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Effect of structure on magnetoelectric properties of CoFe2O4–BaTiO3 multiferroic composites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363 www.elsevier.com/locate/jmmm
Effect of structure on magnetoelectric properties of CoFe2O4–BaTiO3 multiferroic composites Giap V. Duonga,b, R. Groessingera, R. Sato Turtellia, a
b
Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040, Vienna, Austria Faculty of Chemical Engineering, Hanoi University of Technology, No. 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam Available online 7 November 2006
Abstract The 50%CoFe2O4–50%BaTiO3 (in mass) composites with four different building structures, namely: CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core, BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, and BaTiO3–CoFe2O4–BaTiO3 layer structure, have been synthesized and studied. The core–shell structures give higher magnetoelectric (ME) coefficients compared to the other structures. When using CoFe2O4 as core, the ME coefficient is highest, reaching 3.4 mV cm1 Oe1 for the sample pressed at 6 ton/cm2 and sintered at 1250 1C for 12 h. r 2006 Elsevier B.V. All rights reserved. PACS: 75.80.+q; 77.65.j; 77.84.Lf Keywords: Magnetoelectric effect; Multiferroic composite; Effect of structure; Barium titanate; Cobalt ferrite
The origin of the magnetoelectric (ME) effect in ME composites is the coupling between the magnetostrictive and piezoelectric phases [1]. So the micro-structure of the composites which affects the interactions between the two phases as well as some physical properties such as electrical resistance, dielectric constant may play an important role in ME composites. In this work, 50%CoFe2O4–50%BaTiO3 (in mass) composites with four different building structures, namely: CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core, BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, and BaTiO3–CoFe2O4–BaTiO3 layer structure, have been synthesized and studied. The core–shell structure samples were prepared by wet chemical method as described elsewhere [2]. In general, the core was prepared by co-precipitation (CoFe2O4, average grain size of about 10 nm) or sol–gel (BaTiO3, average grain size of about 42 nm) technique, then introduced to Corresponding author. Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040, Vienna, Austria. Tel.: +43 1 58801 13152; fax: +43 1 58801 13899. E-mail addresses: [email protected] (G.V. Duong), [email protected] (R. Sato Turtelli).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.338
the homogeneous solution containing chelating agent and elements that form the shell. This solution evaporated and gelated on the surface of the core during heating and stirring. The obtained products were pre-sintered at 450 1C for 3 h, pressed into pellets under a pressure of 3–7.5 ton/cm2, sintered at 1000–1250 1C for 1–20 h, then cooled down naturally to room temperature (RT). The mixed structure was prepared by simply mixing the two initial powders: CoFe2O4 (average rain size of 10 nm) and BaTiO3 (average grain size of 40 nm), then pressed into pellets under a pressure similar to those for the core–shell structure samples. The layer structure was prepared by casting the powder into the matrix, slightly pressed before casting the other layers, and then the whole powders in the matrix were pressed under a pressure of 6 ton/cm2. The heat treatment of the mixed and layer structure is similar to that of the core–shell structure samples. After heat treatment, all samples were poled under an electric field of 7500 V/cm and painted by silver paste for electrical contacts. X-ray diffraction characterization showed that all composites consisted of two single phases only: CoFe2O4 and BaTiO3. Magnetic studies showed that the magnetic properties of the CoFe2O4 component are similar to those
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G.V. Duong et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363
of the bulk sample: saturation magnetization (Ms) of 72 emu/g and coercivity (Hc) of 460 Oe at RT. For reference, a CoFe2O4 bulk sample prepared by citrate gel method at RT has Ms of 78 emu/g, Hc of 825 Oe and magnetostriction l of 130 and 70 ppm for parallel and perpendicular measurements, respectively. Very similar values are obtained for the other samples. The ME effect was measured using a lock-in technique as described in Ref. [3]. Fig. 1 shows the ME coefficient as function of the DC bias field (aE–HDC) of the sample with CoFe2O4 in core using an AC field of 10 Oe, 270 Hz. This sample is formed by pressing the powder under a pressure of 6 ton/cm2, and then sintered at 1250 1C for 12 h. It is
clear that the curves experience a maximum and show remanence as well as hysteresis behavior. The maximum ME coefficient is 3.4 mV cm1 Oe1 for longitudinal measurement and 2.0 mV cm1 Oe1 for transverse case. The effect of structure on ME coefficient is shown in Fig. 2. It was found that the CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core has the highest ME coefficient: 1.62 mV cm1 Oe1 for sample annealed at 1200 1C for 16 h, which is of about 4.2, 8.1 and 11 times higher than that of the BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, BaTiO3–CoFe2O4–BaTiO3 layer structure, prepared under the same conditions, respectively. The higher ME coefficient of the core–shell structure may be attributed to the better coupling between the two phases due to its larger interface area. Additionally, the higher measured ME coefficients when CoFe2O4 was used as core compared to the case of BaTiO3 in core may be understood also as the discharging effect in the former was lesser. The
Fig. 1. The aE–HDC curves of the CoFe2O4–BaTiO3 composite with CoFe2O4 in core at RT (sample preparation: pressed at 6 ton/cm2 and sintered at 1250 1C for 12 h).
Fig. 2. Effect of structure on aE at RT. Sample index: (1) CoFe2O4 as core and BaTiO3 as shell; (2) BaTiO3 as core and CoFe2O4 as shell; (3) mixed structure; (4) BaTiO3–CoFe2O4–BaTiO3 layer structure. Preparation of samples: pressed at 6 ton/cm2 and sintered at 1250 1C for 16 h.
Fig. 3. Effect of synthesis temperature (a) and duration (b) on the ME coefficient of core–shell structure composites.
ARTICLE IN PRESS G.V. Duong et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363
reason is: the electrical resistance of BaTiO3 is higher than that of CoFe2O4 which is confirmed by electrical resistance measurements on shape normalized samples at RT: 330 MO for sample with CoFe2O4 in core, 50 MO for sample with BaTiO3 in core, 80 MO for mixed structure, 42000 MO for layer structure. It is also worth to remind that, beside the physical properties such as electric resistance, the microstructure of the sample also affects seriously on the ME coefficient. When changing the sintering temperature and duration from 1250 1C and 12 h to 1200 1C and 16 h, the longitudinal aE of the sample with CoFe2O4 in core decreases from 3.4 to 1.62 mV cm1 Oe1 as can be seen in Figs. 1 and 2. The reason is the changes in sample microstructure which is now under investigation. For different structures, the optimum preparation conditions are different, e.g., for the core–shell with CoFe2O4 in core, pressed under 6 ton/ cm2 and sintered at 1250 1C for 4 h suited best (max aE ¼ 3.88 mV cm1 Oe1), but when BaTiO3 used as core, the optimum is to press at 3 ton/cm2 and sintered at 1150 1C for 8 h (max aE ¼ 0.63 mV cm1 Oe1) as shown in Fig. 3. These values are in the range of those reported for
e363
particulate composites: 0.43 mV cm1 Oe1 in 15%Ni0.8Cu0.2Fe2O4+85%Ba0.9Pb0.1Ti0.9Zr0.1O3 (in mole) [4], 3.0–5.58 mV cm1 Oe1 in BaO–TiO–FeO–CoO [5] and 0.19 mV cm1 Oe1 in 50%CoFe2O4–50%BaTiO3 (in mass) mixed composite [6]. This work is supported by the FWF Proj. Nr. P16500–N02, Proj. Nr. P15737 and the Austrian Exchange Service (O¨AD).
References [1] J. Van Suchetelene, Philips Res. Rep. 27 (1972) 28. [2] G.V. Duong, R. Groessinger, R. Sato Turtelli, Magnetoelectric properties of CoFe2O4–BaTiO3 core–shell structure composite, IEEE Trans. Magn. 42 (2006) 3611. [3] G.V. Duong, R. Groessinger, M. Schoenhart, D. Bueno-Basques, The Lock-in technique for Studying Magnetoelectric Effect, J. Magn. Magn. Mater., in print. [4] C.M. Kanamadi, L.B. Pujari, B.K. Chougule, J. Magn. Magn. Mater. 295 (2005) 139. [5] S. Mazuder, G.S. Bhattacharyya, Ceram. Int. 30 (2004) 389. [6] R.P. Mahajan, K.K. Patankar, M.B. Kothale, S.C. Chaudhari, V.L. Mathe, S.A. Patil, Pramana-J. Phys. 58 (2002) 1115.
Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363 www.elsevier.com/locate/jmmm
Effect of structure on magnetoelectric properties of CoFe2O4–BaTiO3 multiferroic composites Giap V. Duonga,b, R. Groessingera, R. Sato Turtellia, a
b
Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040, Vienna, Austria Faculty of Chemical Engineering, Hanoi University of Technology, No. 1 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam Available online 7 November 2006
Abstract The 50%CoFe2O4–50%BaTiO3 (in mass) composites with four different building structures, namely: CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core, BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, and BaTiO3–CoFe2O4–BaTiO3 layer structure, have been synthesized and studied. The core–shell structures give higher magnetoelectric (ME) coefficients compared to the other structures. When using CoFe2O4 as core, the ME coefficient is highest, reaching 3.4 mV cm1 Oe1 for the sample pressed at 6 ton/cm2 and sintered at 1250 1C for 12 h. r 2006 Elsevier B.V. All rights reserved. PACS: 75.80.+q; 77.65.j; 77.84.Lf Keywords: Magnetoelectric effect; Multiferroic composite; Effect of structure; Barium titanate; Cobalt ferrite
The origin of the magnetoelectric (ME) effect in ME composites is the coupling between the magnetostrictive and piezoelectric phases [1]. So the micro-structure of the composites which affects the interactions between the two phases as well as some physical properties such as electrical resistance, dielectric constant may play an important role in ME composites. In this work, 50%CoFe2O4–50%BaTiO3 (in mass) composites with four different building structures, namely: CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core, BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, and BaTiO3–CoFe2O4–BaTiO3 layer structure, have been synthesized and studied. The core–shell structure samples were prepared by wet chemical method as described elsewhere [2]. In general, the core was prepared by co-precipitation (CoFe2O4, average grain size of about 10 nm) or sol–gel (BaTiO3, average grain size of about 42 nm) technique, then introduced to Corresponding author. Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, 1040, Vienna, Austria. Tel.: +43 1 58801 13152; fax: +43 1 58801 13899. E-mail addresses: [email protected] (G.V. Duong), [email protected] (R. Sato Turtelli).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.338
the homogeneous solution containing chelating agent and elements that form the shell. This solution evaporated and gelated on the surface of the core during heating and stirring. The obtained products were pre-sintered at 450 1C for 3 h, pressed into pellets under a pressure of 3–7.5 ton/cm2, sintered at 1000–1250 1C for 1–20 h, then cooled down naturally to room temperature (RT). The mixed structure was prepared by simply mixing the two initial powders: CoFe2O4 (average rain size of 10 nm) and BaTiO3 (average grain size of 40 nm), then pressed into pellets under a pressure similar to those for the core–shell structure samples. The layer structure was prepared by casting the powder into the matrix, slightly pressed before casting the other layers, and then the whole powders in the matrix were pressed under a pressure of 6 ton/cm2. The heat treatment of the mixed and layer structure is similar to that of the core–shell structure samples. After heat treatment, all samples were poled under an electric field of 7500 V/cm and painted by silver paste for electrical contacts. X-ray diffraction characterization showed that all composites consisted of two single phases only: CoFe2O4 and BaTiO3. Magnetic studies showed that the magnetic properties of the CoFe2O4 component are similar to those
ARTICLE IN PRESS e362
G.V. Duong et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363
of the bulk sample: saturation magnetization (Ms) of 72 emu/g and coercivity (Hc) of 460 Oe at RT. For reference, a CoFe2O4 bulk sample prepared by citrate gel method at RT has Ms of 78 emu/g, Hc of 825 Oe and magnetostriction l of 130 and 70 ppm for parallel and perpendicular measurements, respectively. Very similar values are obtained for the other samples. The ME effect was measured using a lock-in technique as described in Ref. [3]. Fig. 1 shows the ME coefficient as function of the DC bias field (aE–HDC) of the sample with CoFe2O4 in core using an AC field of 10 Oe, 270 Hz. This sample is formed by pressing the powder under a pressure of 6 ton/cm2, and then sintered at 1250 1C for 12 h. It is
clear that the curves experience a maximum and show remanence as well as hysteresis behavior. The maximum ME coefficient is 3.4 mV cm1 Oe1 for longitudinal measurement and 2.0 mV cm1 Oe1 for transverse case. The effect of structure on ME coefficient is shown in Fig. 2. It was found that the CoFe2O4–BaTiO3 core–shell structure with CoFe2O4 in core has the highest ME coefficient: 1.62 mV cm1 Oe1 for sample annealed at 1200 1C for 16 h, which is of about 4.2, 8.1 and 11 times higher than that of the BaTiO3–CoFe2O4 core–shell structure with BaTiO3 in core, CoFe2O4–BaTiO3 mixed structure, BaTiO3–CoFe2O4–BaTiO3 layer structure, prepared under the same conditions, respectively. The higher ME coefficient of the core–shell structure may be attributed to the better coupling between the two phases due to its larger interface area. Additionally, the higher measured ME coefficients when CoFe2O4 was used as core compared to the case of BaTiO3 in core may be understood also as the discharging effect in the former was lesser. The
Fig. 1. The aE–HDC curves of the CoFe2O4–BaTiO3 composite with CoFe2O4 in core at RT (sample preparation: pressed at 6 ton/cm2 and sintered at 1250 1C for 12 h).
Fig. 2. Effect of structure on aE at RT. Sample index: (1) CoFe2O4 as core and BaTiO3 as shell; (2) BaTiO3 as core and CoFe2O4 as shell; (3) mixed structure; (4) BaTiO3–CoFe2O4–BaTiO3 layer structure. Preparation of samples: pressed at 6 ton/cm2 and sintered at 1250 1C for 16 h.
Fig. 3. Effect of synthesis temperature (a) and duration (b) on the ME coefficient of core–shell structure composites.
ARTICLE IN PRESS G.V. Duong et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e361–e363
reason is: the electrical resistance of BaTiO3 is higher than that of CoFe2O4 which is confirmed by electrical resistance measurements on shape normalized samples at RT: 330 MO for sample with CoFe2O4 in core, 50 MO for sample with BaTiO3 in core, 80 MO for mixed structure, 42000 MO for layer structure. It is also worth to remind that, beside the physical properties such as electric resistance, the microstructure of the sample also affects seriously on the ME coefficient. When changing the sintering temperature and duration from 1250 1C and 12 h to 1200 1C and 16 h, the longitudinal aE of the sample with CoFe2O4 in core decreases from 3.4 to 1.62 mV cm1 Oe1 as can be seen in Figs. 1 and 2. The reason is the changes in sample microstructure which is now under investigation. For different structures, the optimum preparation conditions are different, e.g., for the core–shell with CoFe2O4 in core, pressed under 6 ton/ cm2 and sintered at 1250 1C for 4 h suited best (max aE ¼ 3.88 mV cm1 Oe1), but when BaTiO3 used as core, the optimum is to press at 3 ton/cm2 and sintered at 1150 1C for 8 h (max aE ¼ 0.63 mV cm1 Oe1) as shown in Fig. 3. These values are in the range of those reported for
e363
particulate composites: 0.43 mV cm1 Oe1 in 15%Ni0.8Cu0.2Fe2O4+85%Ba0.9Pb0.1Ti0.9Zr0.1O3 (in mole) [4], 3.0–5.58 mV cm1 Oe1 in BaO–TiO–FeO–CoO [5] and 0.19 mV cm1 Oe1 in 50%CoFe2O4–50%BaTiO3 (in mass) mixed composite [6]. This work is supported by the FWF Proj. Nr. P16500–N02, Proj. Nr. P15737 and the Austrian Exchange Service (O¨AD).
References [1] J. Van Suchetelene, Philips Res. Rep. 27 (1972) 28. [2] G.V. Duong, R. Groessinger, R. Sato Turtelli, Magnetoelectric properties of CoFe2O4–BaTiO3 core–shell structure composite, IEEE Trans. Magn. 42 (2006) 3611. [3] G.V. Duong, R. Groessinger, M. Schoenhart, D. Bueno-Basques, The Lock-in technique for Studying Magnetoelectric Effect, J. Magn. Magn. Mater., in print. [4] C.M. Kanamadi, L.B. Pujari, B.K. Chougule, J. Magn. Magn. Mater. 295 (2005) 139. [5] S. Mazuder, G.S. Bhattacharyya, Ceram. Int. 30 (2004) 389. [6] R.P. Mahajan, K.K. Patankar, M.B. Kothale, S.C. Chaudhari, V.L. Mathe, S.A. Patil, Pramana-J. Phys. 58 (2002) 1115.