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Influences of annealing temperature on dielectric, magnetic and magnetoelectric coupling properties of 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 ceramics
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
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Research articles
Influences of annealing temperature on dielectric, magnetic and magnetoelectric coupling properties of 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 ceramics ⁎
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L.G. Wang1, X.X. Wang1, C.M. Zhu , G.B. Yu, Y.T. Zhao, Z.H. Huang, W.J. Kong , F.C. Liu, F.Z. Lv College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroics Magnetoelectric coupling Dielectricity Magnetism
Multiferroic 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 ceramics were prepared by sol-gel method with various temperatures ranging from 950 to 1100 °C. The influence of annealing temperature on structure, morphology, dielectric, magnetic and magnetoelectric coupling effect was investigated. The refinement of x-ray diffraction data and the thermogravimetric-differential thermal analysis indicated the coexistence of perovskite and spinel phases with annealing temperature above 950 °C. Increasing grain size in surface morphology measurement was closely related to the increasing annealing temperature. Enhancement in dielectric properties was observed with increasing annealing temperature. The room-temperature ferromagnetic properties were also detected with saturated magnetic hysteresis loops under different annealing temperatures. Finally, magnetoelectric coupling effect was confirmed in the samples. The possible factors for room-temperature multiferroic properties were discussed with increasing annealing temperature. This study showed that annealing temperature could evidently affect the multiferroic properties in 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution.
1. Introduction
written as:[10]
Multiferroic materials can exhibit the simultaneous (anti)ferroelectric, (anti)ferromagnetic, (anti)ferroelastic and ferrotoroidic behaviors. Hence, they attract increasing attention due to the fascinating fundamental physics and potential applications for data storage, spin valves, sensors and spintronics etc [1–3]. In particular, magnetoelectric (ME) multiferroics are known as the coexistence of ferroelectricity and ferromagnetism in the same material and the appearance of ME coupling effect. During ME coupling process, the ferroelectric polarization can be induced by an external magnetic field or the magnetization is induced by an electric field [4–6]. ME multiferroics can be divided into single phase and composite materials. However, the kinds of single phase ME multiferroics are very finite. On the other hand, the weak ME effect and low working temperature also limit the utilization of single phase materials. In contrast, ME multiferroic composites can have the evident ME coupling effect at room temperature because the composites are usually acquired through good magnetic materials combined with ferroelectric/piezoelectric materials [7–9]. ME coupling effect of composites is on account of the magnetostrictive effect in magnetic phases and the piezoelectric effect in piezoelectric phases, which can be
ME effect =
magnetic mechanical eletrical mechanical or × × mechanical eletrical mechanical magnetic (1)
In ME multiferroic composites, the viable ME response in technology can be observed above room temperature, such as the easy control and the high coupling coefficient. Thus, ME multiferroic composites have become the hot research topic on multiferroic materials and been extensively investigated in recent years [11–13]. In order to realize the practical application, one of the important goals for multiferroic composites is improve the multiferroic properties at room temperature. It is well known that the most fundamental factor affecting the multiferroic properties is the material system. In addition, multiferroic properties are also very sensitive to the preparation process, such as the annealing time and annealing temperature. These extrinsic effects, which are mainly related to the inner stress, domain wall motion and defects, usually play a significant role in multiferroic performance [14]. Therefore, from the point of structure–property relationship, it is interesting to discuss improving the multiferroic properties through modifying some extrinsic conditions. Among many
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Corresponding authors. E-mail addresses: [email protected] (C.M. Zhu), [email protected] (W.J. Kong). 1 L.G. Wang and X.X. Wang equally contributed to this work. https://doi.org/10.1016/j.jmmm.2019.165773 Received 20 May 2019; Received in revised form 27 August 2019; Accepted 30 August 2019 Available online 30 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
L.G. Wang, et al.
at 95 °C for 5 h to form the gel. Subsequently, the gel was dried at 150 °C, ground and calcined at 450 °C for 5 h to obtain the precursor powder. Finally, the precursor powder was pressed into disks with 1 mm in thickness and 10 mm in diameter, and annealed for 2 h at 950, 1000, 1050, and 1100 °C respectively to yield the samples. Besides, in order to measure the dielectric and ME coupling properties, electrodes with 6 mm in diameter were applied to both surfaces of the disks with silver paste. The synthesized samples were subjected to various investigations as following. Crystal structure was characterized by x-ray diffraction (XRD, Philips Panalytical X’pert) using the iron-filtered Cu-Kα radiation (λ = 0.15406 nm) in the 2θ range of 10°–90° with a step size of 0.02°. Lattice constants of the samples were determined by refinement with the Rietveld technique using Fullprof method based on the fine XRD data. To confirm the crystallization temperature range, thermogravimetric-differential thermal analysis (TG-DTA) of the gel was performed based on a thermoanalyzer (Labsys evo TG-DTA/DSC) in the temperature range from 27 to 1027 °C at a heating rate of 10 °C/min. Micrographs were observed by scanning electron microscopy (SEM, JSM-5610 V). The dielectric properties were measured by an impedance analyzer (PST-2000H). Magnetic properties were carried out through a physical property measurement system (PPMS, Quantum Design). ME coupling coefficients were analyzed by magnetoelectric coupling test system (Super ME, Quantum Design).
approaches, controlling annealing temperature which has been used in many material systems, is a convenient and economical method to regulate the microstructure, microdomain and further the physical properties of multiferroics [15]. Hence, systemic study of the annealing temperature effect on multiferroic properties is very necessary. For clearly analyzing the influence of annealing temperature on multiferroic properties, we select the solid solution ceramics with a certain proportion of Bi0.5Na0.5TiO3 (BNTO) and NiFe2O4 (NFO). As the essential component for electronic devices, piezoelectric ceramic lead zirconate titanate (Pb(Zr, Ti)O3 or PZT) is the most widely used due to its excellent electric properties. However, the hazardous lead is a serious problem to urgently solve by exploring lead-free piezoelectric ceramics. BNTO which has been studied intensively for decades is a promising candidate with high Curie temperature and large remnant polarization [16,17]. NFO with a typical spinel structure has been chosen as the magnetic phase in our previous experience because of its obvious net magnetization [18]. The ferrimagnetism originates from magnetic moment of Ni2+ ions at octahedral sites and anti-parallel spins between Fe3+ ions at tetrahedral sites [19]. Moreover, xBi0.5Na0.5TiO3-(1-x)NiFe2O4 (xBNTO-(1-x)NFO) with x = 0.7 can display the excellent room-temperature multiferroic properties based on our previous work. Therefore, in this work, we have synthesized 0.7BNTO-0.3NFO solid solution composites annealed at 950, 1000, 1050 and 1100 °C respectively. The effect of annealing temperature on microstructure, morphology, dielectric, magnetic and ME coupling properties has been investigated in detail. The results reveal that annealing temperature can have a significant impact on the room-temperature multiferroic behavior of 0.7BNTO-0.3NFO.
3. Results and discussion Fig. 2 shows the experimental and refined XRD profiles of 0.7BNTO0.3NFO samples annealed at different temperatures. It reveals that all samples annealed above 950 °C present the complete two phase coexistence of perovskite structure BNTO and spinel structure NFO. It can be proved by the positions of Bragg reflections in the refinement results. Moreover, with annealing temperature increasing from 950 to 1100 °C, crystal structure characterization of the samples does not exhibit any observable difference. Through further calculation of the refinement process, lattice constants are obtained as shown in Table 1. The incredibly similar data of both BNTO and NFO phases also illustrate the ignorable influence on crystal structure of different annealing
2. Experimental procedure The 0.7BNTO-0.3NFO ceramics were synthesized through sol–gel method. The flowchart of the detailed experimental procedure is shown in Fig. 1. Analytical reagents of Bi(NO3)3·5H2O, NaNO3, Ni (NO3)2·6H2O, Fe(NO3)3·9H2O, tetrabutyl titanate (C16H36O4Ti) and citric acid (C6H8O7·6H2O) were used as the raw materials. In this experiment, citric acid acted as the complexant. Firstly, nitrates as metal precursors and tetrabutyl titanate were proportionally dissolved in citric acid solution. Then, with strong magnetic stirring at room temperature, pH value of ~7 was adjusted using ammonia. Moreover, stirring for 15 h was still continued at room temperature in order to form the homogeneous sol. Secondly, the sol was heated in water bath
Fig. 2. The experimental (red circle) and calculated (black line) XRD patterns for 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures from 950 to 1100 °C. The short vertical lines with green color below the patterns mark the positions of Bragg reflections. The bottom continuous line is the difference between the observed and calculated intensity.
Fig. 1. Flowchart of the detailed experimental procedure for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 samples. 2
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
L.G. Wang, et al.
be attributed to four kinds of polarization: space charge, dipolar, ionic and electronic. The high value of dielectric constant observed at lower frequencies can be mainly attributed to the space charge polarization caused by the heterogeneity in samples. The heterogeneity generally arises from porosity, grain structure and interfaces between two different dielectric phases such as ferrite and ferroelectric systems [24]. The conducting grain boundaries of interfaces with some defects and oxygen vacancies could lead to the increase in space charges. Under the action of electric field, the space charge polarization mainly occurs in the low frequency region and has a slow polarization form. With the frequency further increasing, the movement of space charges cannot keep up with the change of frequency. The dielectric constant at the higher frequency range just comes from the other polarization forms. Thus, dielectric constant gradually goes down at high frequency range. This dielectric behavior can be understood by the Maxwell-Wagner interfacial polarization [25]. In addition, as annealing temperature increases from 950 to 1100 °C, dielectric constant undergoes the monotonous increasing tendency. Fig. 5b shows the room-temperature dielectric constant of samples with different annealing temperatures at 1000 Hz. It can be seen that the dielectric constant values are respectively 272.6, 356.8, 434.5 and 462.2 as the annealing temperature increases from 950 to 1100 °C. Moreover, dielectric dispersion phenomenon can be also detected in the relationship of dielectric constant and annealing temperature. The variation of dielectric loss (tan δ) with frequency at room temperature under different annealing temperatures is plotted in Fig. 6. Dielectric loss of all the samples is below 0.4 in the range of measurement frequency, which indicates that the samples have attractive potential application in electronic devices. Besides, dielectric loss at lower frequencies is relative higher than that of higher frequency range. With increasing measuring frequency, abnormal peak of dielectric loss appears. As can be seen, the peak of dielectric loss is negligible with the annealing temperature of 950 °C. However, it is gradually evident with annealing temperature increasing from 1000 to 1100 °C. Moreover, the peak shifts to higher frequency range with increasing annealing temperature. Similar to the variation of dielectric constant, frequency dependence of dielectric loss presents more distinct dispersion properties, which also proves the Maxwell-Wagner interfacial polarization in the samples. Maxwell-Wagner polarization usually occurs in heterogeneous dielectrics where different dielectrics have different conductivities. The observed relaxation could be attributed to the heterogeneity of the 0.7BNTO-0.3NFO composites, such as the grain boundaries in the interfaces and other defect areas. Thus, it can be supposed that the appearance of dielectric loss peaks is attributed to Maxwell-Wagner relaxation. Room-temperature magnetic properties of 0.7BNTO-0.3NFO annealed at various temperatures are investigated and shown in Fig. 7a with the applied magnetic field range of ± 30 kOe. It confirms that all the samples have well saturated magnetic hysteresis loops and show ferromagnetic behavior at room temperature because of the rapid saturated state under a small magnetic field. Besides, the magnetic behavior is found to be strongly depended on the annealing temperature. Taking saturation magnetization (MS) as an example, MS is monotonously decreasing with increasing annealing temperature, which can be clearly observed in the right inset of Fig. 7a. Further, the annealing temperature dependence of MS is shown in Fig. 7b. As samples are annealed at 950, 1000, 1050 and 1100 °C, the values of MS are respectively 13.2, 11.7, 10.7 and 10.2 emu/g. The coexistence of ferroelectric BNTO and magnetic NFO phases may give rise to ME coupling effect, which is characterized by ME voltage coefficient (αE), i.e. induced voltage with applied direct current magnetic field. αE is usually calculated according to
Table 1 The lattice constant a of BNTO and NFO as a function of annealing temperature. Annealing temperature (°C)
950
1000
1050
1100
BNTO (Å) NFO (Å)
3.895 8.348
3.895 8.347
3.896 8.348
3.895 8.344
Fig. 3. TG-DTA curves of the 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 gel.
temperatures. TG-DTA curves of 0.7BNTO-0.3NFO gel are shown in Fig. 3. As displayed in TG curve, the weight loss process can be divided into four stages. The first stage is between 60 and 110 °C with a slight weight loss which is mainly caused by the dehydration of residual water and removal of organic solvents. The second stage accompanied by an evident exothermic reaction is in the temperature range of 110–200 °C with the steep weight loss. It can be related to the combustion of volatile organic matter and nitrates. In the third stage, a gentle decrease of weight is observed at the temperature range of 200–780 °C, which should be due to the decomposition of remnant nitrates [20]. Subsequently, the TG curve becomes basically parallel to the temperature axis, indicating no further weight loss above 780 °C. In DTA curve, the exothermic peak at 200 °C confirms the burnout of organic contents. Moreover, two small exothermic peaks of ~870 (T1) and ~940 °C (T2) with unchanged weight can be observed, which implies the crystallization of BNTO and NFO phases [21,22]. Thermal effect is hardly observed above 950 °C, suggesting that no crystallization occurs at the higher temperature range, which is consistent with the TG curve. TG-DTA results of the gel reveal that crystallization temperature is below 950 °C. It also proves the uninfluenced crystal structure in XRD patterns with annealing temperature increasing from 950 to 1100 °C. The surface morphologies of 0.7BNTO-0.3NFO annealed at different temperatures are shown in Fig. 4. It is obvious that the average grain size is gradually increased as the annealing temperature increases. Meanwhile, the loose dispersion at lower annealing temperature such as 950 °C is also improved with increasing temperature. However, agglomeration phenomenon is also observed with increasing annealing temperature. In addition, two kinds of grain sizes can be found in all the SEM images with different annealing temperatures. Compared with the previous researches, it can be concluded that the smaller grains are NFO phase and the larger ones are BNTO phase. It also reveals that the growth rate of BNTO phase is faster than that of NFO phase with annealing temperature increasing [23]. Fig. 5 shows the frequency dependence of dielectric constant (εr) of 0.7BNTO-0.3NFO annealed at different temperatures in the range of 102–106 Hz. As shown in Fig. 5a, dielectric constant decreases steeply at lower frequencies and attains nearly constant at higher frequencies, which is due to the different polarizations contributing to the dielectric constant at different frequencies. The increase in dielectric constant can
αE = (∂E / ∂H ) = V /(dHac )
(2)
where V is ME voltage developed across the samples, d is the thickness of the samples and Hac is the magnitude of the applied alternating 3
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
L.G. Wang, et al.
Fig. 4. SEM morphologies of the 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures.
current magnetic field [26]. To confirm the coupling between BNTO and NFO phases, ME measurement has been studied at 300 K. The measurement method is observing the change of αE under the applied magnetic field varying from 0 to 1000 Oe. The applied alternating current magnetic field is 5 Oe. The variation of αE with direct current magnetic field for 0.7BNTO-0.3NFO annealed at various temperatures is shown in Fig. 8a. As can be seen, all the samples exhibit significant ME coupling effect. The value of αE increases with the increase in magnetic field, reaches a maximum value and then decreases for the higher magnetic field, which is a typical proof of the magnetic-mechanical–electrical interaction between two phases [27]. It is necessary to track the piezomagnetic coupling coefficient
q = dk / dH
Peak position is successively 266.5, 197.7, 281.5 and 300.0 Oe with annealing temperature increasing from 950 to 1100 °C, which is decreasing and then increasing with the minimum value at 1000 °C. Correspondingly, the peak value is 1.03, 1.58, 1.28 and 1.16 mV/ (Oe·cm) with the maximum value of αE at 1000 °C when the annealing temperature is respectively 950, 1000, 1050 and 1100 °C, which presents the opposite variation tendency to that of peak position. It is worth noting that 1000 °C is a special annealing temperature point due to the change trend transition of ME coupling effect. Based on the above characterization and analysis of the crystal structure, morphology and physical properties, it can be considered that the influence of annealing temperature on room-temperature multiferroic behavior of 0.7BNTO-0.3NFO samples is mainly related to the variation of morphology with different annealing temperatures. Combined with Figs. 4 and 5, the increase in dielectric constant with increasing annealing temperature can be ascribed to the gradual increase of grain size. Room-temperature dielectric properties generally result from the combination contribution of intrinsic response and extrinsic factors. The variation of grain size induced by annealing temperature is related to domain walls, phase boundaries and defect contributions, which are the extrinsic factors to influence the dielectric constant. Ferroelectric materials possess multiple ferroelectric domains separated by interfaces known as the domain walls. Dielectric constant depends on the number and mobility of domain walls because of the certain amount of space charge sites inside grain boundaries and domain walls [29]. These space charge sites lead to electric field, which can observably affect the movement of domain walls. The increase of grain size facilitates the formation and growth of domains and reduces the area of the domain boundaries. Thus, the surface area of space charge layer decreases with the increase in grain size. As a result, the
(3)
where k is the magnetostriction [28]. For ferromagnetic phase, when magnetostriction obtains the saturation value, piezomagnetic coupling coefficient q decreases and the piezomagnetic coupling gradually becomes weak, which further results in the weakening of the ME coupling effect [4]. In 0.7BNTO-0.3NFO samples, the initial increase of αE is due to the increase in magnetostriction of NFO phase and the subsequent decrease of αE with increase in applied magnetic field is because of the fact that the magnetostriction coefficient of NFO phase reaches the saturation value at the certain value of magnetic field. Furthermore, the distinct annealing temperature dependence of ME coupling effect can be observed in Fig. 8a, which is mainly reflected in two aspects including the peak value and the peak position of αE. The peak position is the required magnetic field to reach the peak value. In order to describe the influence of annealing temperature on ME coupling effect more intuitively, variation of the peak position and peak value of αE with different annealing temperatures is shown in Fig. 8b. 4
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
L.G. Wang, et al.
Fig. 5. (a) Frequency dependence of the dielectric constant for 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures. (b) The variation of dielectric constant at 1000 Hz with different annealing temperatures.
Fig. 7. (a) Room-temperature magnetic hysteresis loops for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 solid solution annealed at different temperatures. Inset exhibits the magnified region of the loops. (b) Annealing temperature dependence of MS.
the increase in grain size of BNTO phase means the reduction of disordered area and gives rise to an increase of the dielectric constant likewise. All the samples present saturated magnetic hysteresis loops, which is attributed to the NFO phase. However, the decrease in MS with gradual increase of annealing temperature can be caused by two reasons. Firstly, the higher magnetization at lower temperature is due to the disordered crystal site orientation of the ions in spinel structure. When annealing temperature increases, the ordering of ion occupancy leads to the decrease of magnetic moment [31]. Then, as can be seen from Figs. 3 and 4, with annealing temperature increasing, the growth rate of BNTO phase is faster than that of NFO phase. The larger grains of BNTO hinder the growth of NFO, and break the magnetic domain wall motion and domain rotation of NFO phase. Thus, as the annealing temperature increases, the saturation magnetization of the sample gradually decreases. The mechanism of ME coupling effect of 0.7BNTO-0.3NFO samples has been discussed, which is related to the magnetic-mechanical–electrical interaction in the samples with applied external magnetic field. Nevertheless, the influence of annealing temperature on the variation of ME coupling effect should be attributed to the different morphologies of the samples. As shown in Fig. 8b, the increase of peak value with annealing temperature from 950 to 1000 °C can be due to the increasing grain size. ME coupling is on account of the mechanical coupling between two phases. The larger grain size decreases the
Fig. 6. Frequency dependence of the dielectric loss for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 solid solution annealed at different temperatures.
space charge field is weakened. Further, the movement of domain walls tends to fluent, which results in the increase of dielectric constant. It is also consistent with the space charge theory of Okazaki and Nagata [30]. In addition, the domain boundaries with a relatively low dielectric constant are often due to the disordered structure. Therefore, 5
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
L.G. Wang, et al.
4. Conclusions In conclusion, influence of annealing temperature on structure, morphology, dielectric, magnetic and ME coupling properties of 0.7BNTO-0.3NFO ceramics was investigated. With annealing temperature from 950 to 1100 °C, the XRD and TG-DTA results revealed the formation and coexistence of BNTO and NFO phases with ignorable variation of crystal constant. The effect on morphology was evidently shown as the increasing grain size and the appearance of agglomeration with increasing annealing temperature. Monotonous increase in dielectric constant with low dielectric loss was also observed by increasing annealing temperature. The magnetic properties also strongly depended on annealing temperature with MS decreasing from 13.2 to 10.2 emu/g at the annealing temperature range of 950–1100 °C. At last, the room-temperature ME coupling effect was realized and the optimal annealing temperature was deduced at ~1000 °C with the maximum of αE and the minimum of applied magnetic field. The above results indicated that the variation of morphology induced by increasing annealing temperature had important influence on the room-temperature multiferroic properties of 0.7BNTO-0.3NFO solid solution. It also proved that 0.7BNTO-0.3NFO solid solution had potential applications in the field of multifunction devices. Acknowledgements This work was supported by Natural Science Foundation of Guangxi Province (No. 2018GXNSFBA281125), Middle-aged and Young Teachers' Basic Ability Promotion Project of Guangxi (No. 2019KY0096), Innovation and Entrepreneurship Program for College Students of Guangxi (No. 201810602240) and National Natural Science Foundation of China (No. 11164003) Appendix A. Supplementary data
Fig. 8. (a) Variation of magnetoelectric coupling coefficient αE with the applied magnetic field of 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures. (b) Variation of the peak position and peak value of αE with different annealing temperatures.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165773. References
surface ratio of the whole grain accompanied by the increasing magnetic domains and ferroelectric domains, which in turn enhances the reaction of magnetic domains to the applied magnetic field and further affects the deformation of ferroelectric phase. Thus, the ME coupling effect is stronger when annealing temperature is increased. However, as annealing temperature continues to increase, grain size is increased. Meanwhile, the agglomeration phenomenon of the samples also appears due to the different growth rates of BNTO and NFO phases, which is more evident in BNTO phase because of the fast growing grain size. Agglomeration of BNTO phase impedes the growth of magnetic domains in NFO phase, weakening the magnetic properties and interaction between BNTO and NFO phases. Besides, as observed in SEM graphs of Fig. 4, agglomeration is also accompanied by the increasing grain boundaries and porosities of the samples, which can diminish the mechanical coupling and the strain conversion generated by magnetic field. Therefore, the peak value of αE is decreased with annealing temperature increasing from 1000 to 1100 °C. Compared with the variation of peak value, peak position of αE presents the opposite tendency. The required magnetic field of peak value is a critical parameter for the operation of multiferroic-based devices to maximize the energy conversion between electric and magnetic fields. The appearance of peak value is related to the saturation value of magnetostriction coefficient in NFO phase, which is in connection with the certain value of magnetic field (that is the peak position). Therefore, the maximum of peak value is obtained at the minimum of peak position with the annealing temperature of 1000 °C, which also indicates that 1000 °C is the optimal annealing temperature to realize the most significant ME coupling of 0.7BNTO-0.3NFO samples.
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Research articles
Influences of annealing temperature on dielectric, magnetic and magnetoelectric coupling properties of 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 ceramics ⁎
T
⁎
L.G. Wang1, X.X. Wang1, C.M. Zhu , G.B. Yu, Y.T. Zhao, Z.H. Huang, W.J. Kong , F.C. Liu, F.Z. Lv College of Physical Science and Technology, Guangxi Normal University, Guilin 541004, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Multiferroics Magnetoelectric coupling Dielectricity Magnetism
Multiferroic 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 ceramics were prepared by sol-gel method with various temperatures ranging from 950 to 1100 °C. The influence of annealing temperature on structure, morphology, dielectric, magnetic and magnetoelectric coupling effect was investigated. The refinement of x-ray diffraction data and the thermogravimetric-differential thermal analysis indicated the coexistence of perovskite and spinel phases with annealing temperature above 950 °C. Increasing grain size in surface morphology measurement was closely related to the increasing annealing temperature. Enhancement in dielectric properties was observed with increasing annealing temperature. The room-temperature ferromagnetic properties were also detected with saturated magnetic hysteresis loops under different annealing temperatures. Finally, magnetoelectric coupling effect was confirmed in the samples. The possible factors for room-temperature multiferroic properties were discussed with increasing annealing temperature. This study showed that annealing temperature could evidently affect the multiferroic properties in 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution.
1. Introduction
written as:[10]
Multiferroic materials can exhibit the simultaneous (anti)ferroelectric, (anti)ferromagnetic, (anti)ferroelastic and ferrotoroidic behaviors. Hence, they attract increasing attention due to the fascinating fundamental physics and potential applications for data storage, spin valves, sensors and spintronics etc [1–3]. In particular, magnetoelectric (ME) multiferroics are known as the coexistence of ferroelectricity and ferromagnetism in the same material and the appearance of ME coupling effect. During ME coupling process, the ferroelectric polarization can be induced by an external magnetic field or the magnetization is induced by an electric field [4–6]. ME multiferroics can be divided into single phase and composite materials. However, the kinds of single phase ME multiferroics are very finite. On the other hand, the weak ME effect and low working temperature also limit the utilization of single phase materials. In contrast, ME multiferroic composites can have the evident ME coupling effect at room temperature because the composites are usually acquired through good magnetic materials combined with ferroelectric/piezoelectric materials [7–9]. ME coupling effect of composites is on account of the magnetostrictive effect in magnetic phases and the piezoelectric effect in piezoelectric phases, which can be
ME effect =
magnetic mechanical eletrical mechanical or × × mechanical eletrical mechanical magnetic (1)
In ME multiferroic composites, the viable ME response in technology can be observed above room temperature, such as the easy control and the high coupling coefficient. Thus, ME multiferroic composites have become the hot research topic on multiferroic materials and been extensively investigated in recent years [11–13]. In order to realize the practical application, one of the important goals for multiferroic composites is improve the multiferroic properties at room temperature. It is well known that the most fundamental factor affecting the multiferroic properties is the material system. In addition, multiferroic properties are also very sensitive to the preparation process, such as the annealing time and annealing temperature. These extrinsic effects, which are mainly related to the inner stress, domain wall motion and defects, usually play a significant role in multiferroic performance [14]. Therefore, from the point of structure–property relationship, it is interesting to discuss improving the multiferroic properties through modifying some extrinsic conditions. Among many
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Corresponding authors. E-mail addresses: [email protected] (C.M. Zhu), [email protected] (W.J. Kong). 1 L.G. Wang and X.X. Wang equally contributed to this work. https://doi.org/10.1016/j.jmmm.2019.165773 Received 20 May 2019; Received in revised form 27 August 2019; Accepted 30 August 2019 Available online 30 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
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at 95 °C for 5 h to form the gel. Subsequently, the gel was dried at 150 °C, ground and calcined at 450 °C for 5 h to obtain the precursor powder. Finally, the precursor powder was pressed into disks with 1 mm in thickness and 10 mm in diameter, and annealed for 2 h at 950, 1000, 1050, and 1100 °C respectively to yield the samples. Besides, in order to measure the dielectric and ME coupling properties, electrodes with 6 mm in diameter were applied to both surfaces of the disks with silver paste. The synthesized samples were subjected to various investigations as following. Crystal structure was characterized by x-ray diffraction (XRD, Philips Panalytical X’pert) using the iron-filtered Cu-Kα radiation (λ = 0.15406 nm) in the 2θ range of 10°–90° with a step size of 0.02°. Lattice constants of the samples were determined by refinement with the Rietveld technique using Fullprof method based on the fine XRD data. To confirm the crystallization temperature range, thermogravimetric-differential thermal analysis (TG-DTA) of the gel was performed based on a thermoanalyzer (Labsys evo TG-DTA/DSC) in the temperature range from 27 to 1027 °C at a heating rate of 10 °C/min. Micrographs were observed by scanning electron microscopy (SEM, JSM-5610 V). The dielectric properties were measured by an impedance analyzer (PST-2000H). Magnetic properties were carried out through a physical property measurement system (PPMS, Quantum Design). ME coupling coefficients were analyzed by magnetoelectric coupling test system (Super ME, Quantum Design).
approaches, controlling annealing temperature which has been used in many material systems, is a convenient and economical method to regulate the microstructure, microdomain and further the physical properties of multiferroics [15]. Hence, systemic study of the annealing temperature effect on multiferroic properties is very necessary. For clearly analyzing the influence of annealing temperature on multiferroic properties, we select the solid solution ceramics with a certain proportion of Bi0.5Na0.5TiO3 (BNTO) and NiFe2O4 (NFO). As the essential component for electronic devices, piezoelectric ceramic lead zirconate titanate (Pb(Zr, Ti)O3 or PZT) is the most widely used due to its excellent electric properties. However, the hazardous lead is a serious problem to urgently solve by exploring lead-free piezoelectric ceramics. BNTO which has been studied intensively for decades is a promising candidate with high Curie temperature and large remnant polarization [16,17]. NFO with a typical spinel structure has been chosen as the magnetic phase in our previous experience because of its obvious net magnetization [18]. The ferrimagnetism originates from magnetic moment of Ni2+ ions at octahedral sites and anti-parallel spins between Fe3+ ions at tetrahedral sites [19]. Moreover, xBi0.5Na0.5TiO3-(1-x)NiFe2O4 (xBNTO-(1-x)NFO) with x = 0.7 can display the excellent room-temperature multiferroic properties based on our previous work. Therefore, in this work, we have synthesized 0.7BNTO-0.3NFO solid solution composites annealed at 950, 1000, 1050 and 1100 °C respectively. The effect of annealing temperature on microstructure, morphology, dielectric, magnetic and ME coupling properties has been investigated in detail. The results reveal that annealing temperature can have a significant impact on the room-temperature multiferroic behavior of 0.7BNTO-0.3NFO.
3. Results and discussion Fig. 2 shows the experimental and refined XRD profiles of 0.7BNTO0.3NFO samples annealed at different temperatures. It reveals that all samples annealed above 950 °C present the complete two phase coexistence of perovskite structure BNTO and spinel structure NFO. It can be proved by the positions of Bragg reflections in the refinement results. Moreover, with annealing temperature increasing from 950 to 1100 °C, crystal structure characterization of the samples does not exhibit any observable difference. Through further calculation of the refinement process, lattice constants are obtained as shown in Table 1. The incredibly similar data of both BNTO and NFO phases also illustrate the ignorable influence on crystal structure of different annealing
2. Experimental procedure The 0.7BNTO-0.3NFO ceramics were synthesized through sol–gel method. The flowchart of the detailed experimental procedure is shown in Fig. 1. Analytical reagents of Bi(NO3)3·5H2O, NaNO3, Ni (NO3)2·6H2O, Fe(NO3)3·9H2O, tetrabutyl titanate (C16H36O4Ti) and citric acid (C6H8O7·6H2O) were used as the raw materials. In this experiment, citric acid acted as the complexant. Firstly, nitrates as metal precursors and tetrabutyl titanate were proportionally dissolved in citric acid solution. Then, with strong magnetic stirring at room temperature, pH value of ~7 was adjusted using ammonia. Moreover, stirring for 15 h was still continued at room temperature in order to form the homogeneous sol. Secondly, the sol was heated in water bath
Fig. 2. The experimental (red circle) and calculated (black line) XRD patterns for 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures from 950 to 1100 °C. The short vertical lines with green color below the patterns mark the positions of Bragg reflections. The bottom continuous line is the difference between the observed and calculated intensity.
Fig. 1. Flowchart of the detailed experimental procedure for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 samples. 2
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be attributed to four kinds of polarization: space charge, dipolar, ionic and electronic. The high value of dielectric constant observed at lower frequencies can be mainly attributed to the space charge polarization caused by the heterogeneity in samples. The heterogeneity generally arises from porosity, grain structure and interfaces between two different dielectric phases such as ferrite and ferroelectric systems [24]. The conducting grain boundaries of interfaces with some defects and oxygen vacancies could lead to the increase in space charges. Under the action of electric field, the space charge polarization mainly occurs in the low frequency region and has a slow polarization form. With the frequency further increasing, the movement of space charges cannot keep up with the change of frequency. The dielectric constant at the higher frequency range just comes from the other polarization forms. Thus, dielectric constant gradually goes down at high frequency range. This dielectric behavior can be understood by the Maxwell-Wagner interfacial polarization [25]. In addition, as annealing temperature increases from 950 to 1100 °C, dielectric constant undergoes the monotonous increasing tendency. Fig. 5b shows the room-temperature dielectric constant of samples with different annealing temperatures at 1000 Hz. It can be seen that the dielectric constant values are respectively 272.6, 356.8, 434.5 and 462.2 as the annealing temperature increases from 950 to 1100 °C. Moreover, dielectric dispersion phenomenon can be also detected in the relationship of dielectric constant and annealing temperature. The variation of dielectric loss (tan δ) with frequency at room temperature under different annealing temperatures is plotted in Fig. 6. Dielectric loss of all the samples is below 0.4 in the range of measurement frequency, which indicates that the samples have attractive potential application in electronic devices. Besides, dielectric loss at lower frequencies is relative higher than that of higher frequency range. With increasing measuring frequency, abnormal peak of dielectric loss appears. As can be seen, the peak of dielectric loss is negligible with the annealing temperature of 950 °C. However, it is gradually evident with annealing temperature increasing from 1000 to 1100 °C. Moreover, the peak shifts to higher frequency range with increasing annealing temperature. Similar to the variation of dielectric constant, frequency dependence of dielectric loss presents more distinct dispersion properties, which also proves the Maxwell-Wagner interfacial polarization in the samples. Maxwell-Wagner polarization usually occurs in heterogeneous dielectrics where different dielectrics have different conductivities. The observed relaxation could be attributed to the heterogeneity of the 0.7BNTO-0.3NFO composites, such as the grain boundaries in the interfaces and other defect areas. Thus, it can be supposed that the appearance of dielectric loss peaks is attributed to Maxwell-Wagner relaxation. Room-temperature magnetic properties of 0.7BNTO-0.3NFO annealed at various temperatures are investigated and shown in Fig. 7a with the applied magnetic field range of ± 30 kOe. It confirms that all the samples have well saturated magnetic hysteresis loops and show ferromagnetic behavior at room temperature because of the rapid saturated state under a small magnetic field. Besides, the magnetic behavior is found to be strongly depended on the annealing temperature. Taking saturation magnetization (MS) as an example, MS is monotonously decreasing with increasing annealing temperature, which can be clearly observed in the right inset of Fig. 7a. Further, the annealing temperature dependence of MS is shown in Fig. 7b. As samples are annealed at 950, 1000, 1050 and 1100 °C, the values of MS are respectively 13.2, 11.7, 10.7 and 10.2 emu/g. The coexistence of ferroelectric BNTO and magnetic NFO phases may give rise to ME coupling effect, which is characterized by ME voltage coefficient (αE), i.e. induced voltage with applied direct current magnetic field. αE is usually calculated according to
Table 1 The lattice constant a of BNTO and NFO as a function of annealing temperature. Annealing temperature (°C)
950
1000
1050
1100
BNTO (Å) NFO (Å)
3.895 8.348
3.895 8.347
3.896 8.348
3.895 8.344
Fig. 3. TG-DTA curves of the 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 gel.
temperatures. TG-DTA curves of 0.7BNTO-0.3NFO gel are shown in Fig. 3. As displayed in TG curve, the weight loss process can be divided into four stages. The first stage is between 60 and 110 °C with a slight weight loss which is mainly caused by the dehydration of residual water and removal of organic solvents. The second stage accompanied by an evident exothermic reaction is in the temperature range of 110–200 °C with the steep weight loss. It can be related to the combustion of volatile organic matter and nitrates. In the third stage, a gentle decrease of weight is observed at the temperature range of 200–780 °C, which should be due to the decomposition of remnant nitrates [20]. Subsequently, the TG curve becomes basically parallel to the temperature axis, indicating no further weight loss above 780 °C. In DTA curve, the exothermic peak at 200 °C confirms the burnout of organic contents. Moreover, two small exothermic peaks of ~870 (T1) and ~940 °C (T2) with unchanged weight can be observed, which implies the crystallization of BNTO and NFO phases [21,22]. Thermal effect is hardly observed above 950 °C, suggesting that no crystallization occurs at the higher temperature range, which is consistent with the TG curve. TG-DTA results of the gel reveal that crystallization temperature is below 950 °C. It also proves the uninfluenced crystal structure in XRD patterns with annealing temperature increasing from 950 to 1100 °C. The surface morphologies of 0.7BNTO-0.3NFO annealed at different temperatures are shown in Fig. 4. It is obvious that the average grain size is gradually increased as the annealing temperature increases. Meanwhile, the loose dispersion at lower annealing temperature such as 950 °C is also improved with increasing temperature. However, agglomeration phenomenon is also observed with increasing annealing temperature. In addition, two kinds of grain sizes can be found in all the SEM images with different annealing temperatures. Compared with the previous researches, it can be concluded that the smaller grains are NFO phase and the larger ones are BNTO phase. It also reveals that the growth rate of BNTO phase is faster than that of NFO phase with annealing temperature increasing [23]. Fig. 5 shows the frequency dependence of dielectric constant (εr) of 0.7BNTO-0.3NFO annealed at different temperatures in the range of 102–106 Hz. As shown in Fig. 5a, dielectric constant decreases steeply at lower frequencies and attains nearly constant at higher frequencies, which is due to the different polarizations contributing to the dielectric constant at different frequencies. The increase in dielectric constant can
αE = (∂E / ∂H ) = V /(dHac )
(2)
where V is ME voltage developed across the samples, d is the thickness of the samples and Hac is the magnitude of the applied alternating 3
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
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Fig. 4. SEM morphologies of the 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures.
current magnetic field [26]. To confirm the coupling between BNTO and NFO phases, ME measurement has been studied at 300 K. The measurement method is observing the change of αE under the applied magnetic field varying from 0 to 1000 Oe. The applied alternating current magnetic field is 5 Oe. The variation of αE with direct current magnetic field for 0.7BNTO-0.3NFO annealed at various temperatures is shown in Fig. 8a. As can be seen, all the samples exhibit significant ME coupling effect. The value of αE increases with the increase in magnetic field, reaches a maximum value and then decreases for the higher magnetic field, which is a typical proof of the magnetic-mechanical–electrical interaction between two phases [27]. It is necessary to track the piezomagnetic coupling coefficient
q = dk / dH
Peak position is successively 266.5, 197.7, 281.5 and 300.0 Oe with annealing temperature increasing from 950 to 1100 °C, which is decreasing and then increasing with the minimum value at 1000 °C. Correspondingly, the peak value is 1.03, 1.58, 1.28 and 1.16 mV/ (Oe·cm) with the maximum value of αE at 1000 °C when the annealing temperature is respectively 950, 1000, 1050 and 1100 °C, which presents the opposite variation tendency to that of peak position. It is worth noting that 1000 °C is a special annealing temperature point due to the change trend transition of ME coupling effect. Based on the above characterization and analysis of the crystal structure, morphology and physical properties, it can be considered that the influence of annealing temperature on room-temperature multiferroic behavior of 0.7BNTO-0.3NFO samples is mainly related to the variation of morphology with different annealing temperatures. Combined with Figs. 4 and 5, the increase in dielectric constant with increasing annealing temperature can be ascribed to the gradual increase of grain size. Room-temperature dielectric properties generally result from the combination contribution of intrinsic response and extrinsic factors. The variation of grain size induced by annealing temperature is related to domain walls, phase boundaries and defect contributions, which are the extrinsic factors to influence the dielectric constant. Ferroelectric materials possess multiple ferroelectric domains separated by interfaces known as the domain walls. Dielectric constant depends on the number and mobility of domain walls because of the certain amount of space charge sites inside grain boundaries and domain walls [29]. These space charge sites lead to electric field, which can observably affect the movement of domain walls. The increase of grain size facilitates the formation and growth of domains and reduces the area of the domain boundaries. Thus, the surface area of space charge layer decreases with the increase in grain size. As a result, the
(3)
where k is the magnetostriction [28]. For ferromagnetic phase, when magnetostriction obtains the saturation value, piezomagnetic coupling coefficient q decreases and the piezomagnetic coupling gradually becomes weak, which further results in the weakening of the ME coupling effect [4]. In 0.7BNTO-0.3NFO samples, the initial increase of αE is due to the increase in magnetostriction of NFO phase and the subsequent decrease of αE with increase in applied magnetic field is because of the fact that the magnetostriction coefficient of NFO phase reaches the saturation value at the certain value of magnetic field. Furthermore, the distinct annealing temperature dependence of ME coupling effect can be observed in Fig. 8a, which is mainly reflected in two aspects including the peak value and the peak position of αE. The peak position is the required magnetic field to reach the peak value. In order to describe the influence of annealing temperature on ME coupling effect more intuitively, variation of the peak position and peak value of αE with different annealing temperatures is shown in Fig. 8b. 4
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
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Fig. 5. (a) Frequency dependence of the dielectric constant for 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures. (b) The variation of dielectric constant at 1000 Hz with different annealing temperatures.
Fig. 7. (a) Room-temperature magnetic hysteresis loops for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 solid solution annealed at different temperatures. Inset exhibits the magnified region of the loops. (b) Annealing temperature dependence of MS.
the increase in grain size of BNTO phase means the reduction of disordered area and gives rise to an increase of the dielectric constant likewise. All the samples present saturated magnetic hysteresis loops, which is attributed to the NFO phase. However, the decrease in MS with gradual increase of annealing temperature can be caused by two reasons. Firstly, the higher magnetization at lower temperature is due to the disordered crystal site orientation of the ions in spinel structure. When annealing temperature increases, the ordering of ion occupancy leads to the decrease of magnetic moment [31]. Then, as can be seen from Figs. 3 and 4, with annealing temperature increasing, the growth rate of BNTO phase is faster than that of NFO phase. The larger grains of BNTO hinder the growth of NFO, and break the magnetic domain wall motion and domain rotation of NFO phase. Thus, as the annealing temperature increases, the saturation magnetization of the sample gradually decreases. The mechanism of ME coupling effect of 0.7BNTO-0.3NFO samples has been discussed, which is related to the magnetic-mechanical–electrical interaction in the samples with applied external magnetic field. Nevertheless, the influence of annealing temperature on the variation of ME coupling effect should be attributed to the different morphologies of the samples. As shown in Fig. 8b, the increase of peak value with annealing temperature from 950 to 1000 °C can be due to the increasing grain size. ME coupling is on account of the mechanical coupling between two phases. The larger grain size decreases the
Fig. 6. Frequency dependence of the dielectric loss for 0.7Bi0.5Na0.5TiO30.3NiFe2O4 solid solution annealed at different temperatures.
space charge field is weakened. Further, the movement of domain walls tends to fluent, which results in the increase of dielectric constant. It is also consistent with the space charge theory of Okazaki and Nagata [30]. In addition, the domain boundaries with a relatively low dielectric constant are often due to the disordered structure. Therefore, 5
Journal of Magnetism and Magnetic Materials 494 (2020) 165773
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4. Conclusions In conclusion, influence of annealing temperature on structure, morphology, dielectric, magnetic and ME coupling properties of 0.7BNTO-0.3NFO ceramics was investigated. With annealing temperature from 950 to 1100 °C, the XRD and TG-DTA results revealed the formation and coexistence of BNTO and NFO phases with ignorable variation of crystal constant. The effect on morphology was evidently shown as the increasing grain size and the appearance of agglomeration with increasing annealing temperature. Monotonous increase in dielectric constant with low dielectric loss was also observed by increasing annealing temperature. The magnetic properties also strongly depended on annealing temperature with MS decreasing from 13.2 to 10.2 emu/g at the annealing temperature range of 950–1100 °C. At last, the room-temperature ME coupling effect was realized and the optimal annealing temperature was deduced at ~1000 °C with the maximum of αE and the minimum of applied magnetic field. The above results indicated that the variation of morphology induced by increasing annealing temperature had important influence on the room-temperature multiferroic properties of 0.7BNTO-0.3NFO solid solution. It also proved that 0.7BNTO-0.3NFO solid solution had potential applications in the field of multifunction devices. Acknowledgements This work was supported by Natural Science Foundation of Guangxi Province (No. 2018GXNSFBA281125), Middle-aged and Young Teachers' Basic Ability Promotion Project of Guangxi (No. 2019KY0096), Innovation and Entrepreneurship Program for College Students of Guangxi (No. 201810602240) and National Natural Science Foundation of China (No. 11164003) Appendix A. Supplementary data
Fig. 8. (a) Variation of magnetoelectric coupling coefficient αE with the applied magnetic field of 0.7Bi0.5Na0.5TiO3-0.3NiFe2O4 solid solution annealed at different temperatures. (b) Variation of the peak position and peak value of αE with different annealing temperatures.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165773. References
surface ratio of the whole grain accompanied by the increasing magnetic domains and ferroelectric domains, which in turn enhances the reaction of magnetic domains to the applied magnetic field and further affects the deformation of ferroelectric phase. Thus, the ME coupling effect is stronger when annealing temperature is increased. However, as annealing temperature continues to increase, grain size is increased. Meanwhile, the agglomeration phenomenon of the samples also appears due to the different growth rates of BNTO and NFO phases, which is more evident in BNTO phase because of the fast growing grain size. Agglomeration of BNTO phase impedes the growth of magnetic domains in NFO phase, weakening the magnetic properties and interaction between BNTO and NFO phases. Besides, as observed in SEM graphs of Fig. 4, agglomeration is also accompanied by the increasing grain boundaries and porosities of the samples, which can diminish the mechanical coupling and the strain conversion generated by magnetic field. Therefore, the peak value of αE is decreased with annealing temperature increasing from 1000 to 1100 °C. Compared with the variation of peak value, peak position of αE presents the opposite tendency. The required magnetic field of peak value is a critical parameter for the operation of multiferroic-based devices to maximize the energy conversion between electric and magnetic fields. The appearance of peak value is related to the saturation value of magnetostriction coefficient in NFO phase, which is in connection with the certain value of magnetic field (that is the peak position). Therefore, the maximum of peak value is obtained at the minimum of peak position with the annealing temperature of 1000 °C, which also indicates that 1000 °C is the optimal annealing temperature to realize the most significant ME coupling of 0.7BNTO-0.3NFO samples.
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