Tuning the dielectric, ferroelectric and electromechanical properties of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites

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Tuning the dielectric, ferroelectric and electromechanical properties of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites Aditya Jain, Y.G. Wang∗, N. Wang, Y. Li, F.L. Wang College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211106, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Multiferroic Composites Dielectric Ferrite Perovskite

We report the successful synthesis of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites showing significant improvement in electromechanical and magnetoelectric properties. All the composite samples have formed a diphasic perovskite-ferrite composite without the presence of any impurity or intermediate phase. The bare as well as composite samples have shown classical dielectric behavior even at higher ferrite substituted samples. The electrical characteristics of composite samples have shown slight deterioration, which is mainly attributed to non-ferroelectric MnFe2O4. However, the composites still exhibit high enough piezoelectric behavior and the modification in the electromechanical response of composites is mainly caused by a change in applied stress with MnFe2O4 addition. The M-H loops of composites have demonstrated a ferrimagnetic behavior with a substantial increase in saturation magnetization on increasing the ferrite concentration. Further, the composites have shown better coupling between the ferroelectric and ferrimagnetic phases, which has resulted in an improved magnetoelectric characteristic. The role of oxygen vacancies on ferroelectric and magnetic properties of prepared composites has been systematically studied.

1. Introduction Multiferroic ceramics are those materials which simultaneously exhibit at least two out of three ferroic properties, such as ferroelectricity, ferroelasticity and ferromagnetism. An interrelation between these parameters makes these materials a potential candidate for various applications, e.g., ferroelectric transducers, magnetic data storage, piezoelectric sensors, ceramic capacitors, electronic filters, etc. [1–3]. Numerous investigations have been carried out in the past to obtain ferroelectric and ferromagnetic ordering in a single-phase material. However, very few materials possess both the properties in a single phase. Ferroelectricity generally exists in a material having empty d orbitals, whereas, a partially filled d or f orbital is required for ferromagnetism [4]. This rare occurrence exists in BiFeO3, which has been thoroughly investigated for its multiferroic characteristics. However, single-phase materials possess very weak magnetoelectric coupling and low critical temperature. Therefore, the primary challenge in the designing of multiferroic materials is to integrate ferroelectricity and ferromagnetism with improved coupling between the ferroic characteristics, especially around room temperature. Literature reveals that multiferroic ceramic composites that exhibit ferroelectricity and ferro/ ferrimagnetism may result in high magnetoelectric response around room temperature. In composites, the cross interaction between ∗

individual phases results in high magnetoelectric coupling effect. Numerous works have been reported in which various perovskite ferroelectric materials, such as BaTiO3, PbTiO3 and PbZrO3 have been combined with cubic spinal ferro/ferrimagnetic materials namely CoFe2O4, NiFe2O4, NiCo2O4 and MnCo2O4 to improve the multiferroic characteristics of ceramics [5–7]. However, composites generally exhibit agglomerated particles with an irregular distribution, which results in an increase in conductivity of the material and thereby, the sample becomes very lossy which results in very weak magnetoelectric coupling [8,9]. This issue can be overcome by carefully optimizing the calcination and sintering temperature of the individual as well as composite phases. BaTiO3 (BT) possess excellent dielectric, ferroelectric and piezoelectric characteristics and has been investigated thoroughly in the past few decades for various applications in the field of microelectronics. A very weak magnetoelectric coupling has been observed in single-phase BaTiO3, which can be improved by combining it with suitable ferro/ ferrimagnetic dopants or magnetically active guest compounds. The substitution/doping of various isovalent and aliovalent ions at Ba2+ and Ti4+ site can significantly improve its electrical and magnetic properties. In the author's previous research work, the substitution of Ca2+ and Sr2+ at Ba2+ site has resulted in enhanced dielectric, ferroelectric and piezoelectric characteristics [10,11]. Therefore, in the

Corresponding author. E-mail address: [email protected] (Y.G. Wang).

https://doi.org/10.1016/j.ceramint.2019.11.257 Received 11 September 2019; Received in revised form 27 November 2019; Accepted 27 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Aditya Jain, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.257

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a modified mechanochemical activation method. Both the phases were synthesized separately and mixed in an appropriate amount to obtain the final composite materials. At first, Ba0.83Ca0.10Sr0.07TiO3 phase has been prepared using analytical grade BaCO3 (99.99%), SrCO3 (99.99%), CaCO3 (99.95%) and TiO2 (99.99%) as raw materials. The materials were dried to remove any moisture and subsequently, ballmilled at 350 RPM for effective 24 h. An agate jar of 250 ml capacity and ZrO2 balls of 5 mm diameter were used as the milling medium. 3 ml of ethanol has been mixed with the raw mixture to compensate for the heat generated due to friction during the milling process. The obtained powders were calcined at 1050 °C for 4 h. The calcined powder is milled again for 1 h and sintered at 1250 °C to finally obtain the ferroelectric Ba0.83Ca0.10Sr0.07TiO3 (BSCT) phase. Next, MnFe2O4 (MFO) has been prepared by using MnO (99.99%) and Fe2O3 (99.995%) as the raw materials. The materials were ball milled in the same milling conditions as for the BSCT and subsequently calcined at 825 °C for 2 h. The calcined powder is again finely crushed in ball milling for 1 h and sintered at 1050 °C for 2 h to obtain the pure phase MnFe2O4. Finally, BSCT and MFO phases were mixed together in suitable amounts and annealed at 900 °C for 1 h to achieve the final composites. These composite powders were pressed into the form of disk-shaped pellets of thickness nearly 1 mm for electrical measurements. Thereafter, these composites were named BTMF0, BTMF5, BTMF10, BTMF20 and BTMF30 for x = 0.0, 0.05, 0.10, 0.20 and 0.30, respectively.

quest to develop a multifunctional novel composite material, a modified BT composition has been considered to further improve the magnetic characteristics with the help of guest compounds. MnFe2O4 (MFO) is a well-known ferrite material which exhibits high magnetostriction coefficient and, therefore, it can serve as a suitable candidate for obtaining a multiferroic heterostructure with BT. The redistribution of Mn2+ in the octahedral site and Fe3+ at the tetrahedral site of MnFe2O4 ferrite lattice results in the formation of two strong magnetic sublattices in which the ferromagnetism increases because of high inter sublattice super-exchange interaction [12]. The addition of MnFe2O4 in modified BaTiO3 may result in the formation of exchange bias at the interphase between the two phases which may lead to relatively low interfacial reaction (or diffusion) between the spinal and perovskite phase [13]. This further results in strain mediated coupling between magnetic ferrite and ferroelectric perovskites to improve the magnetic anisotropy of parent modified BaTiO3 compound [14,15]. Mechanochemical activation technique is known to be one of the best modern method to produce nanomaterials. This method is cost effective and commercially viable for synthesizing ceramic materials at relatively low temperature compared to conventional solid-state method. Mechanochemical synthesis do not uses heat to cause chemical reaction, which is very necessary for preparing ceramic materials that may degrade or oxidize at high temperature. In the present work, both the constituent phases are synthesized using mechanochemical activation technique. There are many studies, which report the synthesis and characterization of BT based composites. In these composites, BT acts as the dielectric, ferroelectric or piezoelectric component and various ferrites such as CoFe2O4, NiFe2O4 and ZnFe2O4, hexaferrites such as BaFe12O19 and SrFe12O19 and perovskite such as BiFeO3 serve as the magnetostrictive component [9,16–20]. Out of the various combinations, perovskite-ferrite composites are mostly studied due to their higher magnetostriction coefficient than that of the other BT based composites. However, most of the reports in the literature on BT based composites cite much lower ME coupling coefficient than that has been achieved from theoretical predictions. This small value may be attributed to the microstructure of the composite, interface bonding, type of connectivity and influence of elastic coupling as reported in previous literature [21–24]. In this work, by combining the excellent ferroelectric characteristics of modified BT and ferrimagnetic characteristics of MFO, we have synthesized (1-x)Ba0.83Ca0.10Sr0.07TiO3-(x)MnFe2O4 (x = 0.0, 0.05, 0.10, 0.20 and 0.30) and systematically investigated their structural, dielectric, ferroelectric, piezoelectric and magnetic properties. Ba0.83Ca0.10Sr0.07TiO3 (BSCT) and MFO have been selected believing that the combination of these materials will result in the better coupling between the two phases. Another reason for choosing BSCT and MFO in this research work is not only their ferroelectric and magnetic properties, respectively, but also their high electrical resistance around the room temperature, which will prevent the discharging process during the measurement [25,26]. Further, in comparison to thin films, bulk perovskite/ferrite ceramics allow an increased degree of freedom, which can tune the ferroelectric and ferromagnetic phases at nanoscale resulting in an improved magnetoelectric coupling. In addition to this, these materials are chemically and mechanically stable and do not possess any toxicity, which is very important for commercial application and environment point of view. The primary objective of this research work is to develop an environment-friendly novel multiferroic composite material by effectively controlling the synthesis conditions of the materials.

2.2. Characterization For phase and crystallographic study, XRD measurements have been carried out using Bruker D8 Advance X-ray diffractometer. In order to further confirm the phase, X'pert highscore plus software has been used to analyze the XRD patterns. The microstructure of the prepared materials has been examined using Hitachi SU 8010 scanning electron microscope. For the electrical measurement, the ceramic disks were carefully polished to achieve the desired thickness and subsequently coated with silver paste to form metallic electrodes. Agilent 4294A LCR meter has been used to measure the frequency-dependent dielectric properties of pure and composite pellet samples in 27–350 °C temperature range. Electric field vs. polarization loop measurements and electric field vs. strain measurements of pure and composite pellet samples were carried out using Radiant Technology precision premier II ferroelectric tester. The piezoelectric measurement and ME coefficient measurement have been performed on a poled pellet sample. The pellet samples were poled along the thickness direction at an optimized electric field of 18 kV/cm for 30 min. The longitudinal piezoelectric constant (d33) of the samples has been measured using PM300 d33 piezometer system. Koehler automatic dielectric breakdown tester has been used to measure the electrical breakdown strength measurement of pellet samples. Magnetic measurements of powder samples were carried out using VSM (HH-10). The temperature-dependent ME coefficient of poled pellet samples has been measured along the thickness direction using a dynamic field method by an in-house built ME measurement system. 3. Results and discussion 3.1. Phase and crystallographic analysis The X-ray diffraction patterns of bare and composite samples are depicted in Fig. 1(a–f). The bare BTMF0 sample is present in pure phase exhibiting a tetragonal crystal structure with P4mm space group. In all the composite samples, the XRD pattern shows a mixture of two clear phases, which include perovskite Ba0.83Ca0.10Sr0.07TiO3 (BSCT) and MnFe2O4 (MFO) phase. The Rietveld refined XRD patterns of four composite samples confirm that both the phases have retained their identity and there is no intermediate or impurity phase present in the

2. Experimental section 2.1. Synthesis process The multiferroic nanocomposites of (1-x)Ba0.83Ca0.10Sr0.07TiO3xMnFe2O4 (x = 0.0, 0.05, 0.10, 0.20 and 0.30) were synthesized using 2

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Fig. 1. Comparative and Rietveld refined X-ray diffraction patterns of BTMF samples: a) combined XRD pattern of the five synthesized samples, b) BTMF0, c) BTBM5, d) BTMF10, e) BTMF20 and f) BTMF30.

composite samples. The patterns are well fitted with standard powder diffraction data of BaTiO3 (JCPDS files no.: 01-074-1956) and MnFe2O4 (JCPDS files no.: 01-073-1964). In all the four composite samples, BSCT is present in the perovskite phase with tetragonal crystal structure and P4mm space group, whereas MnFe2O4 is present in spinal phase with cubic crystal structure and Fd3m space group. These results further confirm that no significant structural changes have occurred after the incorporation of MFO in BSCT. The Rietveld refinement of composite samples approves the successful preparation of diphasic multiferroic composite ceramics with no or little chemical reaction between the ferroelectric and ferrite phases. Further, it also suggests that there is no diffusion between the two phases during the final heat-treatment process. Such a structure is always desirable to obtain better interaction between ferroelectric and ferromagnetic phases, which subsequently induces large magnetoelectric coupling in composite samples. Moreover, the addition of MFO has resulted in slight changes in lattice parameters of BSCT phase. The detailed Rietveld refined phase and structural parameters are shown in Table 1. The lattice parameters of perovskite and ferrite phases are shown separately to clearly understand the changes in lattice parameters. The lattice parameters ‘a’ and ‘c’ decrease slightly with an increase in MFO concentration, which may be attributed to rearrangement of Ba, Ca, Sr, Ti and O atoms of BSCT lattice after the addition of guest compound [27]. Further, the lattice parameters of MFO phase increase with the increase of MFO concentration in BSCT. These changes in lattice parameters have resulted in compression of BSCT lattice and expansion of MFO lattice with increasing MFO concentration. In addition to this, the modification in lattice parameters also leads to structural rearrangement that induces strain, which consequently affects the electrical characteristics of the materials. Further, it can be observed that the peak intensity of MFO phase increases and BSCT phase decreases with increasing MFO concentration, which

further confirms that the two phases coexist in composite samples. With an increase in MFO concentration, the cell volume of BSCT phase decreases and that of MFO phase increases. This decrease in cell volume of BSCT phase may be ascribed to the compressive strain of MFO phase and is one of the most critical parameters that affect the magnetic as well as magnetoelectric characteristics of composite samples.

3.2. Microstructural studies SEM micrograph of sintered pellets and grain size distribution of all the five samples are shown in Fig. 2(a–j). A significant change in surface morphology has been observed with MFO addition; however, with the increase in MFO concentration, the grain morphology has remained approximately same with a marginal difference in the grain size of the samples. BTMF0 sample has shown the presence of rather small-sized grains and higher density compared to other four composite samples. The grain density results are in good agreement with the Rietveld analysis, where also BTMF0 sample has shown the highest theoretical density. Further, BTMF10 sample exhibit better grain density and much higher grain uniformity among the four composite samples. The decrease in density of composite samples may be ascribed to the lower crystallographic density of MnFe2O4 (4.72 g/cm3) as compared to parent Ba0.83Ca0.10Sr0.07TiO3 (5.96 g/cm3). The theoretical density measured from Rietveld analysis is shown in Table 1. Further, in composite ceramics, the differences in the density of parent and guest materials result in abnormal changes in the surface morphology of final bulk composite, which consequently affects the dielectric, ferroelectric and magnetic characteristics of the material. All the five samples have shown clearly distinguishable grain boundary and the average grain size of the samples increases with MFO addition. The average grain size of the samples has been calculated using ImageJ software and found to be 244, 269, 314, 321 and 366 nm 3

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Table 1 Rietveld refined XRD parameters for bare BSCT and BSCT-MFO composite samples. Sample

Space group

Crystal structure

Lattice parameters (Å)

Cell volume (Å3)

Theoretical density (gm/cm3)

Experimental density (gm/ cm3)

χ2

Rexp

Rf

RBragg

BTMF0

P4mm

Tetragonal

64.9191

5.8629

5.7122

2.93

4.28

5.74

2.26

P4mm

Tetragonal

64.8044

5.3925

5.2785

3.07

5.92

5.18

3.16

Fd3m

Cubic

a = 4.0129 c = 4.0314 a = 4.0101 c = 4.0299 a = 8.4988

P4mm

Tetragonal

64.7477

Fd3m

Cubic

a = 4.0077 c = 4.0312 a = 8.5021

P4mm

Tetragonal

64.7540

Fd3m

Cubic

a = 4.0071 c = 4.0328 a = 8.5089

P4mm

Tetragonal

64.7405

Fd3m

Cubic

a = 4.0043 c = 4.0376 a = 8.5096

BTMF5

BTMF10

BTMF20

BTMF30

BSCT phase MFO phase BSCT phase MFO phase BSCT phase MFO phase BSCT phase MFO phase

613.864

2.78 5.4587

5.3017

3.71

5.18

6.29

614.580

2.84 3.71

5.2194

5.1248

2.87

5.08

5.31

616.056

2.91 3.29

5.1922

616.208

5.0714

3.93

5.14

6.05

3.51 4.82

relaxation process associated with the reduction in polarization [32]. At a low frequency of operation, the molecular dipoles have a sufficient amount of time to switch the ferroelectric domains and subsequently, a higher dielectric constant is obtained. However, at high frequency, the interfacial polarization does not catch up with the changes in frequency and thus, do not respond to the electric field, resulting in the decrease of dielectric constant [33]. In order to ensure that the interfacial polarization can keep up with the variation in frequency, a relatively high transition temperature is required for the dipoles to contribute to the polarization at high frequency and therefore, the dielectric constant peak of samples move towards the high-temperature side with an increase in frequency. Another important aspect with the increase in MFO concentration is the restricted dipole rotation, which hinders the contribution of dipolar polarization and consequently reduced dielectric constant [34]. The diffuseness parameter has been calculated to better understand the phase transition characteristics. The diffuseness parameter has been calculated at an operating frequency of 1 kHz and shown in the insets of dielectric loss curve for the corresponding samples. The phase transition has become increasingly diffused with the increase in MFO concentration, suggesting a slight deviation from classical ferroelectric behavior. Further, the obtained results in this work are comparable or even better compared to previous reports on BT based composites [7,35]. The dielectric loss of composites increases gradually with MFO addition and becomes maximum for BTMF30. This increase in dielectric loss can mainly be attributed to the reformation of conductive networks mainly triggered by the overall increase in conductivity of the material by MFO phase [36]. Further, the increase in dielectric loss at low frequency is associated with low-frequency relaxation of the dipoles. The MFO substitution in BSCT has resulted in slight deterioration of dielectric characteristics. These results are expected as the main objective of this work is to make a multiferroic composite having optimal electrical and magnetic characteristics. Therefore, the achieved values of dielectric constant and dielectric loss are still appreciable and can be used in fabricating electronic devices based on multiferroic materials.

for BTMF0, BTMF5, BTMF10, BTMF20 and BTMF30 with an accuracy of ± 5 nm, respectively. Further, it is observed that a lot of pores exist in all four composite samples, which suggest that the abnormal grain growth has been restricted by the addition of MFO in BSCT. With the increase in MFO concentration beyond x = 0.10, the non-uniformity as well as the grain size of the material gradually increases and is found to be maximum for BTMF30 sample. The increase in grain size with an increase in MFO concentration may be attributed to different thermal characteristics of BSCT and MFO phases. The lower sintering temperature of MFO phase than that of BCST phase results in slight acceleration in the grain growth of the composite samples. 3.3. Dielectric studies The variation of dielectric constant and dielectric loss with temperature and frequency are shown in Fig. 3(a–j). The samples were tested from room temperature to 350 °C at 1 kHz to 1 MHz frequency range. Interestingly, all the sample have shown classical dielectric nature, which is mainly ascribed to parent BSCT phase. The dielectric properties of materials are generally dependent on temperature and frequency as different structure and molecule groups have different response characteristics to these two parameters. Therefore, the dielectric characteristics of composite materials consisting of two dissimilar compounds may show slightly different frequency dependence [28]. Typically, the frequency dependence of dielectric constant in an insulating ceramic composite is relatively weak. However, the previous literature [29,30] suggest that the conducting ceramic composites have strong frequency-dielectric constant dependence, which is ascribed to low-frequency leakage current and electrode polarization; such has been observed in the present case. BTMF0 sample has shown the highest dielectric constant among the five synthesized samples. The dielectric constant decreases with increase in MFO concentration. In addition to this, BSCT/MFO composites have shown frequency dispersion and therefore, different phase transition temperature has been observed at different frequencies. Further, the phase transition temperature of all the composite samples is slightly increased with an increase in frequency, indicating a frequency dependence. This frequency dependence is much more prominent for samples with higher MFO concentration. The interfacial polarization effect also known as Maxwell-Wagner-Sillars effect is mainly responsible for such frequency dependence [31]. This effect arises due to the accumulation of free charge carriers at the BSCT/MFO interface. The changes in frequency from 1 kHz to 1 MHz has resulted in a decrease in dielectric constant value around the phase transition temperature, which is due to

3.4. Ferroelectric studies The electric field dependent polarization characteristics (P-E loops) are shown in Fig. 4(a–b). The P-E loops were measured at room temperature before the appearance of considerable leakage current at higher electric field due to an increase in conductivity of the material. The polarization loop of all the samples has been measured at the highest electric field of ± 42 kV/cm and 10 Hz frequency. As expected, 4

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increasing MFO concentration. The loops have become oval-shaped with an increase in MFO concentration and best quality loops are observed only for BTMF0 and BTMF5 sample. Fig. 4(b) shows the variations of remnant and saturation polarizations for all the five synthesized samples. Further, a higher value of coercivity and a transition from classical ferroelectric to round-shaped loop in higher concentration samples may be attributed to higher loss and increased conductivity in the materials [37,38]. In multiferroic ceramic composites, an increase in the coercive field with an increase in magnetic phase has been previously observed as well [39]. The presence of magnetic phase obstructs the movement of ferroelectric domains which results in lower saturation polarization and thus, the material becomes difficult to be polarized at low electric fields [40,41]. Another reason for lossy ferroelectric loops in the multiferroic composites is due to an increase in leakage current with increasing guest substitution. The higher magnetic phase substitution leads to the development of extra free charge carriers in the matrix and these charge hops from one neighboring site to another resulting in higher leakage current [42,43]. Further, these results indicate a substantial improvement in ferroelectric characteristics compared to previous literature [44,45]. 3.5. S-E and piezoelectric coefficient measurement Fig. 5(a) shows the bipolar S-E curves for bare and composite samples. The S-E curves demonstrated a nonlinear strain hysteresis characteristic on the application of applied DC electric field, which is generally observed in piezoceramic actuators. The electric field dependent strain measurement carried out at the highest electric field of ± 40 kV/cm shows typical well-shaped butterfly like curves for all the samples. Similar to ferroelectric characteristics, the percentage strain is also found to decrease with MFO addition. However, even the sample with x = 0.30 has shown classical strain characteristics, which is necessary for obtaining better piezoelectric characteristics. BTMF0 sample has shown the highest strain value among the five synthesized samples. The strain decreases abruptly even with a small amount (x = 0.05) of MFO addition; however, with further increase in MFO concentration, the reduction in strain has become gradual. Fig. 5(b1) shows the variation of maximum strain with increasing MFO content. The addition of MFO in BSCT may have triggered two problems: (i) increased stiffness of BTMF composites, which have degraded the flexibility of pellet samples and (ii) weak bonding capability between BSCT and MFO atoms. Consequently, there is a reduction in free strain with an increase in MFO concentration. The piezoelectric coefficient (d33) of poled samples has also been carried out in order to better understand the electromechanical response of bare and composite samples. Fig. 5(b) shows the variation of piezoelectric coefficient for the five synthesized samples. Piezoelectric properties are highly reliant on poling conditions due to randomly oriented ferroelectric domains of as prepared composite materials. Typically, a very high electric field is required to obtain perfectly oriented domains, but samples are prone to damage at very high electric field. Therefore, an optimized electric field with optimum time duration has been explored in the present work. An optimized poling electric field of 18 kV/cm and poling duration of 30 min is kept to obtain optimum d33 value. The piezoelectric coefficient was found to decrease with MFO addition. The piezoelectric coefficient results are in complete agreement with dielectric and strain results. These obtained values of piezoelectric coefficient are superior compared to previous reports on BT based composites [46]. Further, the reduction in d33 value may be attributed to the formation of diphasic BTMF composite with a low piezoelectric response due to partial interdiffusion at the grain boundaries. In addition to this, the relatively better crystallinity in BTMF0 sample has resulted in the reduction of pinning effect introduced by charge defects at the grain boundaries, thereby increasing the motion of domain walls and consequently higher piezoelectric response compared to composite samples [47].

Fig. 2. SEM micrographs of sintered pellets and histograms illustrating the variation of grain size for the five synthesized samples: a) and b) BTMF0, c) and d) BTMF5, e) and f) BTMF10, g) and h) BTMF20 and i) and j) BTMF30.

BTMF0 sample has shown classical ferroelectric behavior with highest saturation polarization among the five synthesized samples. Further, MnFe2O4 is a ferrite material and does not possess any significant ferroelectric characteristics due to its low resistivity. It has been observed that the MFO volume fraction has a significant effect on the ferroelectric characteristics of the composite samples. The addition of MFO in BSCT results in a slight increase in remnant polarization of the material. BTMF5 sample has shown improved ferroelectric hysteresis loop with high remnant polarization, however, increase in MFO concentration beyond x = 0.05 has resulted in deterioration of classical ferroelectric loop and an undesirable increase in the coercive electric field of the material. This increase in coercivity of the material suggests that the material is becoming much more difficult to be polarized with 5

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Fig. 3. Temperature-dependent dielectric constant and dielectric loss for bare and composite samples: a) and b) BTMF0, c) and d) BTMF5, e) and f) BTMF10, g) and h) BTMF20 and i) and j) BTMF30; insets of dielectric loss curves show the variation of diffuseness parameter with change in MFO concentration.

Fig. 4. Ferroelectric hysteresis loops of BTMF samples at room temperature: a) Comparative analysis of polarizations vs. electric field for all the five synthesized samples (the inset shows the variation of coercive electric field) and b) variations of remnant and saturation polarizations for the five samples. 6

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Fig. 5. Electromechanical response of bare and composite samples: a) comparative analysis of bipolar piezoelectric hysteresis loops of electric field dependent strain for the five samples, b) variation of local piezoelectric constant (d33) for the five synthesized samples and b1) variation of maximum strain for all the five samples at a maximum applied electric field of ± 40 kV/cm.

charge carriers gathered at the BSCT/MFO interface to leave this zone and to move towards the MFO matrix. This phenomenon results in a reduction in electrical breakdown strength of composites with increasing MFO concentration compared to BTMF0 sample.

3.6. Breakdown strength measurement Fig. 6 shows the variation of electrical breakdown strength (Ebd) of all the five synthesized samples. For each sample, 9 identical pellets with approximate thickness of 0.9 mm and 10 mm diameter have been tested to obtain the average value of dielectric breakdown strength. In order to obtain high energy density, it is desirable that the material should exhibit high electrical breakdown strength. BTMF0 sample has shown highest Ebd among the five samples. The Ebd decreases with MFO substitution and the minimum appears for BTMF30 sample. However, it is worth to notice that MFO is a ferrite material particularly known for exhibiting better magnetic property and therefore, obtaining a value of 196 kV/cm for a magnetic composite is still appreciable. These obtained results are still higher than previously reported on BT based magnetic composites [48,49]. Further, the primary reason for the decrease in Ebd may be attributed to a reduction in Young's modulus value with the addition of MFO in BSCT lattice [50]. Another reason for the decrease in breakdown strength of composite materials can be ascribed to a persistent increase in electric conductivity of the samples. MFO with relatively high inborn conductivity results in significant conductivity difference between the highly insulating BSCT and conductive MFO, which leads to an accumulation of higher amount of charge carriers at the BSCT/MFO interface. Therefore, when breakdown strength measurement is performed, a low electric field will drive those

3.7. Magnetic properties Fig. 7 illustrates the magnetization vs. magnetic field hysteresis loops for the as-prepared composite samples at room temperature by applying a maximum DC magnetic field of ± 10 kOe. The figure does not show the M-H loop of BTMF0 sample as it does not possess any significant magnetic properties. The inset of Fig. 7 shows the variation of saturation magnetization with an increase in MFO concentration. The loops clearly indicate the presence of an ordered magnetic structure in the mixed diphasic perovskite-ferrite composite. Pure MnFe2O4 exhibit slim ferromagnetic loops with low remnant and high saturation magnetization combined with the smaller coercive field. The four composite samples have also demonstrated similar behavior, where an increase in remnant and saturation magnetization is observed with increase in MFO concentration. An S-shaped ferromagnetic hysteresis loop has been observed for all the four samples, indicating a significant increase in magnetic characteristics with MFO addition in BSCT. The observed characteristics clearly indicate the role of interfacial contact between the ferroelectric and magnetic domains and it will be maximum for

Fig. 7. Variation of magnetic hysteresis loops for the composite samples and the inset shows the variation of saturation magnetization for the four-composite sample.

Fig. 6. Electrical breakdown strength (Ebd) of bare and composite samples at room temperature. 7

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Fig. 8. DC magnetic field dependent magnetoelectric coupling coefficient (αME) at five different temperatures for the four composite samples: a) BTMF5, b) BTMF10, c) BTMF20 and d) BTMF30; insets show the variations of maximum ME coefficient at various temperatures.

V ∂E ⎞ αME = ⎛ = dHac ⎝ ∂H ⎠

BTMF30 sample as it presents the highest heterogeneity among the four composite phases. Further, the highly saturated M-H loops in composite samples indicate a substantial magnetic interaction. A smaller remnant magnetization (Mr) and coercive magnetic field (Hc) may be attributed to short-range magnetic interaction in the composites [51,52]. The linear rise in saturation magnetization also suggests a reduction in paramagnetic components, which are mainly responsible for the degradation of magnetic characteristics [53]. The decrease in paramagnetic components is related to a decrease in the number of Ba2+, Ca2+, Sr2+ and Ti4+ ions, which are not ferromagnetically coupled with oxygen vacancies. These ions fully occupied in the ferroelectric region and do not contribute to the overall magnetic moment. Further, with the addition of MFO in BSCT, there is an increase in the number of Fe3+ ions leading to a higher number of oxygen vacancies, which are particularly beneficial for improving magnetic properties in BaTiO3 based composites. The presence of oxygen vacancies in composites introduces polaronic effects [53]. Due to polaronic effect, the electrons will get trapped in the oxygen vacancies and subsequently leads to the formation of an F-center (an anionic vacancy in the crystal occupied by one or more unpaired electrons). These polaronic electrons occupy an orbital in the F-center by effectively overlapping the d shells of surrounding magnetic ions [54]. These F-center bound magnetic polarons are the primary reason for the enhancement in magnetic properties of composite samples [55]. Therefore, the presence of a magnetic phase coupled with oxygen vacancies formed by the doping is mainly responsible for ferrimagnetism observed in BSCT-MFO composites.

(1)

where, V is the induced ME voltage across the sample, d represents the thickness of the sample and Hac is the applied AC magnetic field. To confirm the ME coupling between the BSCT and MFO phases, the temperature-dependent ME coefficient (αME) of all four composite samples have been measured by applying the fixed AC magnetic field of 10 Oe at 7 kHz frequency. The measurement has been carried out in 27–200 °C temperature range with DC magnetic field of up to 15 kOe. The figure does not show the magnetoelectric coefficient of BTMF0 for same reason addressed in section 3.7. Initially, in order to obtain an optimized operating frequency, the samples were tested at room temperature in 100 Hz–10 kHz frequency range and keeping a fixed AC magnetic field of 10 Oe and DC magnetic field of 5 kOe. The ME coefficient rises linearly with increase in frequency up to 7 kHz and the samples have shown optimum ME coefficient at 7 kHz and, therefore, further temperature-dependent measurements have been carried out at the optimized frequency. Unlike the pure BSCT sample, abrupt changes in ME response has been observed on the application of the DC magnetic field. The ME coefficient was found to decrease with increasing temperature in all four composite samples. A gradual increase in ME coefficient has been observed with increase in ME concentration in BSCT lattice and found to be maximum for BTMF30 sample, which may be attributed to increased heterogeneity as compared to other three samples. A non-linear increase in ME coefficient with applied DC magnetic field can be seen and a maximum ME coefficient has been found around 3.5 kOe. After achieving maxima, the ME coefficient found to decrease and became nearly saturated at higher DC magnetic field. The insets of Fig. 8(a–d) shows the variation of ME coefficient with the increase in temperature for the corresponding samples. The presence of ferroelectric ordering combined with relatively high magnetic properties have boosted a significant improvement in ME coefficient compared to ferroelectric BSCT. Further, the elastic effect is known to be one of the main reasons for the induction of magnetoelectric effect in BT based magnetic composites [58]. Due to this effect, the magnetostriction in composites induced by the DC magnetic field

3.8. Magnetoelectric characteristics This section describes the magnetoelectric (ME) coupling performance of BTMF composites. The ME coupling in composites arises due to domain movement in the magnetic phase. The presence of ferroelectric BSCT and ferrimagnetic MFO may give rise to ME coupling effect. The ME effect can be described by the ME voltage coefficient (αME), which can be calculated by the following relation [56,57]:

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can be transferred to the ferroelectric phase and results in the occurrence of polarization in the material. Further, the occurrence of substantial magnetoelectric voltage may be ascribed to improved mechanical coupling between the piezoelectric and magnetic phases [59]. The ME coupling in these ferroelectric-magnetic composites mainly arises due to magnetic-electric-mechanical transform via stress-mediated transfer at the interface. Further, the obtained value of magnetoelectric coefficient is significantly higher than previously reported on perovskite-ferrite composites [60–62].

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4. Conclusions In this work, we demonstrated that the ferromagnetic and magnetoelectric properties can be significantly improved without hampering the ferroelectric characteristics of Ba0.83Ca0.10Sr0.07TiO3 ceramics. XRD analysis confirmed the formation of diphasic composites without the presence of any impurity or intermediate phase. SEM studies show clearly distinguishable quasi-spherical shaped grains, which are homogeneously distributed in the observed area. The composite samples have shown a slight reduction in dielectric, ferroelectric and piezoelectric properties with MnFe2O4 addition. However, there is a significant increase in magnetic and magnetoelectric properties. Magnetic studies have shown the formation of ferrimagnetic hysteresis loops with considerable saturation magnetization for all the composite samples compared to pure BSCT. In addition to this, a substantial increase in magnetoelectric response from the coupling of ferroelectric and ferrimagnetic phase has been observed. Therefore, it can be established that the addition of MFO in BSCT has resulted in the emergence of a new multiferroic material. The relatively uniform grain size combined with superior electrical and magnetic properties of BTMF heterostructure composites can prove to be a potential candidate for multiferroic applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We thank Prof. Ying Yang and Prof. Kunjun Zhu for their help in carrying out ferroelectric measurement at Nanjing University of Aeronautics and Astronautics, Nanjing, China. References [1] S. Ravi, Multiferroism in Pr2FeCrO6 perovskite, J. Rare Earths 36 (2018) 1175–1178. [2] V.A. Khomchenko, V.V. Shvartsman, P. Borisov, W. Kleemann, D.A. Kiselev, I.K. Bdikin, J.M. Vieira, A.L. Kholkin, Effect of Gd substitution on the crystal structure and multiferroic properties of BiFeO3, Acta Mater. 57 (2009) 5137–5145. [3] M. Sahoo, Z. Yajun, J. Wang, R. Choudhary, Composition control of magnetoelectric relaxor behavior in multiferroic BaZr0.4Ti0.6O3/CoFe2O4 composites, J. Alloy. Comp. 657 (2016) 12–20. [4] S.W. Cheong, M. Mostovoy, Multiferroics: a magnetic twist for ferroelectricity, Nat. Mater. 6 (2007) 13–20. [5] R.N. Bhowmik, M.C. Aswathi, Modified dielectric and ferroelectric properties in the composite of ferrimagnetic Co1.75Fe1.25O4 ferrite and ferroelectric BaTiO3 perovskite in comparison to Co1.75Fe1.25O4 ferrite, Compos. B Eng. 160 (2019) 457–470. [6] H.H. Wang, Z.Q. Guo, W.T. Hao, L. Sun, Y.J. Zhang, E.S. Cao, Ethanol sensing characteristics of BaTiO3/LaFeO3 nanocomposite, Mater. Lett. 234 (2019) 40–44. [7] P. Phansamdaeng, J. Khemprasit, Study on magnetic and dielectric properties of BaTiO3/MnCr0.2Fe1.8O4 composite material, J. Alloy. Comp. 776 (2019) 105–110. [8] Y.C. Qing, L.Y. Ma, X.C. Hu, F. Luo, W.C. Zhou, NiFe2O4 nanoparticles filled BaTiO3 ceramics for high-performance electromagnetic interference shielding applications, Ceram. Int. 44 (2018) 8706–8709. [9] A. Jain, A.K. Panwar, A.K. Jha, Significant enhancement in structural, dielectric, piezoelectric and ferromagnetic properties of Ba0.9Sr0.1Zr0.1Ti0.9O3-CoFe2O4 multiferroic composites, Mater. Res. Bull. 100 (2018) 367–376.

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