Journal of Nanomaterials
Volume 2015 (2015), Article ID 613565, 5 pages
http://dx.doi.org/10.1155/2015/613565

Research Article

Multiferroic Properties of Nanopowder-Synthesized Ferroelectric-Ferromagnetic 0.6BaTiO₃-0.4NiFe₂O₄ Ceramic

Xinye Xu,¹ Zheng Wu,² Yanmin Jia,³ Weijian Li,³ Yongsheng Liu,⁴ Yihe Zhang,⁵ and A’Xi Xue³

¹Chuyang Honors College, Zhejiang Normal University, Jinhua 321004, China
²College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China
³Department of Material Physics, Zhejiang Normal University, Jinhua 321004, China
⁴Department of Physics, Shanghai University of Electric Power, Shanghai 200090, China
⁵School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China

Received 6 March 2015; Accepted 27 April 2015

Academic Editor: Donglu Shi

Copyright © 2015 Xinye Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Multiferroic 0.6BaTiO₃-0.4NiFe₂O₄ dense nanoceramic composites were synthesized via a powder-in-sol precursor hybrid chemical synthesis route and a ceramic sintering process. At the measured frequency range (1 kHz~1 MHz), the relative dielectric constant is 150~1670 and the dielectric loss is 0.05~0.70. The composite ceramic showed obvious coexistence of ferroelectric and ferromagnetic phases. With the increase of temperature, the saturation ferromagnetic magnetization decreases, while the ferroelectric polarization increases.

1. Introduction

Multiferroic materials have been formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously and have stimulated much scientific and technological interest, owing not only to potential applications for devices taking advantage of the multiferroic coupling, but also to the interesting physics manifested by these materials [1]. It was first presumed to exist by Curie in 1894 based on the crystal symmetry considerations [2]. The multiferroic effect was first experimentally observed in Cr₂O₃ in 1961 [3, 4], and over the recent decades, a lot of monophase materials have been investigated [5]. However, due to low Neel temperature, it is difficult to use them in practical devices [3–5]. As a possible alternative, laminated ferromagnetic/ferroelectric composites have received much interest with regard to product properties [6, 7]. The multiferroic effect in a composite ceramic or laminated structure has a multiferroic coupling constant two or three orders higher than that in single phase structures [6, 7]. However, the laminated structure is limited by the feature size, miniaturizing difficulties, and brittleness. Composites ceramics are limited by their high dielectric loss, often higher than 1, which is not good for practical applications [7]. The high dielectric loss should be attributed to the interface reaction at high sintering temperature and the low resistance of the magnetic phase. Therefore, choosing the suitable ferroelectric and ferromagnetic phases and decreasing the sintering temperature to reduce the reactions at the phase interfaces, are crucial for the excellent multiferroic properties of ceramic composites.

In our research, we select the conventional barium titanate BaTiO₃ as the ferroelectric phase and use nickel ferrite NiFe₂O₄ as the ferromagnetic phase, due to its low anisotropy and high initial permeability. We also prepared nanopowders to synthesize BaTiO₃ (BT) phase and nickel ferrite phase in order to decrease the sintering temperature and reduce the interface reactions. A chemical method was developed to mix ferromagnetic and ferroelectric phases uniformly at the microscale, which also favors the multiferroic coupling. The temperature dependence on the multiferroic properties of 0.6BaTiO₃-0.4NiFe₂O₄ nanoceramic was presented in this paper. The combination of nickel ferrite and BaTiO₃ leads to a good multiferroic property.

2. Experimental Section

The 0.6BaTiO₃-0.4NiFe₂O₄ composites were made of two individual phases. The nano-BaTiO₃ powders were synthesized via a hydrothermal method [8]. Fe(NO₃)₃9H₂O, Ni(NO₃)₂6H₂O, C₆H₈O₇H₂O, and NH₄OH were used to synthesize NiFe₂O₄ ferrite. Highly dense microparticulate ceramics of the 0.6BaTiO₃-0.4NiFe₂O₄ compound were synthesized via a chemical powder-in-sol precursor hybrid synthesis route and ceramic sintering process. The process for original mixture powder was shown in Figure 1. Nano-BaTiO₃ powders were mixed directly with ethanol and ball-milled for 4 hours. Appropriate amounts of metal nitrates Ni(NO₃)₂·6H₂O (5.7260 g); Fe(NO₃)₃·9H₂O (15.8314 g), and citric acid were first dissolved into a beaker in a minimum amount of deionized water (150 mL). The pH of the solution was approximately 2. Appropriate amounts of ball-milled BaTiO₃ (pH-9) were added into the beaker. A small amount of ammonia and deionized water was added to the solution to adjust the pH value to about 7.5. The mixed solution was then put into an oven to dry for 72 hours at 130°C. In this process, the solution was heated and stirred to transform into a xerogel. The dried gel was burnt in a self-propagating combustion manner until all the gel was burnt out completely to form loose powder.

Figure 1: Processing route for preparing uniformly distributed slurry.

A simultaneous thermal analyzer (NETZSCH STA 449C) was used for thermogravimetric analysis and differential scanning calorimetry (TG and DSC) of the combustion process. The decomposition behavior of the resulting dried powder was studied at a heating rate of 10°C/min from the room temperature to 1300°C. On the basis of the TG and DSC results, the synthesized 0.6BaTiO₃-0.4NiFe₂O₄ powder was pelletized at a pressure of 220 MPa using polyvinyl alcohol as a binder. The pellets were sintered at 1150°C for 4 hours.

The density of sintered pellet was determined by Archimedes’ method. The phase structure was analyzed using a Cu Kα (0.15406 nm) X-ray Diffractometer (XRD) (PW-1830, Philips, Netherlands). A scanning electron microscopy (SEM, 1530 YP, Leo Co., Germany) was used for analyzing the microstructure of the ceramic sample. The complex dielectric permittivity and loss over a broad frequency range (1 kHz~1 MHz) were measured in vacuum with a HP 4192A impedance analyzer. The low temperature data were obtained starting from liquid nitrogen temperature. The magnetization hysteresis was performed by superconducting quantum interference device (SQUID). Ferroelectric performance was measured using a TF analyzer 2000 FE-Module (at 1 Hz).

3. Results and Discussion

The sintered 0.6BaTiO₃-0.4NiFe₂O₄ ceramic is dense with a density of 5.409 g/cm³, which is equivalent to 93.92% of the theoretical density (the half of the sum of BaTiO₃ density of 6.02 g/cm³ and NiFe₂O₄ density of 5.368 g/cm³).

Figure 2 shows the decomposition behavior of the synthesized powder obtained by DSC and TG measurements. The decomposition reaction proceeds mainly via three stages. The initial TG mass loss up to 130.5°C corresponds to the dehydration of the precursor. Next mass loss in the temperature range 200~400°C corresponds to the decarboxylation of oxalates, which results in an exothermic peak in the DSC curve at 268.9°C. Further weight loss in the temperature range of 400~600°C is due to the decomposition of residual Fe(NO₃)₃ and Ni(NO₃)₂ with an exothermic peak at 487.7°C. The total mass change is approximately 49.89%.

Figure 2: TG and DSC curves of the composite.

Figure 3 shows the XRD patterns. The particulate composite contains both the BaTiO₃ phase with a ferroelectric tetragonal perovskite structure and the NiFe₂O₄ phase with a ferromagnetic cubic spinel structure and no other impure phase is observed from the XRD results, which suggests that there is no significant chemical reaction during the sintering.

Figure 3: The results of XRD analysis for the composite.

Figure 4 shows the surface SEM of the dense diphase microstructure of 0.6BaTiO₃-0.4NiFe₂O₄ multiferroic ceramic. The average grain size of the composite ceramic is about 1 micrometer.

Figure 4: Microstructure of surface area of the sintered composite by SEM.

The result of dielectric constant and dielectric loss tangent as a function of frequency was shown in Figure 5. With the increase of frequency, both the dielectric constant and the dielectric loss tangent decrease continuously. The high dielectric loss tangent (0.05~0.70) at low frequencies are mainly attributed to the dipoles which resulted from the change in valance of cations, such as Fe³⁺/Fe²⁺ and Ni²⁺/Ni³⁺  [9, 10].

(a)

(b)

Figure 5: Dielectric constant (a) and dielectric loss tangent (b) as a function of frequency.

Nickel ferrite possesses a low magnetic anisotropy and a high initial permeability. Figure 6 shows magnetic hysteresis (M~H) of the composite at 5 K and 290 K, respectively. The sample exhibits typical magnetic hysteresis behavior at 290 K, which indicates the ferromagnetic characterization of the 0.6BaTiO₃-0.4NiFe₂O₄ ceramic. The saturation magnetizations are ~5 emu/g and ~2.8 emu/g at the temperatures of 5 K and 290 K, respectively. With the increase of temperature, the saturation ferromagnetic magnetization of the composite ceramic decreased and was much lower than that of pure NiFe₂O₄, which corresponded with the high nonmagnetic phase (ferroelectric) in this composite [11].

Figure 6: Ferromagnetic hysteresis M-H curve at different temperatures (5 K, 290 K).

The small hysteresis in Figure 6 indicates the week ferromagnetism of the composite. The magnetic properties are mainly determined by the content of NiFe₂O₄ phase. In this composite, the nonmagnetic BaTiO₃ phase prevents the interaction of the magnetic poles in the magnetic particles.

Quasistatic ferroelectric measurements indicate ferroelectric hysteresis as shown in Figure 7. NiFe₂O₄ ferrite phase has a low resistivity compared to the BaTiO₃ ferroelectric phase, which leads to a large conductivity of the multiferroic composite ceramic [12]. With the increase of temperature, the ferroelectric loop becomes lossy due to the influence of the relatively high conductivity in NiFe₂O₄ phase, which may originate from the mixed valence of the magnetic ions (e.g., Fe²⁺ and Fe³⁺) or from oxygen vacancies during the sintering process of the biphase ceramic [13, 14]. High conductivity usually leads to a pseudohysteresis loop and even low electric field endurance. At room temperature, the curve looks like a Lissajous plot, which is not a reliable ferroelectric loop. The ferroelectric remnant polarization strength of the 0.6BaTiO₃-0.4NiFe₂O₄ ceramic is approximately 105 μC/cm² at 300 K. The addition of the nonferroelectric NiFe₂O₄ phase hinders and pins the domain wall motion of ferroelectric regions and leads to the increase of the remanent polarization in the biphase ceramic (the remanent polarization of the pure BaTiO₃ ceramic is ~2 μC/cm²) [15–17].

Figure 7: Ferroelectric hysteresis loops of the 0.6BaTiO₃-0.4NiFe₂O₄ composite.

4. Conclusions

Multiferroic ceramic of 0.6BaTiO₃-0.4NiFe₂O₄ was synthesized via a chemical sol-gel precursor technique and ceramic sintering process. On basis of the XRD results, the nanopowder-synthesized ceramic contains both the ferroelectric tetragonal BaTiO₃ phase and the ferromagnetic cubic NiFe₂O₄ phase. The ferromagnetic M-H hysteresis and ferroelectric P-E hysteresis properties demonstrate its multiferroicity.

Conflict of Interests

The authors do not have any conflict of interests in their submitted paper.

Acknowledgments

This work was supported by the Natural Science Foundation of Zhejiang Province (LY14E020003), Zhejiang Provincial Xinmiao Project of China (2015R404001 and 2014R404015), the National Natural Science Foundation of China (no. 11374204), “Shu Guang” Project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation (no. 13SG52), and the Science and Technology Commission of Shanghai Municipality (nos. 12JC1404400 and 14520501000).

References

P. Fischer, M. Polomska, I. Sosnowska, and M. Szymanski, “Temperature dependence of the crystal and magnetic structures of BiFeO₃,” Journal of Physics C: Solid State Physics, vol. 13, no. 10, pp. 1931–1940, 1980.

P. Curie, “Sur la symétrie dans les phénomènes physiques, symétrie d'un champ électrique et d'un champ magnétique,” Journal de Physique Théorique et Appliquée, vol. 3, no. 1, pp. 393–415, 1894.

G. T. Rado and V. J. Folen, “Observation of the magnetically induced magnetoelectric effect and evidence for antiferromagnetic domains,” Physical Review Letters, vol. 7, no. 8, pp. 310–311, 1961.

V. J. Folen, G. T. Rado, and E. W. Stalder, “Anisotropy of the magnetoelectric effect in Cr₂O₃,” Physical Review Letters, vol. 6, no. 11, pp. 607–608, 1961.

M. Clin, J. P. Rivera, and H. Schmid, “Linear and quadratic magnetoelectric effect in boracite Co₃B₇O₁₃Br,” Ferroelectrics, vol. 79, no. 1, pp. 173–176, 1988.

Y. M. Jia, H. S. Luo, X. Y. Zhao, and F. F. Wang, “Giant magnetoelectric response from the combination of piezoelectric/magnetostrictive laminated composite with a piezoelectric transformer,” Advanced Materials, vol. 20, no. 24, pp. 4776–4779, 2008.

Y. M. Jia, A. X. Xue, Z. H. Zhou et al., “Magnetostrictive/piezoelectric drum magnetoelectric transducer for H₂ detection,” International Journal of Hydrogen Energy, vol. 38, no. 34, pp. 14915–14919, 2013.

H. Y. Tian, J. Q. Qi, Y. Wang, J. Wang, H. L. W. Chan, and C. L. Choy, “Core-shell structure of nanoscaled Ba0.5Sr0.5TiO₃ self-wrapped by MgO derived from a direct solution synthesis at room temperature,” Nanotechnology, vol. 16, no. 1, pp. 47–52, 2005.

J. Wang, X. G. Tang, H. L. W. Chan, C. L. Choy, and H. S. Luo, “Dielectric relaxation and electrical properties of 0.94Pb (Fe1/2 Nb1/2) O₃–0.06PbTiO₃ single crystals,” Applied Physics Letters, vol. 86, no. 15, Article ID 152907, pp. 1–3, 2005.

Y. Jia, H. Luo, S. W. Or, Y. Wang, and H. L. W. Chan, “Dielectric behavior and phase transition in perovskite oxide Pb(Fe1/2Nb1/2)1−xTixO₃ single crystal,” Journal of Applied Physics, vol. 105, no. 12, Article ID 124109, 4 pages, 2009.

J. D. Burton and E. Y. Tsymbal, “Prediction of electrically induced magnetic reconstruction at the manganite/ferroelectric interface,” Physical Review B—Condensed Matter and Materials Physics, vol. 80, no. 17, Article ID 174406, 2009.

N. A. Pertsev, H. Kohlstedt, and B. Dkhil, “Strong enhancement of the direct magnetoelectric effect in strained ferroelectric-ferromagnetic thin-film heterostructures,” Physical Review B—Condensed Matter and Materials Physics, vol. 80, no. 5, Article ID 054102, 2009.

J. M. Xu, G. M. Wang, H. X. Wang, D. F. Ding, and Y. He, “Synthesis and weak ferromagnetism of Dy-doped BiFeO₃ powders,” Materials Letters, vol. 63, no. 11, pp. 855–857, 2009.

G. Srinivasan, C. P. de Vreugd, V. M. Laletin et al., “Resonant magnetoelectric coupling in trilayers of ferromagnetic alloys and piezoelectric lead zirconate titanate: the influence of bias magnetic field,” Physical Review B, vol. 71, no. 18, Article ID 184423, 2005.

S. Ren, M. Laver, and M. Wuttig, “Nanolamellar magnetoelectric BaTiO₃-CoFe₂ O₄ bicrystal,” Applied Physics Letters, vol. 95, no. 15, Article ID 153504, 2009.

K. Sen, S. Thakur, K. Singh, A. Gautam, and M. Singh, “Room-temperature magnetic studies of La-modified BiFeO₃ ceramic,” Materials Letters, vol. 65, no. 12, pp. 1963–1965, 2011.

K. C. Verma and R. K. Kotnala, “Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO₃-0.3BaTiO₃ nanocrystals,” Solid State Communications, vol. 151, no. 13, pp. 920–923, 2011.