Magneto-electric properties of xNi0.7Zn0.3Fe2O4 – (1-x)BaTiO3 multiferroic composites

Author’s Accepted Manuscript Magneto-electric properties of xNi0.7Zn0.3Fe2O4 – (1-x)BaTiO3 multiferroic composites A.S. Dzunuzovic, M.M. Vijatovic Petrovic, J.D. Bobic, N.I. Ilic, M. Ivanov, R. Grigalaitis, J. Banys, B.D. Stojanovic www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)32169-7 https://doi.org/10.1016/j.ceramint.2017.09.229 CERI16400

To appear in: Ceramics International Received date: 15 June 2017 Revised date: 14 September 2017 Accepted date: 28 September 2017 Cite this article as: A.S. Dzunuzovic, M.M. Vijatovic Petrovic, J.D. Bobic, N.I. Ilic, M. Ivanov, R. Grigalaitis, J. Banys and B.D. Stojanovic, Magneto-electric properties of xNi0.7Zn0.3Fe2O4 – (1-x)BaTiO3 multiferroic composites, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.09.229 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 xNi0.7Zn0.3Fe2O4 – (1-x)BaTiO3 multiferroic composites A.S. Dzunuzovica, M.M. Vijatovic Petrovica, J.D. Bobica, N.I. Ilica, M. Ivanovb, R. Grigalaitisb, J. Banysb, B.D. Stojanovica a

Institute for Multidisciplinary Research University of Belgrade, Belgrade, Serbia b Faculty of Physics, Vilnius University, Lithuania

Abstract Di-phase

ceramic

composites,

with

general

formula

xNi0.7Zn0.3Fe2O4

-

(1-

x)BaTiO3(x=0.9,0.7,0.5,0.3,0.1), were prepared by a mixing method. X-ray analysis, for powder and ceramics, indicated the formation of ferrite and barium titanate phases without the presence of the impurities. SEM analysis indicated that the composite morphology contained two types of grains, polygonal and rounded. Homogeneous microstructure and the smallest grain size were obtained in ceramics with 70 % of barium titanate. The electrical properties of these materials were investigated using impedance spectroscopy, dielectric and ferroelectric measurements. The NZF-BT(30-70) composite has shown better electrical properties in comparison to other investigated ceramics, confirmed by dielectric and ferroelectric data analysis. Saturation magnetization and coercive field decreased with the increase of the content of ferroelectric phase.

Keywords: B. Composites, C. Impedance, C. Magnetic properties, Auto-combustion

Corresponding author: Tel: +381 11 2085 039, Fax: +381 11 2085062 e-mail: [email protected] (Mirjana Vijatovic Petrovic)

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1. Introduction Multiferroic materials have been in a focus of scientific community because of their potential application in multifunctional devices. These nanostructured materials have proven to be very attractive and promising materials due to their potential application in magnetic field sensors and electrically tunable microwave devices such as filters, oscillators and phase shifters [1, 2]. The term multiferroic has been firstly used by H. Schmid in 1994 [3]. Today this term represents materials which exhibit at least two of ferroic properties (ferroelectricity, ferromagnetism and ferroelasticity) at the same time. As a result, they have a spontaneous magnetization that can be switched by an external electric field, and spontaneous polarization that can be switched by an applied magnetic field. Recently, there has been a growing interest in so-called ferrotoroidic properties of multiferroic materials [4]. There are two types of multiferroics: single-phase such as bismuth ferrite and composites which consist of two or more phases. Various magnetoelectric bulk composites have been produced and investigated, such as (NiZn)Fe2O4-BaTiO3, BaSrTiO3- Ni,ZnFe2O4, Ni(Co, Mn)Fe2O4-BaTiO3, CoFe2O4-BaTiO3, NiFe2O4-BaTiO3, Ni(Cu,Zn)Fe2O4-BaTiO3, etc. [5-8]. The magnetic properties of spinel nickel zinc ferrite depend on the local configuration and the organization of the constituent atoms. Ferroelectrics possess anomalous dielectric properties, nonlinearities and disappearance of ferroelectricity above Curie temperature [9, 10]. From the application point of view, nickel zinc ferrite has a lot of opportunities due to its high electrical resistivity, high value of magnetization and high chemical stability [1]. BaTiO3 owing

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to its high permittivity, low dielectric loss and high tunability, has become quite significant material for the high speed and nonvolatile memory devices [11]. Studies of magneto-electric systems have greatly contributed to the appearance of composite materials due to the strong coupling of magnetic and electrical properties. It means, the induction of magnetization by an electric field or the polarization by a magnetic fiel. It is a “product property” and which is absent in individual constituent phases. Magnetic phase possesses the magnetostrictive effect while ferroelectric phase possesses the piezoelectric effect. The ME effect is considered as an intrinsic effect. These two phenomena are coupled via elastic interaction [12, 13]. Magneto-electric response can be displayed with the equation: ΔP =α ΔH or ΔE = αEΔH,

(1)

where E denotes electric field and α is the effective ME coefficient. Magnetodielectric effect can be used to change the dielectric constant with the magnetic field.

Multiferroic

materials are interesting due to their potential applications in devices such as sensors, transducers, filters, oscillators, phase shifters, memory devices, etc. [14, 15]. This paper describes the preparation and characterization of composites xNi0.7Zn0.3Fe2O4(1-x)BaTiO3 (x=0.9, 0.7, 0.5, 0.3, 0.1), where Ni0.7Zn0.3Fe2O4 (NZF) represents the magnetic phase and BaTiO3 (BT) represents ferroelectric phase. Both phases were synthesized by an autocombustion method which is considered to be a very attractive method because of short reaction time and low costs compared to the conventional methods, such as a solid state reaction, etc. Previous research results have shown that when nickel was changed with zinc in nickel ferrite lattice, the improvement of the properties of nickel ferrite ceramics was noticed [16]. Detailed structural analysis has shown the influence of the Zn addition on crystallite size, particle size and Fe/Ni/Zn-O bonds formed in the nickel zinc ferrite. Furthermore, the Ni0.7Zn0.3Fe2O4 was found

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to be the material with optimal magnetic properties comparing to other analyzed compositions and thus it was chosen as a part of multiferroic composite in this study. Hence, an attempt is made here to investigate the multiferroic composites consisted of nickel zinc ferrite (Ni0.7Zn0.3Fe2O4 ) and pure barium titanate in the various mass ratios, without secondary phases, and with optimal electrical and magnetic properties.

2. Material and methods The multiferroic composites with general formula xNi0.7Zn0.3Fe2O4-(1x)BaTiO3(x=0.9,0.7,0.5,0.3,0.1) were obtained using auto-combustion method. NZF(70-30) powder was prepared starting from iron nitrate (Fe(NO3)3 · 9H2O, AlfaAesar, 98.0–101.0%), nickel nitrate (Ni(NO3)2 · 6H2O, AlfaAesar, 99.9985%), zinc nitrate (Zn(NO3)2 · 6H2O, Alfa Aesar, 99%), citric acid (C6H8O7 · H2O, CarloErba,99.5–100.5%) and ammonium hydroxide (NH4OH, LachNer,25%), as a raw materials [16]. Precursor powder was thermally treated at 1000 °C for 1h, and after the calcination pure NZF powder was obtained. BaTiO3, denoted as BT, was synthesized by a modified auto-combustion method. Starting reagents used for BT synthesis were Ti(OCH(CH3)2)4 (TTIP) (AlfaAesar, 98.0 – 101.0%), C6H8O7 · H2O (Carlo Erba, 99.5–100.5%), Ba(NO3)2 and NH4OH (Lach Ner, 25%). The precursor powder of barium titanate was further calcined at 900 °C for 2h, and pure BT powder was obtained. Composites were prepared by mixing and homogenizing NZF and BT powders in a planetary ball mill for a 24h in iso-propil alcohol as a milling medium. The mass ratio of BT and NZF(70-30) was varied and the multiferroic composites were denoted according to the mass ratio of NZF and BT namely as NZF-BT(90-10), NZF-BT(70-30), NZF-BT(50-50), NZF-BT(30-

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70), NZF-BT(10-90). The composite powders were uniaxially pressed at 196 MPa into pellets and sintered under different sintering conditions in the air atmosphere in order to obtain composites with no secondary phase. One series of composites was sintered at the temperature of 1170 ºC, while the others were sintered at the temperatures of 1120 ºC. XRD patterns of the powders and ceramics composites were recorded using X-ray diffraction technique (Model Phillips PW1710 diffractometer, with Co Kα radiation, 0.5°/min). The FT-IR spectra of all investigated samples were collected using a Bruker Equinox-55 spectrometer. Scanning electron microscope (SEM) (Model TESCAN SM-300) was used for morphological and microstructural characterization of the powders and ceramics. The grain size was determined using the ImageJ program. Impedance spectroscopy of the sintered pellets was performed in the frequency and temperature range from 100 Hz to 1 MHz and 150-300 °C, respectively, using an LCR meter (model 9593-01, HIOKI HITESTER). Collected data were analyzed using the commercial software package Z-view. Dielectric permittivity measurements were carried out using an LCR meter (model 4284A, Hewllet-Packard) in the temperature range from -155 °C to 220 °C. The magnetic moments were measured using a superconducting quantum interferometric magnetometer SQUID (Quantum Design).

3. Results and discussion 3.1. Structure and microstructure analysis The XRD patterns of NZF-BT(90-10), NZF-BT(70-30), NZF-BT(50-50), NZF-BT(3070), NZF-BT(10-90) powders are given in Fig 1. All peaks correspond to both of the constituent phases which are spinel cubic NZF (JCPDS file no. 10-0325) and perovskite tetragonal BT phase (JCPDS files no. 05-0626). The obtained powders were highly crystalline. In the composite

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powders consisted of higher concentration of barium titanate (50, 70 and 90 mass %) a small amount of barium carbonate was noticed, while in other samples the presence of any intermediate phase or impurities was not detected. FT-IR spectra of the obtained composite powders, recorded in the wave number range of 400-4000 cm-1 at room temperature, are presented in Fig 2. Absorption bands near 1000 and 1500 cm-1 can be observed in FT-IR spectra of NZF [17], while absorption bands near 500 and 870 cm-1 correspond to the BT phase [18]. The band at 3500 cm-1 corresponds to stretching and bending vibrations of O–H bonds [19]. XRD patterns of ceramics of all investigated composites are given in Fig. 3. The formation of both ferrite and barium titanate phases without the presence of the impurities, except in the case of NZF-BT (10-90) where small quantities of secondary barium ferrite phase, denoted as BF, are noticeable. In order to obtain composites with only two phases and best possible densities, various sintering regimes were applied depending on composite mass ratio. High sintering temperatures caused the formation of the secondary phase due to interphase reactions, while lower sintering temperatures caused significant decrease of the density. Sintering was conducted in the temperature range from 1200 to 1100 ºC, with different maximum sintering temperatures and different retention time at desired sintering temperature. Optimum sintering conditions were determined as follows: -

4 h at the temperature of 1170 ºC and the sintering rate of 5 ºC/min for the composites NZF-BT (90-10), NZF-BT (70-30) and NZF-BT (10-90)

-

1 h at the temperature of 1120 ºC and the sintering rate of 10 ºC/min for the composites NZF-BT (50-50) and NZF-BT (30-70).

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SEM and backscattered electron (BSE) images of obtained composites are given in Fig. 4. (insets of the upper right corner of the images represent BSE micrographs). The composite morphology is contained of two types of grains: polygonal nickel zinc ferrite, and more rounded grains of barium titanate. From the BSE images, a homogeneous phase distribution in obtained composites can be noticed. The average grain size is at the nanoscale and presented as follows: ~ 296 nm for the sample NZF – BT (90-10); ~ 276 nm for the sample NZF – BT (70-30); ~ 228 nm for the sample NZF – BT (50-50); ~ 120 nm for the sample NZF – BT (30-70); ~ 186 nm for the sample NZF – BT (10-90). The average grain size decreases with the increase of the barium titanate amount, up to the mass ratio of 10-90. This occurs as the consequence of the sintering conditions. Composites sintered under identical sintering conditions show smaller grain size depending on the amount of barium titanate phase. The reason for this is most probably the fact that the primary particles of barium titanate are finer than the particles of nickel zinc ferrite starting powder. It can be noticed that the sample NZF-BT (30-70) showed the best homogeneity and the smallest grain size. The densities of obtained ceramic composites were found to be quite low and not consistent with mass ratio between the phases. This occurrence is possibly due to the compositional mismatch between smaller and round barium titanate grains and bigger polygonal ferrite grains. The density values were found to be 81.5, 80.2, 65.5, 70.2 and 82,5 % of the theoretical value for the 90, 70, 50, 30 and 10 mass % of NZF in the composite, respectively.

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3.2. Dielectric properties The real part of permittivity (ε′) of the investigated composites as a function of temperature are shown in Fig. 5. The BT phase transitions peaks cannot be clearly noticed in these composites. Only in the case of NZF-BT (30-70) some evidence of BT phase transition can be observed, one at 120 °C (from cubic to tetragonal) and the other around 30 °C (from tetragonal to orthorhombic). However, an anomaly in behavior occurred in the sample with the largest amount of BT, NZF-BT (10-90). It was expected for this sample to have clear phase transitions, but it was not the case in the obtained ceramics. This may be due to low density or the presence of small amounts of secondary barium ferrite phase in this sample. BF indicated low levels of electrical permittivity and this may be the reason for concealing the peaks in the obtained sample [20]. Since the sample NZF-BT(30-70) showed the most prominent phase transitions at frequencies up to 1 MHz, the additional measurements at higher frequencies were performed for this sample (Fig. 6.). Two peaks of the real part of permittivity at about 120 ºC (from cubic to tetragonal) and 40 ºC (from tetragonal to orthorhombic) can be observed indicating the phase transitions of barium titanate. The peak at about 40 ºC was moved in regard to phase transition peak of barium titanate at 5 ºC. This enhancement of the temperature of phase transition can be explained by partial substitution of Ba with Zn or Ti with Fe3+ ions, changing the ion size in the lattice of barium titanate which affect dielectric properties of the material. This also contributed to broadening of the phase transition peaks which becomes barely noticeable [21]. These peaks were more prominent at higher frequencies. It is well known that peaks of barium titanate corresponding to phase transitions from rhombohedral to orthorhombic, orthorhombic to tetragonal, and tetragonal to cubic structure, are usually sharp and abrupt [22]. Peaks derived

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from dielectric measurements for this composite were flat and wide, and the permittivity was reduced, which can be attributed to the grain size (below 300 nm), as well as the possible presence of impurities and defects [23]. Some amount of Fe3+ from nickel zinc ferrite can act as a dopant in the BT lattice, although this very low concentration of impurities can not be seen in XRD. Also, the presence of nickel zinc ferrite can reduce the values of permittivity, ie. the dielectric constant at Curie temperature. This temperature was attributed to the transition from paraelectric to ferroelectric state, due to the influence of the concentration of the ferrite phase in the value of the phase transformation temperature. The increase of permittivity up to the Curie temperature can be explained by the increase of surface polarization and conductivity with the temperature. The increase of permittivity at high temperatures is related to Maxwell-Wagner relaxation and increasing electrical conductivity. This can be caused by the increase in the mobility of the charge carriers with the temperature. After the phase transformation dielectric permittivity decrease due to the disappearance of domains [11]. For frequencies between 333 kHz and 1 MHz, there was a shift of the maximum in the permittivity towards lower temperature with increasing frequency. This indicated the occurrence of the relaxation, i.e. thermally activated processes. The dielectric permittivity as a function of frequency for the sample NZF-BT(30-70) at different temperatures is presented in Fig. 7. The values of the dielectric permittivity decrease with the increase of frequency and decrease of temperature. At the value of frequency lower than 50 kHz, the dispersion of the dielectric permittivity appeared, while at frequencies above 50 kHz the value of ε remained constant up to the temperature of 353 K, when this phenomena moved towards higher values of frequencies. This may be due to the presence of dipoles, which is conditioned by changes of valent state of cations. These dipoles disappeared at higher

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frequencies because of the influence of an applied permanent alternating electric field [24]. This occurs in the case of the heterogeneous structure of the composite, consisting of relatively conductive grains surrounded by the grain boundaries with high resistance, which is based on the Maxwell-Wagner relaxation [25]. High dielectric response at the lowest frequencies could be also influenced by the interaction of the conduction electrons and the dielectric polarization [26]. Fig. 8. shows variation of tangent loss as a function of frequency at higher temperatures. At higher temperatures, there was a rapid decrease of the dielectric losses with increasing frequency, indicating the mechanism of polarization due to the presence of Fe2+ and Fe3+ in the ferrite phase. Electron hopping from one to another crystallographic equivalent place, causes high dielectric losses. This conduction mechanism at higher temperatures causes the dielectric losses to be greater than unity, which is a disadvantage for a material with good magneto-electric effect [27-29]. At higher frequency region this hopping electron no longer has an impact because it cannot follow the applied electric field, resulting in the constant value of the dielectric losses [11]. At room and lower temperatures, values of the loss tangent are significantly lower, below 0.7. Improvement in dielectric characteristics was observed because the following conditions were satisfied: loss tangent is less than 1 at lower frequencies, while at frequencies above 104 Hz is less than 0.05. Consequently, this material can be recommended for applications where the main problem is the reduction of conductivity at lower frequencies and high temperatures in order to achieve a high magneto-electric response [30]. 3.3. Impedance analysis Fig. 9. shows the complex impedance plane plot for NZF – BT (90-10), NZF – BT (7030), NZF – BT (50-50), NZF – BT (30-70), NZF – BT (10-90) ceramic composites. Due to the high conductivity of nickel zinc ferrite in the composite sample with the largest amount of this

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component, it was not possible to determine the true value of the resistivity of the sample so these results were omitted from further analysis. The values of the grain, grain boundary and total electrical resistivity for all other obtained ceramic composites are presented in the Table 1. In composite materials the appearance of one or more semicircular arc can represent the presence of different relaxation processes such as grain and grain boundary, but nevertheless also different crystallographic phases (in our case NZF and BT). For all investigated composite ceramics, two semicircular arcs were observed. In our case, we can assume that the existence of these two semicircular arcs are possibly due to two different crystallographic phases. From the resistivity values of pure NZF and BT can be pointed that more resistive part which is BT dominates entirely and makes resolution of the less resistive response (NZF) difficult. The change of the total electrical resistivity with reducing the amount of magnetic phase was expected. The increase of barium titanate content above 70 % of mass ratio led to the increase of total electrical resistivity, after which total electrical resistivity starts to decrease. Having in mind that the sample with the largest amount of barium titanate phase should show the highest resistance, such behavior has not occurred. The reason for this may be the presence of small amounts of secondary barium ferrite phase (Fig. 3.), because this phase has a lower resistivity in comparison with barium titanate that could influence the decrease of the total resistivity of the composites [31, 32]. The temperature dependence of the resistance can be presented by the equation: Ea ö ÷÷ è k bT ø æ

s = s 0 exp çç -

(2)

The activation energies of the samples were determined from the slopes of the curves, presented in Fig. 10. , and were calculated, for the one phase, to be 0.19 eV, 0.32 eV, 0.45 eV, 0.62 eV for NZF-BT(70-30), NZF-BT(50-50), NZF-BT(30-70), NZF-BT(10-90) samples, 11

respectively, and for the another phase, 0.31 eV, 0.37 eV, 0.21 eV, 0.47 eV for the same order of the samples. Generally, the activation energies for the oxide ion conductors are >0.9 eV. Activation energies for electron hopping are lower than for hole hopping. It is usually, less then 0.2eV for n type polaronic conduction and above 0.2eV for polaronic conduction of the holes. For these composites, the activation energies were approximately from 0.2 eV up to 0.5 eV could suggested mechanism of polaronic conduction of both types, depending on a composite material [33]. Fig. 11. shows the variation of the imaginary part of impedance as a function of frequency at different temperatures. The value of Z″ was reaching a maximum at different frequencies for different temperatures. Height of the peaks decreased toward the higher frequency side indicating the possible presence of space charge polarization or accumulation at the grain boundaries. The peak were shifting towards higher frequencies with the temperature, indicating the temperature dependent electrical relaxation in the material, due to the contribution of immobile parts in the material at lower temperatures and mobile defects and vacancies at higher temperatures [34]. For calculating the corresponding capacitance, the maximum value of the Z″ peak and associated fmax value were found and the C and R were obtained, using the relationships C = 1 / 2πfmaxR, where R = 2Z″max. The obtained values are presented in Table 1. For all samples, with the increase of temperature the value of capacitance decrease. The scaling behavior of the samples was studied by plotting normalized parameters Z"/Zmax as a function of f/fmax (Fig. 12.) It can be noticed that all data collapse into a single master curve and almost perfectly overlap at different temperatures. This is a suggestion that the distribution of relaxation time is temperature independent.

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The complex electric modulus analysis can be used to provide information about homogeneity of polycrystalline samples. Also, it can be used to separate the electrode polarization effect from the grain boundary conduction process [35]. Complex modulus plots (M′ vs. M″) of the investigated composites at different temperatures are shown in Fig. 13. These diagrams enable establishing the correlation between the dielectric properties of materials and their microstructure and also to separate the grain, grain boundary and interface contributions [36]. Two semicircular arcs were observed in the measured temperature range, a smaller one at lower frequencies and larger at higher frequencies indicating the influence of two different relaxation processes in the material. One arc was observed in the temperature of 300 ºC indicating that a single relaxation process remained. In order to determine these different electroactive regions in the modulus, plots M″-f were calculated. Modulus plots M″-f for investigated ceramics composites are presented in Fig. 14. From the diagram it can be observed that there is a presence of two dielectric relaxations, indicating the contribution of two different phases (NZF and BT) to the conduction process in the material. For the sample NZF-BT(90-10), there was no dielectric relaxation observed because this sample had the highest mass contribution of nickel zinc ferrite phase which is a highly conductive phase. For other samples at lower temperatures there was a noticeable relaxation at the frequency below 10 kHz, as a maximum which moves to higher frequencies with increasing temperature. This behavior suggested that the dielectric relaxation is thermally activated and also confirmed hopping mechanism of charge carriers in these materials. The asymmetric broadening of the peak can be noticed, indicating that the relaxation in these materials is of non-Debye type [37]. Charged particles can move from one place to the neighboring which is energetically favorable (long distances). The second maxima occurred at frequencies between 50 and 700 kHz, where

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the charged particles are spatially confined by the potential barrier and cannot move (short distances). The variation of normalized parameters Z"/Zmax and M"/Mmax as a function of frequency at 250 ºC for NZF-BT(10-90) are shown in Fig. 15. The mismatch between the peak frequency of the normalized M″ and Z″ at low frequencies indicates the short-range conduction of charge carriers. This difference indicated that polarization process is due to localized conduction of multiple carriers [38]. The results of the room temperature polarization-electric field (P-E) measurements for all investigation composites are given in Fig. 16. The look of the hysteresis loop indicates the dilution of ferroelectric properties in all multiferroic composites. With the increase of the amount of nickel zinc ferrite phase, as a very conductive phase in this composite, the shape of the curve differs from the conventional ferroelectric materials. The values of the remnant polarization were: 0.14, 0.44, 0.30, 0.25, 0.19 µC / cm2 for the composites NZF-BT(90-10), NZF-BT(70-30), NZF-BT(50-50), NZF-BT(30-70), NZF-BT(10-90) respectively, and for the coercive field were 7.95, 7.86, 5.35, 3.73, 1.50 kV / cm. These values were slightly less than for NZF-BT that was synthesized by Mondal et al [6]. Remnant polarization, Pr, decrease in composite materials is usually connected with electromechanical coupling, dielectric losses increase, heterogeneous conduction between ferroelectric and ferromagnetic phases interfaces. The nickel zinc ferrite acts as an obstacle for the movement of domains due to hindered and pinned domain wall motion. When domain walls are being pinned by different kind of defects, in this case with Vo or complexes Fe, Ti-Vo the polarization is being difficult and Pr decrease and Ec increases. The value of coercive field decreases with the increase of the barium titanate phase amount. It can be noticed that the sample NZF-BT(30-70) possesses the higher value of remnant polarization and

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saturation polarization in comparison to the sample NZF-BT(10-90). These measurements have confirmed anomalies in the behavior of the sample with higher amount of barium titanate phase, previously observed by dielectric measurements. 3.4. Magnetic properties Magnetic hysteresis loops of all investigated composites are presented in Fig. 17. These curves are typical for soft magnetic materials [39], since in these composites the only carrier of the magnetic properties is nickel zinc ferrite. These curves also indicated the presence of an ordered magnetic structure. In these composites, BT leads to a decrease of magnetic properties (Table 2.). This behavior was also observed in other composites investigated previously [5]. Remnant magnetization and coercitive field have shown the increase with the increase in the mass ratio of barium titanate up to 50-50. With further increase of the amount of barium titanate Mr and Hc tended to decrease. The value of magnetization saturation for obtained composite decreased, in comparison with pure NZF phases, 78.68 emu/g [16]. The observed values for saturation magnetization for NiZnFe2O4-BaTiO3 (50-50) (~37.6 emu/g) were higher than for the MgFe2O4-BaTiO3 (~17 emu/g) and CoFe2O4-BaTiO3 (~ 28.9 emu/g) synthesized by the hybrid chemical process and wet chemical method [40, 41]. As in the case of pure nickel zinc ferrite, spontaneous magnetization in these composites originates from the unbalanced antiparallel spins of ferromagnetic character [42]. The values of the coercive field for obtained composite were higher than for pure NZF-BT(70-30), which is 0.018 Oe. This can be understood by considering the fact that the composites have a higher anisotropy field in relation to pure ferrite which is a constituent part of the composites [43]. It can be seen that the saturation magnetization of composite materials decreases linearly with nickel zinc ferrite amount and the coercivity is being enhanced suggesting that the presence of barium titante non-magnetic phase and the interface

15

effects does not affect magnetic interactions they are just proportional with the amount of nickel zinc ferrite. Also, small values of the coercivity and the saturation fields are conditioned by the unbalanced antiparallel spins [44].

Conclusion The multiferroic composites of nickel zinc ferrite – barium titanate were obtained by a mixing method. XRD confirmed the presence of two phases, nickel zinc ferrite and barium titanate, for all synthesized powder and ceramics. SEM analysis indicated that the average grain size decreased with the increase of the barium titanate amount, up to the mass ratio of 10-90. Conduction mechanism in investigated composites was found to obey polaron hopping at the high temperature. Interface effects played important roles in the modification of the dielectric properties, which caused space charge effect and the Maxwell-Wagner relaxation at low frequencies and high temperatures. Impedance measurements showed that the increase of barium titanate content led to the increase of total resistivity, up to the composition of NZF-BT (30-70) while with farther increase of barium titanate amount, the decrease of the total resistivity was observed. The saturation magnetization of composite materials decreased linearly with nickel zinc ferrite amount and the coercivity was enhanced, suggesting that the presence of barium titanate non-magnetic phase and the interface effects does not affect magnetic interactions they are just proportional with the amount of nickel zinc ferrite. The sample NZF-BT(30-70) showed better electrical properties, microstructure homogeneity and density compared to other investigated ceramics. Considering that both phases in composite materials retained their characteristic properties (magnetic and dielectric), these composites could be applied as good ME materials.

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Acknowledgements

The authors gratefully acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (project III 45021), COST IC 1208 and MP 1308. Special thanks to Dr. Darko Makovec from the Institute “Jozef Stefan”, Ljubljana, Slovenia, for the magnetic measurements.

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[27] W. Zhu, Z. Ye, Effects of chemical modification on the electrical properties of 0.6BiFeO30.33PbTiO3 ferroelectric ceramics, Ceram. Int. 30 (2004) 1435-1442. [28] T. Lupeiko, I. Lisnevskaya, M. Chkheidze, B. Zvyagintsev, Laminated magnetoelectric composites based on nickel ferrite and PZT materials, Inorg. Mater. 31 (1995) 1139-1142. [29] X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, S. Buddhudu, A ferroelctric ferromagnetic composite material with significant permeability and permittivity, Adv. Funct. Mater. 9 (2004) 920-926. [30] L. Mitoseriu, V. Buscaglia, M. Viviani, M. Buscaglia, I. Pallecchi, C. Harnagea, A. Testino, V. Trefiletti, P. Nanni, A. Siri, BaTiO3– (Ni0.5Zn0.5)Fe2O4 ceramic composites with ferroelectric and magnetic properties, J. Eur. Ceram. Soc. 27 (2007) 4379–4382. [31] A. Dubey, K. Kakimoto, Impedance spectroscopy and mechanical response of porous nanophase hydroxyapatite–barium titanate composite, Mat. Sci. Eng. C. 63 (2016) 211-221 [32] R. Almeida, W. Paraguassu, D. Pires, R. Correa, C. Paschoal, Impedance spectroscopy analysis of BaFe12O19 M-type hexaferrite obtained by ceramic method, Ceram. Inter. 35 (2009) 2443–2447. [33] M. Idrees, M. Nadeem, M. Atif, M. Siddique, M. Mehmood, M. Hassan, Origin of colossal dielectric response in LaFeO3, Acta Mater. 59 (2011) 1338–1345. [34] K. Verma, S.Sharma, Impedance spectroscopy and dielectric behavior in barium strontium titanate-nickel zinc ferrite composites, Phys. Status Solidi B. 249 (2012) 209–216. [35] M. Dar, V. Verma, S. Gairola, W. Siddiqui, R. Singh, R. Kotnala, Low dielectric loss of Mg doped Ni–Cu–Zn nano-ferrites for power applications, Appl. Surf. Sci. 258 (2012) 5342–5347. [36] A. Jonscher, The ‘universal’ dielectric response, Nature, 267 (1977) 673-679. [37] A. Chaouchi, S. Kennour, Impedance spectroscopy studies on (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 ceramics, Process. Appl. Ceram. 6 [4] (2012) 201–207.

lead

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[38] S. Sen, S. Mishra, S. Palit, S. Das, A. Tarafdar, Impedance analysis of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 ceramic, J. Alloy. Compd. 453 (2008) 395-400. [39] N. Spaldin, Magnetic Materials, Fundamentals and Applications, second ed., Cambridge University Press, Cambridge, 2011. [40] R. Tadi, Y. Kim, D. Sarkar, C. Kim, K. Ryu, Magnetic and electrical properties of bulk BaTiO3 + MgFe2O4 composite, J. Magn. Magn. Mater. 323 (2011) 564-568. [41] V. Corral-Flores, D. Bueno-Baques, R. Ziolo, Synthesis and characterization of novel CoFe2O4–BaTiO3 multiferroic core–shell-type nanostructures, Acta Mater. 58 (2010) 764–769.

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[42] C. Ciomaga, M. Airimioaei, V. Nica, L. Hrib, O. Caltun, A. Iordan, C. Galassi, L. Mitoseriu, M. Palamaru, Preparation and magnetoelectri properties of NiFe2O4-PZT composites obtained in-situ by gel-combustion method, J. Eur. Ceram. Soc. 32 (2012) 3325-3337. [43] U. Acevedo, T. Gaudisson, R. Ortega-Zempoalteca, S. Nowak, S. Ammar, R. Valenzuela, Magnetic properties of ferrite-titanate nanostructured composites synthesized by the polyol method and consolidated by spark plasma sintering, J. Appl. Phys. 113 (2013) 17B519-17B522. [44] A. Testino, L. Mitoseriu, V. Buscaglia, M. Buscaglia, I. Pallecchi, A. Albuquerque, V. Calzona, D. Marre, A. Siri, P. Nanni, Preparation of multiferroic composites of BaTiO3Ni0.5Zn0.5Fe2O4 ceramics, J. Eur. Ceram. Soc. 26 (2006) 3031-3036. FIGURE CAPTIONS Figure 1. The X-ray diffraction patterns of a) NZF-BT(90-10), b) NZF-BT(70-30), c) NZFBT(50-50), d) NZF-BT(30-70), e) NZF-BT(10-90) powders. Figure 2. FTIR spectra of: a) NZF-BT(90-10), b) NZF-BT(70-30), c) NZF-BT(50-50), d) NZF-BT(30-70), e) NZF-BT(10-90) powders. Figure 3. XRD diffractograms of a) NZF-BT(90-10), b) NZF-BT(70-30), c) NZF-BT(50-50), d) NZF-BT(30-70), e) NZF-BT(10-90) ceramics. Figure 4. SEM and BSE images of a) NZF-BT(90-10), b) NZF-BT(70-30), c) NZF-BT(50-50), d) NZF-BT(30-70), e) NZF-BT(10-90) ceramics. Figure 5. Dielectric measurements: a) ε′ vs T for all investigated samples b) ε′ vs f for NZFBT(30-70) at different temperatures. Figure 6. Variation of real part of permittivity with temperature measured at different frequencies for sample NZF-BT(30-70). Figure 7. Frequency dependence of dielectric permittivity at different temperatures for sample NZF-BT(30-70). Figure 8. Frequency dependance of tangent loss at different temperatures for sample NZFBT(30-70). Figure 9. Impedance spectra of composite ceramics measured at 300 °C. Figure 10. Arhenius plots of grain and grain boundary conductivity for NZF-BT(70-30), NZFBT(50-50), NZF-BT(30-70), NZF-BT(10-90). Figure 11. Imaginary parts of the impedance spectra (Z″) as a function of frequency of a) NZFBT(70-30), b) NZF-BT(50-50), c) NZF-BT(30-70), d) NZF-BT(10-90) at different temperatures.

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Figure 12. Frequency dependence Z"/Zmax of a) NZF-BT(70-30), b) NZF-BT(50-50), c) NZFBT(30-70), d) NZF-BT(10-90) at different temperatures. Figure 13. Complex electric modulus plane plot (M″ vs M′) at different temperatures for a) NZF-BT(70-30), b) NZF-BT(50-50), c) NZF-BT(30-70), d) NZF-BT(10-90) composites. Figure 14. Frequency dependence of imaginary parts of modulus of a) NZF-BT(70-30), b) NZF-BT(50-50), c) NZF-BT(30-70), d) NZF-BT(10-90) composites. Figure 15. Frequency dependence Z"/Zmax and M"/Mmax for NZF-BT(10-90) at 250 ºC. Figure 16. Hysteresis P-E loops for all investigated composites. Figure 17. Magnetic measurements for investigated samples at room temperature. TABLE CAPTIONS Table 1. Grain resistance, grain boundary resistance, total resistance and capacitanse for measured samples. Table 2. Saturation magnetization moment, saturation fields, residual magnetization and coercive field for yNi0.7Zn0.3Fe2O4-(1-y)BT (y= 0.9, 0.7, 0.5, 0.3, 0.1) composites samples.

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Figure 4a

Figure 4b

Figure 4c

Figure 4d

Figure 4e

Figure 5

Figure 6a

Figure 6b

Figure 6c

Figure 7

Figure 8

Figure 9a

Figure 9b

Figure 10

Figure 11a

Figure 11b

Figure 11c

Figure 11d

Figure 12a

Figure 12b

Figure 12c

Figure 12d

Figure 13a

Figure 13b

Figure 13c

Figure 13d

Figure 14a

Figure 14b

Figure 14c

Figure 14d

Figure 15

Figure 16a

Figure 16b

Figure 17

Table captions

TABLE CAPTIONS Table 1. Grain resistance, grain boundary resistance, total resistance and capacitanse for measured samples. Table 2. Saturation magnetization moment, saturation fields, residual magnetization and coercive field for yNi0.7Zn0.3Fe2O4-(1-y)BT (y= 0.9, 0.7, 0.5, 0.3, 0.1) composites samples.

Tables

Table 1. Sample

NZF-BT(70-30)

NZF-BT(50-50)

NZF-BT(30-70)

NZF-BT(10-90)

N0.7Z0.3F

T(°C) 150 200 250 300 150 200 250 300 150 200 250 300 150 200 250 300 Troom

from Z"-Z' R1(Ωm) R2(Ωm) 100 850 50 220 40 160 24 86 300 5200 2800 6200 100 5200 400 1050 90000 210000 19000 46000 1000 70000 3200 36800 15300 35000 2900 4800 1250 2900 140 1060 242 725

Rtotal(Ωm) 950 270 200 110 5500 9000 5300 1450 300000 65000 8000 40000 50300 7700 4150 1200 967

from Z"-f Rgb(Ωm) Cgb nF/m 338 8.37 165 6.24 60 5.17 47 5.14 4500 3.74 4928 3.1 2045 1.63 895 1.23 209687 18.06 45390 15.61 68553 13.46 28036 5.54 33980 73.34 3932 26.42 2003 20.63 576 12.67

>108

BT

Table 2. Sample

Msat (emu/g)

Hsat (kOe) Mr (emu/g)

Hc (Oe)

NZF-BT(90-10)

67.6

2.16

4.44

31.55

NZF-BT(70-30)

55.5

2.07

5.04

39.17

NZF-BT(50-50)

37.6

1.98

7.21

68.03

NZF-BT(30-70)

20.8

1.81

3.43

54.47

NZF-BT(10-90)

4.1

1.31

0.48

48.67

NZF

78.7

1.61

2.64

0.018