz. To induce piezoelectric properties in the composites the sintered composites were electroded and poled by applying a high dc. voltage in a silicone oil bath for 1hr. After 24h aging the poled samples were characterized for its piezoelectric properties (d33 and g33) using a Piezometer PM 300 (Piezotest, UK). The elastic stiffness constant C33 was also measured using an Agilent E4980A precision LCR meter from the resonance and anti-resonance frequencies in the frequency range 100 - 500 kHz on poled samples. Magnetoelectric (ME) measurements for all the CFOBCZT composites were carried out by a lock-in technique. A static magnetic field up to 5
±5 kOe and an alternative magnetic field of 3Oe at 1kHz frequency, generated by Helmholtz coils, was applied along the thickness of the sample. The voltage output generated from the composites was recorded using a lock-in amplifier (Stanford Research Systems - SR 830). 3. Results and discussion 3.1. Phase analysis of composite samples Figure 1(a) shows the X-ray diffraction (XRD) patterns of [xCFO-(1-x)BCZT] (x = 30,40 and 50 wt%) samples sintered at 1300°C. XRD peaks show the characteristics of cubic spinel ferrite (CFO) and perovskite BCZT phases. The individual phases were indexed using standard JCPDS and the diffraction peaks show no unidentified peaks, which suggests that there could be no reaction between the two phases during sintering. It can also be seen that as the CFO content increases from 30 to 50wt% the intensity of the ferrite peaks also increases. 3.2. Density and Microstructural analysis The densities of the sintered composites were measured using Archimedes’ method following the ASTM standard (C 373 – 88). The obtained dimensions of sintered composites having 50, 40 and 30 wt.% of CFO are 8.97 × 1.46, 8.99 × 1.60 and 9.11 × 1.62 mm2 (dia. × thickness) respectively. Figure 1b shows the increasing density with increasing CFO content of the sintered samples. The density increase could be attributed to the better densification characteristics of the CFO compared to BCZT in the sintering protocol followed to densify the composites. BCZT normally requires a higher sintering temperature of ≥1450°C to densify itself compared to that required for CFO (~1300°C) [9, 15]. Figures 1(c-e) show the FESEM micrographs of sintered [xCFO-(1-x)BCZT] (x = 30, 40 and 50 wt%) samples. Back scattered mode imaging of the composites show 6
distinct presence of two phases, where CFO grains appeared dark in color and BCZT grains are brighter due to large difference in their molecular weights. 3.3. Magnetization properties Figure 2a shows the magnetic hysteresis loops of (x)CoFe2O4-(1-x)BCZT composites measured at room temperature and the obtained hysteresis behavior is basically due to the presence of CoFe2O4 phase in the composite samples. Maximum magnetization (Mmax) at ~1.5 T field and coercivity (HC) for pure CoFe2O4 were observed to be 70.3 emu/g and 328 Oe respectively, which are in good agreement with the values reported earlier for CFO [16]. From the inset of Fig. 2(a), it can also be seen that maximum magnetization (Mmax) decreases from ~34.7 emu/g to ~17 emu/g and coercivity (HC) increases from 336 Oe to 362 Oe with increasing piezoelectric phase in the composition. Magnetizations of composite samples having different weight fractions of ferrite content were calculated based on maximum magnetization of pure CFO. Marginal decrease in measured magnetization was observed compared to the estimated (~35.2 emu/g to 21.1 emu/g for x = 50 to 30 wt% of ferrite) magnetization values. This could be due to the fact that in composite samples BCZT phase acts as a non-magnetic inclusion in the ferrimagnetic matrix (CFO) and reduces the magnetic interaction between the individual ferrite grains, resulting in degradation of magnetization of the composite samples with increasing BCZT content. Domain wall pinning increases as the non-magnetic BCZT phase increases, which results in increase in coercivity as shown in table 1. 3.4. Magnetostrictive properties Magnetostriction curves of (x)CoFe2O4-(1-x)BCZT composites measured at room temperature are presented in Fig. 2(b). For pure cobalt ferrite maximum 7
magnetostriction (λmax) of ~180ppm was observed, which are in agreement with the literature values [12,13]. The magnetostriction amplitude (λmax) is observed to decrease from -88 ppm for 50CFO-50BCZT to -53ppm for 30CFO-70BCZT. Dilution of the magnetic CFO phase with BCZT phase interrupts the magnetic contact, which results in decreasing magnetostriction with increasing non-magnetic BCZT phase. Piezo-magnetic coefficient (dλ/dH) obtained from the magnetostriction curve is shown in Fig. 2(c). The maximum piezo-magnetic coefficient for pure CFO is observed to be ~50 x 10-9 Oe-1, correlates well with the literature value [10,12]. Maximum dλ/dH decreases from ~41 x 10-9 Oe-1 for 50CFO-50BCZT to ~19 x 10-9 Oe-1 for 30CFO-70BCZT. As the nonmagnetic phase increases it impedes the domain wall motion, which is the main cause for magnetostriction behaviour at lower fields. Anisotropy of the system increases with increasing domain wall pinning, resulting in decreasing piezo-magnetic coefficient. 3.5. Dielectric properties Dielectric constants of (x)CFO-(1-x)BCZT composite samples were determined as a function of frequency at room temperature and the variations are shown in Fig. 3(a). From fig. 3(a), the dielectric constant is seen to decrease with increasing frequency and then reaches a constant value for the composites containing 40 and 30 wt% of CFO. However, for the composite containing 50 wt% of CFO the rapid decrease in dielectric constant is observed. This variation in dielectric constant can be explained on the basis of Maxwell and Wagner two layer model [17]. Due to the hopping of electrons at lower frequencies, electrons pile up at the grain boundaries because of its resistive nature and produce more interfacial polarization. At higher frequencies, however, the interfacial polarization decreases, because the electron exchange rate between ions cannot cope up with the change in external applied field, resulting in decrease in dielectric constant with 8
increasing frequency. CFO has low dielectric constant compared to BCZT; hence it resulted in decrease in dielectric constant with increase in CFO concentration in the composite samples except for 50CFO-50BCZT composition. The large value of dielectric constant (ε) for 50CFO-50BCZT sample than pure BCZT at lower frequencies is attributed to the additional space charge polarization, which is the result of accumulation of space charge carriers at insulating grain boundaries produced from the electron hopping between Fe+2 ↔ Fe+3. The variation in dielectric loss as a function of frequency is shown in the inset of Fig.3(a). It is observed that the dielectric loss decreases with increasing frequency and this could be due to the fact that, as the frequency increases, beyond a certain frequency, the space charge carriers cannot follow the field and the alternation of their direction lags behind the change in field direction. The dielectric loss is seen to increase with increasing ferrite concentration and it is also observed that dielectric loss of 50CFO50BCZT composite sample shows abnormal peaking behaviour at higher frequencies. This type of resonance peak occurs, when the applied field frequency matches the hopping frequency of electrons between Fe+2 and Fe+3 ions in the sample [18]. Figs. 3(b) and (c) show the temperature dependence of dielectric constant of (x)CFO-(1-x)BCZT, (x = 50, 30 wt%) composites measured at varying frequencies. Dielectric anomaly at two different temperatures is clearly observed for all composite samples. The first dielectric anomaly appears around 115 -126˚C with varying ferrite concentration from 30% to 50%. These temperatures correspond to the ferroelectric transition temperature of BCZT phase. The second peak at ~520˚C indicates the magnetic transition temperature of CFO phase. Similar behaviour in dielectric constant has been reported for BZT+CMFO composite samples in the literature [19]. Increase in 9
dielectric constant with increasing ferrite content could be attributed to increasing interfacial polarization as observed earlier in fig. 3(a). From figs. 3(b) and (c) it is also observed that maximum dielectric constant is increased with increasing temperature. In general, the dielectric constant of a material is mainly due to interfacial, dipolar, ionic and electronic polarizations. At lower frequencies interfacial and dipolar polarizations are dominant mechanisms and these polarizations are strongly dependent on temperature [20]. Dipolar polarization decreases with increasing temperature; hence interfacial polarization has more pronounced effect on increasing the dielectric constant with increasing temperature. The observed trend in dielectric constant (ɛ) with temperature (T) can be explained on the basis of electron hopping between neighbouring ions as discussed below. In presence of an external electric field a local distortion of the electron cloud (associated with the electron hopping between Co+2 and Fe+3 ions) takes place in the direction of the applied field, which affects the polarization of the system. As the temperature increases thermal activation energy required for the excitation of charge carriers increases, which results in increase in the dielectric constant of the samples [21]. The decreasing dielectric constant beyond transition temperature could possibly be attributed to the increased randomness in the orientation of the charge carriers and their inability to orient in the direction of the applied field. The transition temperature of BCZT phase is observed to shift towards the high temperature side with increasing CFO concentration in the composite samples. In the composite samples, incorporation of CFO (non-ferroelectric) in BCZT phase dilutes the ferroelectric interaction, resulting in remarkable dispersion in the transition temperature of BCZT phase, similar to what observed in relaxor ferroelectrics. Variation in transition temperature with composition is shown in Fig. 3(d). From figs. 3(b) and (c), it is clearly 10
seen that the FWHMs of the dielectric constant peaks are increased. This indicates a relaxor behaviour, which can be explained based on the Curie-Weiss law, ! ɛ
!
− ɛ = $(% − %& )' #
(1)
where C is the Curie-Weiss constant, which describes the width of the diffuse phase transition. ‘γ’ is an empirical parameter and describes the degree of relaxation and can vary between 1 and 2. For a perfect normal ferroelectric g = 1 and for a perfect relaxor ferroelectric the value of g is 2 [22]. Inset of fig. 3(d) shows the plot of ln ((1/ɛ)-(1/ ɛm)) as a function of ln (T-Tm) at 10 kHz for (x)CFO-(1-x)BCZT composite samples. The value of g can be obtained from the slope of linear fitting of these plots. From fig. 3(d), it is observed that the value of g is nearly 2 for all composite samples, meaning all the samples are exhibiting relaxor behavior. The observed relaxor behaviour in composite samples could be due to the increased chemical inhomogeneity in the system. Such type of relaxor behaviour has previously been reported for some magnetoelectric composite samples [23]. 3.6. Piezoelectric properties Figure 4 shows the room temperature polarization (P)-electric field (E) hysteresis loops of the (x)CFO-(1-x)BCZT magnetoelectric composites. It is clearly seen from Fig. 4, that even under the application of a high electric field (~30 kV/cm), the hysteresis loops of the composite samples were not saturated and also the loops for the samples with high CFO content exhibited some curvature near saturation, which could be attributed to the large leakage current through the low resistive grains of CFO phase. Similar kind of behavior was reported for (1-x) [0.94Bi0.5Na0.5TiO3– 0.06BaTiO3]–xCoFe2O4 magnetoelectric ceramics by L.H. Pang et al. [24]. Variations of remanent polarization and coercive field as a function of CFO content are shown in 11
inset of Fig. 4. The hysteresis loops are observed to broaden with increasing concentration of CFO phase in the composite samples, which indicates the increase in dielectric loss as the ferrite concentration increases and this is in accordance with the result shown in inset of Fig. 3(a). Highest density pellets of each composition of (x)CFO-(1-x)BCZT composites were subjected to electrical poling for the process of orienting the ferroelectric domains. The piezoelectric properties, g33 and d33, of all the composite samples were obtained from the Piezometer (PM300) after aging for 24h after poling. The lower panel of figure 5 shows the variation of d33 (left Y- axis) and g33 (right Y-axis), as a function of composition. A maximum piezoelectric charge coefficient, d33, of ~60pC/N and piezoelectric voltage constant, g33, of ~8.1mV/m.N were obtained for 40CFO-60BCZT. ME composites show very low piezoelectric properties compared to pure piezoelectric phase, which is primarily attributed to the presence of the magnetic phase (CFO), which is conductive in nature compared to BCZT. Hence, composite containing magnetic phase cannot be poled properly even at a high electric field due to its low resistive nature, which is clearly indicated from the dielectric measurements. The value of the piezoelectric coupling factor kt and elastic stiffness C33 were measured for all the composites by resonance method. The upper panel of figure 5 shows the variation in C33 (right Y-axis) and kt (left Y-axis), respectively, as a function of CFO content of the composites. The resonance (fr) and antiresonance (fa) frequencies were obtained from the impedance peaks against frequency scan ranging from 100 kHz to 500 kHz of the optimally poled samples for all the compositions. These peak values were used to evaluate the elastic stiffness for all the composite samples. A maximum elastic stiffness (C33) of 0.66 x1010 N/m2 and a piezoelectric coupling factor kt of 0.243 12
were obtained for the 50CFO-50BCZT composite. Due to the difference in sintering behavior of CFO and BCZT, low density was observed for all the composite samples. Because of the presence of high porosity and low resistive ferrite phase proper poling was not achieved, which resulted in degrading of piezoelectric properties of the composites compared to pure BCZT. In our previous studies d33 and g33 of ~637 pC/N and ~29 mV.m/N, respectively, were achieved for highly dense BCZT sample sintered at 1550°C [15]. In the present study, however, maximum d33 and g33 of ~60 pC/N and ~8.1 mV.m/N, respectively, were observed for 40CFO-60BCZT composite sample. 3.7. Magnetoelectric effect Magnetoelectric property of a material is expressed in terms of the magnetoelectric coefficient αME of the system. An expression for the ME voltage coupling coefficient (αME) was given by Zubkov [4] as, 12
+,- = mv .g33 C33 / .1-mv / 0 4 p
13 m
(2)
where, mv is the volume fraction of the piezoelectric phase (p), g33 is the piezoelectric voltage coefficient, C33 is the elastic stiffness and dl/dH is the strain derivative. From equation (2) it is observed that for a system to show high magnetoelectric effect, the piezoelectric parameters (g33 and C33) and piezomagnetic coefficient (dλ/dH) should be high. Hence, in this study the above mentioned parameters were measured. Figures 6 (a) and (b) show the Magnetoelectric (αME) curves of xCFO-(1x)BCZT composites measured at magnetic field up to ±5000 Oe using the lock-in technique. A superimposed alternating field of 3Oe was simultaneously applied by using Helmholtz coils. A maximum αME value of about 7.75 mVcm−1Oe−1 was observed for 40CFO-60BCZT sample at an applied static dc field of 2560Oe with 1kHz frequency of the a.c. magnetic field. All the composites showed magnetoelectric 13
behaviour and the ME values initially increase with increase in the magnetic field followed by a decrease with further increase in the field. This behaviour is attributed to the high striction developed by the magnetic phase which is transferred to the piezoelectric phase developing a voltage output. The αME value reported in this investigation is higher than those obtained for NBT-NFO [25], NBT-CFO [26] and comparable to BTO-CFO [27, 28] lead free based particulate composites. 40CFO60BCZT showed a maximum αME of 161mV/cm.Oe at its resonance frequency of 365 kHz with fixed dc field of 2560 Oe. For the CFO-BCZT particulate composite prepared by mechanical mixing method (which is communicated separately), the maximum αME at its resonance frequency is observed to be 118mV/cm.Oe. ~35% increase in magnetoelectric coefficient is observed for CFO-BCZT particulate composites prepared by in-situ synthesis technique compared to mechanical mixing method. For a composite to have large ME coefficient perfect coupling between the magnetic and electric order parameters leading to proper strain transfer from one phase to the other is expected. But, in the presence of internal defects such as, porosity the elastic stiffness of the composite will be reduced, which will suppress the coupling between the ferroic phases leading to reduction in ME coefficient. 4. Conclusions Magnetoelectric properties of in-situ prepared xCoFe2O4 – (1x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (x = 30, 40, 50 wt.%) particulate composites were investigated and reported in this work. The magnetisation (Ms, Hc) and magnetostrictive (λ) properties were found to vary with the magnetic content of the composites. A maximum saturation magnetization of 34.7 emu/g and high strain sensitivity dλ/dH of 41x10-9Oe-1 were measured for the 50CFO-50BCZT composite. All 14
the composites showed enhanced magnetoelectric effect and the maximum magnetoelectric coupling coefficient, αME = 7.75mV/cm.Oe, was measured for 40CFO60BCZT composite and a maximum magnetoelectric coupling coefficient, αME = 161mV/cm.Oe, was measured for the same composite at its resonance frequency.
Acknowledgements The authors are grateful to the financial support received for this research work from Science and Engineering Research Board (SERB), Department of Science and Technology (DST) through the grant number, SR/S3/ME/0012/2011. The support received from DST Purse & UPE grants for consumables, contingencies and infrastructural supports are greatly appreciated. The assistance received from Mr. P. Suresh, School of Physics, University of Hyderabad and Mr. Dinesh Kumar, Department of Physics, Indian Institute of Technology, Madras in carrying out the dielectric and magnetoelectric measurements, respectively, is gratefully acknowledged.
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[21] M.R. Anantharaman, S. Sindhu, S. Jagatheesan, K.A. Malini, P. Kurian, Dielectric properties of rubber ferrite composites containing mixed ferrites, J Phys D: Appl Phys 32 (1999) 1801-1810 [22] C.R. Zhou, X.Y. Liu, W.Z. Li, C.L. Yuan, Dielectric relaxor behavior of A-site complex ferroelectrics of Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BiFeO3, Solid State Commun. 149 (2009) 481 [23] Cao Thi My Dung, Nhu Hoa Tran Thi, Kieu Hanh Thi Ta, Vinh Cao Tran, Bao Thu Le Nguyen, Van Hieu Le, Phuong Anh Do, Anh Tuan Dang, Heongkyu Ju, Bach Thang Phan, Journal of Magnetics 20 (2015) 353-359 [24] Ling-Hua Pang, Wei-Jing Ji, Yi Zhang, Lei Wang, Shao-Tao Zhang, Zhen-Lin Luo, Yan-Feng Chen, Multiferroic properties of (1−x)[0.94Bi0.5Na0.5TiO3– 0.06BaTiO3]–xCoFe2O4 ceramics, J. Phys. D: Appl. Phys. 42 (2009) 045304 [25] S. Narendra babu, J.H. Hsu, Y.S. Chen, J.G. Lin, Magnetoelectric response in lead-free multiferroic NiFe2O4–Na0.5Bi0.5TiO3 composites, J. Appl. Phys. 109 (2011) 07D904. [26] C.E. Ciomaga, M. Airimioaei, V. Nica, L.M. Hrib, O.F. Caltun, A.R. Iordan, C. Galassi, L. Mitoseriu, M.N. Palamaru, Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method, J. Eur. Ceram. Soc. 32 (2012) 3325. [27] V. Corral-Flores, D. Bueno-Baques, D. Carrillo-Flores, J. A. Matutes-Aquino, Enhanced magnetoelectric effect in core-shell particulate composites, J. Appl. Phys. 99 (2006) 08J503.
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[28] Giap V. Duong, R. Groessinger, Effect of preparation conditions on magnetoelectric properties of CoFe2O4–BaTiO3 magnetoelectric composites, J. Magn. Magn. Mater. 316 (2007) e624–e627.
Figure 1. (a) X-ray diffraction spectra of sintered (1300°C) [xCFO-(1-x)BCZT] (x = 30, 40, 50 wt.%) particulate composite; (b) variation of sintered density as a function of CFO content; (c-e) FESEM micrographs of sintered xCFO-(1-x)BCZT particulate composites. Figure 2. (a) Magnetization vs magnetic field – hysteresis loops of [xCFO-(1-x)BCZT] (x = 30, 40, 50 wt.%) particulate composites sintered at 1300°C, Inset of (a). Variation of saturation magnetization and coercivity as a function of CFO content; (b). The effect of magnetic field on magnetostriction of [xCFO-(1-x)BCZT] and (c). variation of strain sensitivity as a function of magnetic field for [xCFO-(1-x)BCZT] particulate composites Figure 3. (a) Frequency dependence of dielectric constant and loss factor (Inset) of [xCFO-(1-x)BCZT] composites; (b, c) Variation of Dielectric constant with temperature of [xCFO-(1-x)BCZT] (x = 50 and 30 wt% respectively) composites, measured at 1 kHz to 1 MHz; (d) variation of transition temperature and degree of relaxation as a function of composition, Inset (d). ln((1/ε ) - (1/εm)) as a function of ln(T - Tm) performed at 10 kHz for the [xCFO-(1-x)BCZT] samples.
19
Figure 4. Polarization (P)-electric field (E) hysteresis loops of the (x)CFO-(1-x)BCZT magnetoelectric composites measured at room temperature; Inset. Variation of remnant polarization (Pr) and coercive field (Ec) as a function of CFO Figure 5. Variation of piezoelectric charge coefficient (d33-left Y-axis, lower panel), piezoelectric voltage coefficient (g33-Right Y-axis, lower panel), elastic stiffness (kt-left Y-axis, upper panel) and electromechanical coupling coefficient (C33-Right Y-axis, upper panel) as a function of CFO content of the CFO-BCZT composites Figure 6 (a). Magnetic field dependence of magnetoelectric coupling coefficients of CFO–BCZT sintered particulate composites, Inset. (a) Variation of maximum ME coefficient as a function of CFO content; (b). Frequency dependence of ME coefficient of 40CFO-60BCZT at fixed Hdc ~2560 Oe.
Table 1. Summary of the physical, Piezoelectric and magnetic properties of [xCFO-(1x)BCZT] composites. Density
d33
g33
C33
λmax
dλ/dH
(% TD)
(pC/N)
mV.m/N
x1010Nm-2
(ppm)
x10-9Oe-1
αME (mV/cm.Oe)
92
31
3.9
0.61
88
41
5.9
40CFO-60BCZT
88
60
8.1
0.47
78
35
7.75
30CFO-70BCZT
87
42
4.2
0.41
53
19
3.45
Composition
50CFO-50BCZT
20
Figure 1
Figure 2
Figure 3(a)
Figure 3(b)
Figure 3(c)
Figure 3(d)
Figure 4
Figure 5
Figure 6
PII: DOI: Reference:
S0272-8842(16)31419-5 http://dx.doi.org/10.1016/j.ceramint.2016.08.112 CERI13564
To appear in: Ceramics International Received date: 20 June 2016 Revised date: 4 August 2016 Accepted date: 18 August 2016 Cite this article as: Vinitha Reddy Monaji, J Praveen Paul, N. Shara Sowmya, A. Srinivas and Dibakar Das, Magneto-electric properties of in-situ prepared xCoFe2O4-(1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.112 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 galley proof before it is published in its final citable 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.
Magneto-electric properties of in-situ prepared xCoFe2O4-(1x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 particulate composites Vinitha Reddy Monajia, Paul Praveen Ja, N. Shara Sowmyab, A. Srinivasb, Dibakar Dasa* a
b
School of Engineering Sciences & Technology, University of Hyderabad, Hyderabad, 500046, India
Defence Metallurgical Research Laboratory, Advanced Magnetics Group, Hyderabad 500 058, India *
Corresponding author: Tel:- +91 4066794454. E-mail address : [email protected]
Abstract Magneto-electric particulate composites, [xCoFe2O4 – (1-x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3] (x = 30, 40, 50 wt.%), were synthesized in-situ by a modified sol-gel method. This novel synthesis method has led to uniform distribution of the magnetostrictive and piezoelectric phases in the composite samples and improved magnetoelectric properties. Using autocombustion method CFO nanoparticles were synthesized and were added to the precursor gel of BCZT, further the composite powders were calcined at 800°C and sintered at 1300°C. Composite containing 50wt% CFO showed a maximum magnetostriction (λmax) of ~ 88ppm and strain sensitivity (dλ/dH) of ~ 41 x 10-9 Oe-1. 40CFO-60BCZT composite showed a high piezoelectric voltage constant (g33) of 8x103
V.m/N. All the composites showed magnetoelectric effect and the maximum
magnetoelectric coupling coefficient (dE/dH = αME) was measured ~ 161mV/cm.Oe for 40CFO-60BCZT sample at its resonance frequency. The effect of microstructure on the magnetoelectric properties of [(x)CFO-(1-x)BCZT] composites has been studied and reported in this investigation as a function of its piezoelectric (BCZT)/ferrite (CoFe2O4) content. Keywords: A. Sintering; C. Dielectric properties; D. Ferrites; magnetostriction; magnetoelectric 1
1. Introduction Multiferroics are a class of materials comprising of more than one ferroic order. The coupling between different order parameters in these materials results in a new type of effect, such as magnetoelectric (ME) effect. Magnetoelectric response from a material is the appearance of an electric polarization (P) under an applied magnetic field (H) and vice versa [1,2,3] Various materials have been investigated for magnetoelectric response which includes single phase multiferroics and composites. Composites with different connectivity schemes such as, particulate composites (0-3), laminated composites (2-2), nano pillars (1-3) and multilayer composites containing thin films were studied [4,5]. The ME response of single phase multiferroic materials such as BiFeO3 and various manganites is either relatively week or occurs at low temperatures. In contrast, multiferroic composites, which consist of ferroelectric and ferri-/ferromagnetic phases, exhibit high magnetoelectric effect, which makes them potential candidate for multifunctional device applications [4,5,6]. Magnetoelectric response of a material is expressed by the magnetoelectric coupling coefficient, αME = dE/dH, where E and H are strength of the electric and magnetic fields respectively. In order to achieve a high magnetoelectric coupling coefficient (αME) in a composite, it is important to select a magnetic phase with large magnetostriction and high strain sensitivity and a piezoelectric phase with large piezoelectric response [7,8]. Cobalt ferrite (CoFe2O4, CFO) is a soft magnetic material with cubic spinel structure and shows very high magnetostriction and strain sensitivity compared to other cubic ferrites [9,10]. (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3, [BCZT] is an 2
efficient lead free piezoelectric material, which shows excellent piezoelectric properties [11]. Hence, in this study CFO and BCZT have been used as the magnetic and piezoelectric phase in the composite, respectively. Agglomeration of the magnetic phase is a major issue during synthesis of particulate composites. To avoid it suitable dispersion technique can be adopted to synthesize particulate composites which should lead to enhancement in the ME output. In this study, therefore, an in-situ synthesis technique is being employed to synthesis CFO-BCZT multiferroic ceramic particulate composites with varying CFO content (30 to 50 wt%) to avoid agglomeration of the CFO phase. Microstructural, magnetic, dielectric, piezoelectric properties and the magnetoelectric coupling behavior of the ME particulate composites have been systematically studied and correlated with variation in the magnetic/piezoelectric phase content. 2. Experimental procedure CoFe2O4–(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 composites with varying CoFe2O4 (CFO) content (30 to 50 wt%) were synthesized in-situ by a modified sol-gel method. Cobalt ferrite (CFO) was prepared by auto-combustion method [12, 13]. Co(NO3)2.6H2O and Fe(NO3)3.9H2O were used as source of Co+2 and Fe+3 ions respectively and glycine was used as fuel in the combustion reaction. For the stoichiometric reaction of cobalt ferrite the glycine (which acts as fuel) to nitrate (which act as oxidizer) ratio has to be 0.3, but in this experiment glycine to nitrate ratio has been taken as 0.5, which can be considered as fuel rich ratio [14]. Required amount of nitrates and glycine have been weighed and taken into a beaker. 50-100ml of water was added to the mixture and was stirred continuously to obtain homogeneous aqueous solution. The temperature of the solution was maintained at 80°C. The complete evaporation of water led to the formation of a 3
viscous gel. Auto-ignition of this viscous gel was initiated by increasing the temperature to 200⁰C. The ignition process was accompanied by the evolution of large volume of gases along with cobalt ferrite powder in the form of black ash. Since the time for the ignition process is very small, traces of decomposition products were removed by calcining the powder at 800°C for 2hrs to obtain desired single phase. (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 was prepared by sol-gel synthesis method [15]. Barium acetate (Ba(CH3COO)2), zirconium oxychloride (ZrOCl2·8H2O), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) and titanium isopropoxide (C12H28O4Ti) were used as starting materials. Stoichiometric amounts of (ZrOCl2·8H2O) and (Ca(NO3)2·4H2O) were added to ethanol and (Ba(CH3COO)2) was taken in acetic acid and stirred separately until they got dissolved completely. After dissolution, these three solutions were mixed together followed by the addition of stoichiometric amount of titanium isopropoxide to this solution. Calcined CFO powder in appropriate weight ratios was added to this precursor solution of BCZT and the resultant solution so obtained was continuously stirred and subjected to heating at 90°C until the viscous gel was formed. The resultant gel was dried and was calcined at 800°C for 2hrs for complete phase formation of BCZT. BCZT-CFO ME composite thus prepared by the modified sol-gel method is different than that prepared by simple mechanical mixing of calcined BCZT and CFO powder. Particulate composite prepared by mechanical mixing might contain aggregates of magnetic phase because of the magnetic interaction between the CFO particles. These aggregates form a conducting path, which reduces the effect of poling and thus the piezoelectric properties. In situ mixing of CFO particles in BCZT gel will certainly help in avoiding direct contact of the ferrite particles between them and is expected to eliminate the low conducting path there by reducing the leakage current 4
problem. The calcined ME composite powders were compacted into circular pellets and were subjected to sintering at 1300°C for 4 hrs. The calcined powders and sintered pellets were subjected to x-ray diffraction studies using a Bruker D8 Advance X-ray diffractometer with θ–2θ geometry using Cu Kα radiation in the 2θ range in between 20°-80°. The microstructural analyses of the sintered composites were performed by Carl Zeiss Ultra 55 Field Emission Scanning Electron Microscope (FESEM). The sintered composite pellets were polished and electroded using silver paste to make electrical contact for the dielectric and piezoelectric properties measurements. To ensure the proper electrical contacts, silver paste coated composite pellets were heat treated at 500°C for 15 min. The magnetic hysteresis measurement (M vs. H) was carried out using a Lakeshore (Model 7407) Vibrating Sample Magnetometer (VSM) up to 15kOe at room temperature (~ 298 K). Magnetostriction curves were obtained in a dc magnetic field setup with standard strain gauges (350W) at room temperature. The polarization versus electric field (P-E) hysteresis loops were measured at 1Hz using a Radiant Precision Premier II P-E loop tracer. The dielectric constants as a function of frequency and temperature were measured using Agilent E4980A precession LCR meter in the frequency range 20Hz to 2MHz. To induce piezoelectric properties in the composites the sintered composites were electroded and poled by applying a high dc. voltage in a silicone oil bath for 1hr. After 24h aging the poled samples were characterized for its piezoelectric properties (d33 and g33) using a Piezometer PM 300 (Piezotest, UK). The elastic stiffness constant C33 was also measured using an Agilent E4980A precision LCR meter from the resonance and anti-resonance frequencies in the frequency range 100 - 500 kHz on poled samples. Magnetoelectric (ME) measurements for all the CFOBCZT composites were carried out by a lock-in technique. A static magnetic field up to 5
±5 kOe and an alternative magnetic field of 3Oe at 1kHz frequency, generated by Helmholtz coils, was applied along the thickness of the sample. The voltage output generated from the composites was recorded using a lock-in amplifier (Stanford Research Systems - SR 830). 3. Results and discussion 3.1. Phase analysis of composite samples Figure 1(a) shows the X-ray diffraction (XRD) patterns of [xCFO-(1-x)BCZT] (x = 30,40 and 50 wt%) samples sintered at 1300°C. XRD peaks show the characteristics of cubic spinel ferrite (CFO) and perovskite BCZT phases. The individual phases were indexed using standard JCPDS and the diffraction peaks show no unidentified peaks, which suggests that there could be no reaction between the two phases during sintering. It can also be seen that as the CFO content increases from 30 to 50wt% the intensity of the ferrite peaks also increases. 3.2. Density and Microstructural analysis The densities of the sintered composites were measured using Archimedes’ method following the ASTM standard (C 373 – 88). The obtained dimensions of sintered composites having 50, 40 and 30 wt.% of CFO are 8.97 × 1.46, 8.99 × 1.60 and 9.11 × 1.62 mm2 (dia. × thickness) respectively. Figure 1b shows the increasing density with increasing CFO content of the sintered samples. The density increase could be attributed to the better densification characteristics of the CFO compared to BCZT in the sintering protocol followed to densify the composites. BCZT normally requires a higher sintering temperature of ≥1450°C to densify itself compared to that required for CFO (~1300°C) [9, 15]. Figures 1(c-e) show the FESEM micrographs of sintered [xCFO-(1-x)BCZT] (x = 30, 40 and 50 wt%) samples. Back scattered mode imaging of the composites show 6
distinct presence of two phases, where CFO grains appeared dark in color and BCZT grains are brighter due to large difference in their molecular weights. 3.3. Magnetization properties Figure 2a shows the magnetic hysteresis loops of (x)CoFe2O4-(1-x)BCZT composites measured at room temperature and the obtained hysteresis behavior is basically due to the presence of CoFe2O4 phase in the composite samples. Maximum magnetization (Mmax) at ~1.5 T field and coercivity (HC) for pure CoFe2O4 were observed to be 70.3 emu/g and 328 Oe respectively, which are in good agreement with the values reported earlier for CFO [16]. From the inset of Fig. 2(a), it can also be seen that maximum magnetization (Mmax) decreases from ~34.7 emu/g to ~17 emu/g and coercivity (HC) increases from 336 Oe to 362 Oe with increasing piezoelectric phase in the composition. Magnetizations of composite samples having different weight fractions of ferrite content were calculated based on maximum magnetization of pure CFO. Marginal decrease in measured magnetization was observed compared to the estimated (~35.2 emu/g to 21.1 emu/g for x = 50 to 30 wt% of ferrite) magnetization values. This could be due to the fact that in composite samples BCZT phase acts as a non-magnetic inclusion in the ferrimagnetic matrix (CFO) and reduces the magnetic interaction between the individual ferrite grains, resulting in degradation of magnetization of the composite samples with increasing BCZT content. Domain wall pinning increases as the non-magnetic BCZT phase increases, which results in increase in coercivity as shown in table 1. 3.4. Magnetostrictive properties Magnetostriction curves of (x)CoFe2O4-(1-x)BCZT composites measured at room temperature are presented in Fig. 2(b). For pure cobalt ferrite maximum 7
magnetostriction (λmax) of ~180ppm was observed, which are in agreement with the literature values [12,13]. The magnetostriction amplitude (λmax) is observed to decrease from -88 ppm for 50CFO-50BCZT to -53ppm for 30CFO-70BCZT. Dilution of the magnetic CFO phase with BCZT phase interrupts the magnetic contact, which results in decreasing magnetostriction with increasing non-magnetic BCZT phase. Piezo-magnetic coefficient (dλ/dH) obtained from the magnetostriction curve is shown in Fig. 2(c). The maximum piezo-magnetic coefficient for pure CFO is observed to be ~50 x 10-9 Oe-1, correlates well with the literature value [10,12]. Maximum dλ/dH decreases from ~41 x 10-9 Oe-1 for 50CFO-50BCZT to ~19 x 10-9 Oe-1 for 30CFO-70BCZT. As the nonmagnetic phase increases it impedes the domain wall motion, which is the main cause for magnetostriction behaviour at lower fields. Anisotropy of the system increases with increasing domain wall pinning, resulting in decreasing piezo-magnetic coefficient. 3.5. Dielectric properties Dielectric constants of (x)CFO-(1-x)BCZT composite samples were determined as a function of frequency at room temperature and the variations are shown in Fig. 3(a). From fig. 3(a), the dielectric constant is seen to decrease with increasing frequency and then reaches a constant value for the composites containing 40 and 30 wt% of CFO. However, for the composite containing 50 wt% of CFO the rapid decrease in dielectric constant is observed. This variation in dielectric constant can be explained on the basis of Maxwell and Wagner two layer model [17]. Due to the hopping of electrons at lower frequencies, electrons pile up at the grain boundaries because of its resistive nature and produce more interfacial polarization. At higher frequencies, however, the interfacial polarization decreases, because the electron exchange rate between ions cannot cope up with the change in external applied field, resulting in decrease in dielectric constant with 8
increasing frequency. CFO has low dielectric constant compared to BCZT; hence it resulted in decrease in dielectric constant with increase in CFO concentration in the composite samples except for 50CFO-50BCZT composition. The large value of dielectric constant (ε) for 50CFO-50BCZT sample than pure BCZT at lower frequencies is attributed to the additional space charge polarization, which is the result of accumulation of space charge carriers at insulating grain boundaries produced from the electron hopping between Fe+2 ↔ Fe+3. The variation in dielectric loss as a function of frequency is shown in the inset of Fig.3(a). It is observed that the dielectric loss decreases with increasing frequency and this could be due to the fact that, as the frequency increases, beyond a certain frequency, the space charge carriers cannot follow the field and the alternation of their direction lags behind the change in field direction. The dielectric loss is seen to increase with increasing ferrite concentration and it is also observed that dielectric loss of 50CFO50BCZT composite sample shows abnormal peaking behaviour at higher frequencies. This type of resonance peak occurs, when the applied field frequency matches the hopping frequency of electrons between Fe+2 and Fe+3 ions in the sample [18]. Figs. 3(b) and (c) show the temperature dependence of dielectric constant of (x)CFO-(1-x)BCZT, (x = 50, 30 wt%) composites measured at varying frequencies. Dielectric anomaly at two different temperatures is clearly observed for all composite samples. The first dielectric anomaly appears around 115 -126˚C with varying ferrite concentration from 30% to 50%. These temperatures correspond to the ferroelectric transition temperature of BCZT phase. The second peak at ~520˚C indicates the magnetic transition temperature of CFO phase. Similar behaviour in dielectric constant has been reported for BZT+CMFO composite samples in the literature [19]. Increase in 9
dielectric constant with increasing ferrite content could be attributed to increasing interfacial polarization as observed earlier in fig. 3(a). From figs. 3(b) and (c) it is also observed that maximum dielectric constant is increased with increasing temperature. In general, the dielectric constant of a material is mainly due to interfacial, dipolar, ionic and electronic polarizations. At lower frequencies interfacial and dipolar polarizations are dominant mechanisms and these polarizations are strongly dependent on temperature [20]. Dipolar polarization decreases with increasing temperature; hence interfacial polarization has more pronounced effect on increasing the dielectric constant with increasing temperature. The observed trend in dielectric constant (ɛ) with temperature (T) can be explained on the basis of electron hopping between neighbouring ions as discussed below. In presence of an external electric field a local distortion of the electron cloud (associated with the electron hopping between Co+2 and Fe+3 ions) takes place in the direction of the applied field, which affects the polarization of the system. As the temperature increases thermal activation energy required for the excitation of charge carriers increases, which results in increase in the dielectric constant of the samples [21]. The decreasing dielectric constant beyond transition temperature could possibly be attributed to the increased randomness in the orientation of the charge carriers and their inability to orient in the direction of the applied field. The transition temperature of BCZT phase is observed to shift towards the high temperature side with increasing CFO concentration in the composite samples. In the composite samples, incorporation of CFO (non-ferroelectric) in BCZT phase dilutes the ferroelectric interaction, resulting in remarkable dispersion in the transition temperature of BCZT phase, similar to what observed in relaxor ferroelectrics. Variation in transition temperature with composition is shown in Fig. 3(d). From figs. 3(b) and (c), it is clearly 10
seen that the FWHMs of the dielectric constant peaks are increased. This indicates a relaxor behaviour, which can be explained based on the Curie-Weiss law, ! ɛ
!
− ɛ = $(% − %& )' #
(1)
where C is the Curie-Weiss constant, which describes the width of the diffuse phase transition. ‘γ’ is an empirical parameter and describes the degree of relaxation and can vary between 1 and 2. For a perfect normal ferroelectric g = 1 and for a perfect relaxor ferroelectric the value of g is 2 [22]. Inset of fig. 3(d) shows the plot of ln ((1/ɛ)-(1/ ɛm)) as a function of ln (T-Tm) at 10 kHz for (x)CFO-(1-x)BCZT composite samples. The value of g can be obtained from the slope of linear fitting of these plots. From fig. 3(d), it is observed that the value of g is nearly 2 for all composite samples, meaning all the samples are exhibiting relaxor behavior. The observed relaxor behaviour in composite samples could be due to the increased chemical inhomogeneity in the system. Such type of relaxor behaviour has previously been reported for some magnetoelectric composite samples [23]. 3.6. Piezoelectric properties Figure 4 shows the room temperature polarization (P)-electric field (E) hysteresis loops of the (x)CFO-(1-x)BCZT magnetoelectric composites. It is clearly seen from Fig. 4, that even under the application of a high electric field (~30 kV/cm), the hysteresis loops of the composite samples were not saturated and also the loops for the samples with high CFO content exhibited some curvature near saturation, which could be attributed to the large leakage current through the low resistive grains of CFO phase. Similar kind of behavior was reported for (1-x) [0.94Bi0.5Na0.5TiO3– 0.06BaTiO3]–xCoFe2O4 magnetoelectric ceramics by L.H. Pang et al. [24]. Variations of remanent polarization and coercive field as a function of CFO content are shown in 11
inset of Fig. 4. The hysteresis loops are observed to broaden with increasing concentration of CFO phase in the composite samples, which indicates the increase in dielectric loss as the ferrite concentration increases and this is in accordance with the result shown in inset of Fig. 3(a). Highest density pellets of each composition of (x)CFO-(1-x)BCZT composites were subjected to electrical poling for the process of orienting the ferroelectric domains. The piezoelectric properties, g33 and d33, of all the composite samples were obtained from the Piezometer (PM300) after aging for 24h after poling. The lower panel of figure 5 shows the variation of d33 (left Y- axis) and g33 (right Y-axis), as a function of composition. A maximum piezoelectric charge coefficient, d33, of ~60pC/N and piezoelectric voltage constant, g33, of ~8.1mV/m.N were obtained for 40CFO-60BCZT. ME composites show very low piezoelectric properties compared to pure piezoelectric phase, which is primarily attributed to the presence of the magnetic phase (CFO), which is conductive in nature compared to BCZT. Hence, composite containing magnetic phase cannot be poled properly even at a high electric field due to its low resistive nature, which is clearly indicated from the dielectric measurements. The value of the piezoelectric coupling factor kt and elastic stiffness C33 were measured for all the composites by resonance method. The upper panel of figure 5 shows the variation in C33 (right Y-axis) and kt (left Y-axis), respectively, as a function of CFO content of the composites. The resonance (fr) and antiresonance (fa) frequencies were obtained from the impedance peaks against frequency scan ranging from 100 kHz to 500 kHz of the optimally poled samples for all the compositions. These peak values were used to evaluate the elastic stiffness for all the composite samples. A maximum elastic stiffness (C33) of 0.66 x1010 N/m2 and a piezoelectric coupling factor kt of 0.243 12
were obtained for the 50CFO-50BCZT composite. Due to the difference in sintering behavior of CFO and BCZT, low density was observed for all the composite samples. Because of the presence of high porosity and low resistive ferrite phase proper poling was not achieved, which resulted in degrading of piezoelectric properties of the composites compared to pure BCZT. In our previous studies d33 and g33 of ~637 pC/N and ~29 mV.m/N, respectively, were achieved for highly dense BCZT sample sintered at 1550°C [15]. In the present study, however, maximum d33 and g33 of ~60 pC/N and ~8.1 mV.m/N, respectively, were observed for 40CFO-60BCZT composite sample. 3.7. Magnetoelectric effect Magnetoelectric property of a material is expressed in terms of the magnetoelectric coefficient αME of the system. An expression for the ME voltage coupling coefficient (αME) was given by Zubkov [4] as, 12
+,- = mv .g33 C33 / .1-mv / 0 4 p
13 m
(2)
where, mv is the volume fraction of the piezoelectric phase (p), g33 is the piezoelectric voltage coefficient, C33 is the elastic stiffness and dl/dH is the strain derivative. From equation (2) it is observed that for a system to show high magnetoelectric effect, the piezoelectric parameters (g33 and C33) and piezomagnetic coefficient (dλ/dH) should be high. Hence, in this study the above mentioned parameters were measured. Figures 6 (a) and (b) show the Magnetoelectric (αME) curves of xCFO-(1x)BCZT composites measured at magnetic field up to ±5000 Oe using the lock-in technique. A superimposed alternating field of 3Oe was simultaneously applied by using Helmholtz coils. A maximum αME value of about 7.75 mVcm−1Oe−1 was observed for 40CFO-60BCZT sample at an applied static dc field of 2560Oe with 1kHz frequency of the a.c. magnetic field. All the composites showed magnetoelectric 13
behaviour and the ME values initially increase with increase in the magnetic field followed by a decrease with further increase in the field. This behaviour is attributed to the high striction developed by the magnetic phase which is transferred to the piezoelectric phase developing a voltage output. The αME value reported in this investigation is higher than those obtained for NBT-NFO [25], NBT-CFO [26] and comparable to BTO-CFO [27, 28] lead free based particulate composites. 40CFO60BCZT showed a maximum αME of 161mV/cm.Oe at its resonance frequency of 365 kHz with fixed dc field of 2560 Oe. For the CFO-BCZT particulate composite prepared by mechanical mixing method (which is communicated separately), the maximum αME at its resonance frequency is observed to be 118mV/cm.Oe. ~35% increase in magnetoelectric coefficient is observed for CFO-BCZT particulate composites prepared by in-situ synthesis technique compared to mechanical mixing method. For a composite to have large ME coefficient perfect coupling between the magnetic and electric order parameters leading to proper strain transfer from one phase to the other is expected. But, in the presence of internal defects such as, porosity the elastic stiffness of the composite will be reduced, which will suppress the coupling between the ferroic phases leading to reduction in ME coefficient. 4. Conclusions Magnetoelectric properties of in-situ prepared xCoFe2O4 – (1x)(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (x = 30, 40, 50 wt.%) particulate composites were investigated and reported in this work. The magnetisation (Ms, Hc) and magnetostrictive (λ) properties were found to vary with the magnetic content of the composites. A maximum saturation magnetization of 34.7 emu/g and high strain sensitivity dλ/dH of 41x10-9Oe-1 were measured for the 50CFO-50BCZT composite. All 14
the composites showed enhanced magnetoelectric effect and the maximum magnetoelectric coupling coefficient, αME = 7.75mV/cm.Oe, was measured for 40CFO60BCZT composite and a maximum magnetoelectric coupling coefficient, αME = 161mV/cm.Oe, was measured for the same composite at its resonance frequency.
Acknowledgements The authors are grateful to the financial support received for this research work from Science and Engineering Research Board (SERB), Department of Science and Technology (DST) through the grant number, SR/S3/ME/0012/2011. The support received from DST Purse & UPE grants for consumables, contingencies and infrastructural supports are greatly appreciated. The assistance received from Mr. P. Suresh, School of Physics, University of Hyderabad and Mr. Dinesh Kumar, Department of Physics, Indian Institute of Technology, Madras in carrying out the dielectric and magnetoelectric measurements, respectively, is gratefully acknowledged.
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Figure 1. (a) X-ray diffraction spectra of sintered (1300°C) [xCFO-(1-x)BCZT] (x = 30, 40, 50 wt.%) particulate composite; (b) variation of sintered density as a function of CFO content; (c-e) FESEM micrographs of sintered xCFO-(1-x)BCZT particulate composites. Figure 2. (a) Magnetization vs magnetic field – hysteresis loops of [xCFO-(1-x)BCZT] (x = 30, 40, 50 wt.%) particulate composites sintered at 1300°C, Inset of (a). Variation of saturation magnetization and coercivity as a function of CFO content; (b). The effect of magnetic field on magnetostriction of [xCFO-(1-x)BCZT] and (c). variation of strain sensitivity as a function of magnetic field for [xCFO-(1-x)BCZT] particulate composites Figure 3. (a) Frequency dependence of dielectric constant and loss factor (Inset) of [xCFO-(1-x)BCZT] composites; (b, c) Variation of Dielectric constant with temperature of [xCFO-(1-x)BCZT] (x = 50 and 30 wt% respectively) composites, measured at 1 kHz to 1 MHz; (d) variation of transition temperature and degree of relaxation as a function of composition, Inset (d). ln((1/ε ) - (1/εm)) as a function of ln(T - Tm) performed at 10 kHz for the [xCFO-(1-x)BCZT] samples.
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Figure 4. Polarization (P)-electric field (E) hysteresis loops of the (x)CFO-(1-x)BCZT magnetoelectric composites measured at room temperature; Inset. Variation of remnant polarization (Pr) and coercive field (Ec) as a function of CFO Figure 5. Variation of piezoelectric charge coefficient (d33-left Y-axis, lower panel), piezoelectric voltage coefficient (g33-Right Y-axis, lower panel), elastic stiffness (kt-left Y-axis, upper panel) and electromechanical coupling coefficient (C33-Right Y-axis, upper panel) as a function of CFO content of the CFO-BCZT composites Figure 6 (a). Magnetic field dependence of magnetoelectric coupling coefficients of CFO–BCZT sintered particulate composites, Inset. (a) Variation of maximum ME coefficient as a function of CFO content; (b). Frequency dependence of ME coefficient of 40CFO-60BCZT at fixed Hdc ~2560 Oe.
Table 1. Summary of the physical, Piezoelectric and magnetic properties of [xCFO-(1x)BCZT] composites. Density
d33
g33
C33
λmax
dλ/dH
(% TD)
(pC/N)
mV.m/N
x1010Nm-2
(ppm)
x10-9Oe-1
αME (mV/cm.Oe)
92
31
3.9
0.61
88
41
5.9
40CFO-60BCZT
88
60
8.1
0.47
78
35
7.75
30CFO-70BCZT
87
42
4.2
0.41
53
19
3.45
Composition
50CFO-50BCZT
20
Figure 1
Figure 2
Figure 3(a)
Figure 3(b)
Figure 3(c)
Figure 3(d)
Figure 4
Figure 5
Figure 6