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Effects of niobium donor doping on the phase structures and magnetic properties of Fe-doped BaTiO3 ceramics
Journal of Alloys and Compounds 492 (2010) L79–L81
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Effects of niobium donor doping on the phase structures and magnetic properties of Fe-doped BaTiO3 ceramics Guo-Ping Du ∗ , Zhi-Juan Hu, Qi-Feng Han, Xiao-Mei Qin, Wang-Zhou Shi Key Laboratory of Optoelectronic Materials and Devices, Department of Physics, Shanghai Normal University, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 16 November 2009 Available online 30 December 2009 Keywords: Ceramics Oxide materials Sintering Phase transitions X-ray diffraction
a b s t r a c t Magnetic ions such as Fe3+ are usually doped into ferroelectric oxides in order to obtain multiferroic properties. Fe3+ as an acceptor dopant (substituted for Ti4+ ) in BaTiO3 ceramics usually promotes the formation of the hexagonal phase of BaTiO3 . Based on the consideration of charge compensation for the acceptor dopant Fe3+ in Fe-doped BaTiO3 ceramics, we introduced donor dopants into the ceramics, and investigated the effects of donor doping on the phase structures and magnetic properties of the ceramics. We demonstrated that the phase structures of the Fe-doped BaTiO3 can be readily controlled by donor doping. In this work, we prepared Fe-doped BaTiO3 ceramics at a doping concentration of 10 at% (or Ba(Ti0.9 Fe0.1 )O3 ) as a model system, and employed Nb5+ as the donor dopant for a charge compensation for the acceptor dopant Fe3+ . It was found that the formation of the hexagonal phase in Ba(Ti0.9 Fe0.1 )O3 was consistently suppressed by Nb5+ donor doping. The magnetic properties of the Ba(Ti0.9 Fe0.1 )O3 ceramics were strongly influenced by Nb5+ doping. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials have received intensive attention in recent years [1–5]. This is mainly due to their potential applications based on the magnetoelectric coupling effect. To introduce ferromagnetism into a ferroelectric material originally with no ferromagnetism by itself, one of the common approaches is to dope magnetic impurities into the ferroelectric host material [4]. The other approach is to combine a ferroelectric material and a ferromagnetic material together into a composite material, which subsequently exhibits multiferroic properties. Tetragonal BaTiO3 , a well-known ferroelectric material with a perovskite structure, is a good candidate for this purpose. This is because that the B-site Ti4+ can be easily substituted by other transition metal ions including the magnetic ions, such as Fe3+ and others [6–10]. Tetragonal BaTiO3 with ferroelectric properties has found broad applications in the electronics and electrical industry. BaTiO3 has a Curie temperature at about 120 ◦ C, under which it exhibits good ferroelectricity [11]. At temperatures higher than the Curie temperature, BaTiO3 will lose ferroelectricity and experience a phase transformation from tetragonal to cubic and then to hexagonal crystal structure at a much higher temperature. In order to introduce ferromagnetism to BaTiO3 , magnetic
∗ Corresponding author. Tel.: +86 13052003288; fax: +86 21 64332508. E-mail address: [email protected] (G.-P. Du). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.031
ions Fe3+ have been used to substitute Ti4+ , and room temperature ferromagnetism has been observed in Fe-doped BaTiO3 [9,10]. The doping of Fe3+ to BaTiO3 can stabilize the hightemperature phase, the hexagonal phase, to room temperature [12], and therefore the Fe-doped BaTiO3 ceramics are usually polymorphic, consisting of both tetragonal and hexagonal phases. It has been shown that both higher Fe3+ doping concentrations and higher processing temperatures promote the formation of hexagonal phase in BaTiO3 [6]. Since the hexagonal phase is not ferroelectric, the formation of the hexagonal phase consequently weakens the ferroelectricity of BaTiO3 to a large extent. Fe3+ as an acceptor dopant (substituted for Ti4+ ) generally results in the formation of oxygen vacancies in BaTiO3 . It is believed [8,13] that the oxygen vacancies are responsible for inducing the formation of the hexagonal phase in Fe-doped BaTiO3 ceramics. In this work, we intended to introduce donor dopants into the Fe-doped BaTiO3 for the purpose of a charge compensation with the Fe3+ acceptor dopants, and it was hoped that such a charge compensation will effectively suppress the formation of the hexgaonal phase in Fe-doped BaTiO3 . No systematic investigation with regards to the above doping effects for the Fe-doped BaTiO3 materials has been known in the literature. Here, we chose Fe3+ doping concentration at 10 at%, or the Ba(Ti0.9 Fe0.1 )O3 system, and Nb5+ as the donor dopant substituting for Ti4+ of Ba(Ti0.9 Fe0.1 )O3 . The effects of Nb5+ doping on the phase transition and ferromagnetism of the Ba(Ti0.9 Fe0.1 )O3 ceramics were systematically investigated.
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Fig. 1. The XRD patterns of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%.
2. Experimental High-purity BaCO3 , TiO2 , Fe2 O3 , and Nb2 O5 powders were weighed and mixed according to the formulae Ba(Ti0.9−x Fe0.1 Nbx )O3 , with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. The mixed powders were thoroughly ground and calcined in air at 1200 ◦ C. Two cycles of calcination (each cycle for 10 h) were conducted, and the powders were ground once more between the two cycles of calcination. The calcined powders were pressed into pellets, which were then sintered in air at 1250 ◦ C for 2 h, and then cooled down to room temperature with the furnace. The phase structures were analyzed with the X-ray powder diffraction (XRD, Bruker D8 Focus) technique. The ferroelectric properties were tested with the TF Analyzer 2000, while the ferromagnetic properties were measured with a vibrating sample magnetometer (VSM, LakeShore Model 7404).
3. Results and discussion Fig. 1 shows the XRD patterns of the Nb-doped Ba(Ti0.9 Fe0.1 )O3 ceramics for Nb5+ doping concentrations at x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. As expected, the Ba(Ti0.9 Fe0.1 )O3 ceramics consisted of both tetragonal and hexagonal phase structures, and the hexagonal phase component was dominant. With the increase of x, the XRD diffraction peaks for the tetragonal phase became stronger in a consistent manner, and they were stronger than those for the hexagonal phase component when the Nb5+ doping concentration was at x = 5 at% (Fig. 1). At x = 8 at%, no hexagonal phase was observed in the XRD patterns (Fig. 1). By using pure tetragonal and hexagonal BaTiO3 powder mixtures, the phase composition of the Ba(Ti0.9 Fe0.1 )O3 ceramics can be estimated. The dependence of the phase composition on the Nb5+ doping concentration is shown in Fig. 2. In this work, we also doped higher concentrations (such as
Fig. 2. The phase composition of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%.
Fig. 3. The M(H) hysteresis loops of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. The upper inset: is for x = 8 at% measured at 100 K. The lower inset shows the dependence of MS and HC on x.
x = 10 at% and higher) of Nb5+ into the Ba(Ti0.9 Fe0.1 )O3 ceramics, and it was found that the XRD patterns are similar to that at 8 at% of Nb5+ doping. This implies that 8 at% of Nb5+ doping is enough to suppress the formation of the hexagonal phase. From these results, it can be found that Nb5+ doping can effectively suppress the formation of the hexagonal phase in the Ba(Ti0.9 Fe0.1 )O3 ceramics. This indicates that the phase structures of the Fe-doped BaTiO3 ceramics can be readily controlled by donor doping. Fe3+ dopant results in oxygen vacancies in BaTiO3 , and it is believed [8] that when the concentration of oxygen vacancies in Fedoped BaTiO3 is sufficiently high, hexagonal phase can be formed and stabilized to room temperature. Formation of hexagonal phase has also been found in other acceptor-doped (such as Mn, Cu, Cr, etc.) BaTiO3 ceramics [13]. The reason for the suppression of hexagonal phase formation by Nb5+ doping in Ba(Ti0.9 Fe0.1 )O3 ceramics (Fig. 1) should be due to the effect of charge compensation for Fe3+ dopants, which in turn lowered the concentration of oxygen vacancies to such a low level that the hexagonal phase can not be formed. One can see that the doping concentration of Nb5+ at 8 at% is lower than the Fe3+ concentration which is 10 at%, and therefore there still exist some oxygen vacancies in the Ba(Ti0.9 Fe0.1 )O3 ceramics. This may imply that there exists a critical concentration of oxygen vacancies in BaTiO3 , beyond which the formation of the hexagonal phase will occur. Further investigation will be needed to support this point. It is reasonable to believe that tetragonal BaTiO3 crystal can tolerate a certain concentration of oxygen vacancies. Presence of oxygen vacancies causes lattice distortion in BaTiO3 . When the concentration of oxygen vacancies in tetragonal BaTiO3 crystal exceeds a critical value, the lattice distortion caused by the oxygen vacancies is so large that the tetragonal structure of BaTiO3 can not be preserved, i.e. a phase transition from tetragonal to hexagonal occurs. Fig. 3 shows the room temperature magnetization (M) vs. applied magnetic field (H) hysteresis loops of the Ba(Ti0.9 Fe0.1 )O3 ceramics with different Nb5+ doping concentration x. Except for the sample with x = 8 at%, all samples showed room temperature ferromagnetism. As shown in the upper inset of Fig. 3, the sample with x = 8 at%, however, exhibited ferromagnetism at 100 K. The lower inset of Fig. 3 shows the dependence of the saturation
G.-P. Du et al. / Journal of Alloys and Compounds 492 (2010) L79–L81
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within the crystal lattice of BaTiO3 are the variables and can be optimized for obtaining desired properties. 4. Conclusions
Fig. 4. The M(T) curves for the tetragonal Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 8 and 10 at%.
magnetization (MS ) and magnetic coercivities (HC ) on x at room temperature. MS increased with x to reach a maximum value at about 0.12 emu/g when x = 3 at%, then followed by a decrease. HC , on the other hand, varied little in the range between 3200 Gauss and 3300 Gauss for x = 0–3 at%, and then quickly decreased to 2200 Gauss for x = 5 at%. At x = 8 at% of Nb5+ doping when the Ba(Ti0.9 Fe0.1 )O3 ceramics became tetragonal (Fig. 1), it can be seen in Fig .3 that the Curie temperature is below room temperature. In order to investigate the influence of the Nb5+ doping concentration on the Curie temperature of the tetragonal Fe-doped BaTiO3 ceramics, we conducted M(T) measurements for x = 8 and 10 at% of Nb5+ doping. Fig. 4 shows the M(T) curves for the tetragonal Fe-doped BaTiO3 ceramic samples with x = 8 and 10 at%. As shown in Fig. 4, the Curie temperature of the sample with x = 8 at% of Nb5+ doping is about 130 K, while for x = 10 at% the Curie temperature is below 100 K. In tetragonal Fe-doped BaTiO3 ceramics, we supposed that the exchange interaction among magnetic Fe3+ ions is responsible for the presence of ferromagnetism, and this exchange interaction is established through the neighboring octahedrons. Each Fe3+ ion is located at the center of an octahedron. The exchange interaction will be weakened as more Nb5+ ions are doped and occupying more octahedrons. As a result, the Curie temperature will therefore decrease accordingly. For the interest of practical applications, however, it is desirable for a material to exhibit ferromagnetism at room or higher temperatures. This is our aim of next research effort. The types of donor doping ions, the processing conditions, and the donor doping sites
In summary, we demonstrated that the phase structures of the Fe-doped BaTiO3 ceramics can be effectively controlled by donor doping based on the consideration of charge compensation between donor and acceptor dopants. A model material system with 10 at% of Fe3+ doping, or the Ba(Ti0.9 Fe0.1 )O3 ceramic system, was synthesized. Nb5+ , as a donor dopant, was employed to substitute Ti4+ of the Ba(Ti0.9 Fe0.1 )O3 ceramics. Nb5+ doping consistently suppressed the formation of the hexagonal phase in the Ba(Ti0.9 Fe0.1 )O3 ceramics, and no hexagonal phase was observed when the doping concentration was increased to 8 at%. The ferromagnetism of Fe-doped BaTiO3 ceramics were strongly influenced by Nb5+ donor doping. Room temperature ferromagnetism was exhibited in all samples except for the ones with x = 8 at%, which had a Curie temperature about 130 K. Acknowledgements This work was financially supported by the Leading Academic Discipline Project (NO. DZL804), the Program for Innovative Research Team (NO. DXL902), and the Frontier Research Project (NO. DYL701) of the Shanghai Normal University, and the Innovation Program of Shanghai Municipal Education Commission (NO. 09YZ151). References [1] A.A. Belik, M. Azuma, T. Saito, Y. Shimakawa, M. Takano, Chem. Mater. 17 (2005) 269. [2] H.F. Zhang, S.W. Or, H.L.W. Chan, Mater. Res. Bull. 44 (2009) 1339. [3] C.W. Nan, N. Cai, L. Liu, J. Zhai, Y. Ye, Y. Lin, J. Appl. Phys. 94 (2003) 5930. [4] M.P. Singh, W. Prellier, L. Mechin, C. Simon, B. Raveau, J. Appl. Phys. 99 (2006), 024105. [5] M.A. Zurbuchen, T. Wu, S. Saha, J. Mitcell, S.K. Streiffer, Appl. Phys. Lett. 87 (2005), 232908. [6] I.E. Grey, C. Li, L.M.D. Cranswick, R.S. Roth, T.A. Vanderah, J. Solid State Chem. 135 (1998) 312. [7] E. Mashkina, C. McCammon, F. Seifert, J. Solid State Chem. 177 (2004) 262. [8] R. Bottcher, H.T. Langhammer, T. Muller, H.P. Abicht, J. Phys. :Condens. Matter 20 (2008), 505209. [9] S. Ray, P. Mahadevan, S. Mandal, S.R. Krishnakumar, C.S. Kuroda, T. Sasaki, T. Taniyama, M. Itoh, Phys. Rev. B 77 (2008) 104416. [10] F.T. Lin, D.M. Jiang, X.M. Ma, W.Z. Shi, J. Magn. Magn. Mater. 320 (2008) 691. [11] K.C. Kao, Dielectric Phenomena in Solids, Elsevier Academic Press, London, 2004. [12] R.M. Glaister, H.F. Kay, Proc. Phys. Soc. 76 (1960) 763. [13] H.T. Langhammer, T. Muller, A. Polity, K.H. Felgner, H.P. Abicht, Mater. Lett. 26 (1996) 205.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Effects of niobium donor doping on the phase structures and magnetic properties of Fe-doped BaTiO3 ceramics Guo-Ping Du ∗ , Zhi-Juan Hu, Qi-Feng Han, Xiao-Mei Qin, Wang-Zhou Shi Key Laboratory of Optoelectronic Materials and Devices, Department of Physics, Shanghai Normal University, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 16 November 2009 Available online 30 December 2009 Keywords: Ceramics Oxide materials Sintering Phase transitions X-ray diffraction
a b s t r a c t Magnetic ions such as Fe3+ are usually doped into ferroelectric oxides in order to obtain multiferroic properties. Fe3+ as an acceptor dopant (substituted for Ti4+ ) in BaTiO3 ceramics usually promotes the formation of the hexagonal phase of BaTiO3 . Based on the consideration of charge compensation for the acceptor dopant Fe3+ in Fe-doped BaTiO3 ceramics, we introduced donor dopants into the ceramics, and investigated the effects of donor doping on the phase structures and magnetic properties of the ceramics. We demonstrated that the phase structures of the Fe-doped BaTiO3 can be readily controlled by donor doping. In this work, we prepared Fe-doped BaTiO3 ceramics at a doping concentration of 10 at% (or Ba(Ti0.9 Fe0.1 )O3 ) as a model system, and employed Nb5+ as the donor dopant for a charge compensation for the acceptor dopant Fe3+ . It was found that the formation of the hexagonal phase in Ba(Ti0.9 Fe0.1 )O3 was consistently suppressed by Nb5+ donor doping. The magnetic properties of the Ba(Ti0.9 Fe0.1 )O3 ceramics were strongly influenced by Nb5+ doping. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Multiferroic materials have received intensive attention in recent years [1–5]. This is mainly due to their potential applications based on the magnetoelectric coupling effect. To introduce ferromagnetism into a ferroelectric material originally with no ferromagnetism by itself, one of the common approaches is to dope magnetic impurities into the ferroelectric host material [4]. The other approach is to combine a ferroelectric material and a ferromagnetic material together into a composite material, which subsequently exhibits multiferroic properties. Tetragonal BaTiO3 , a well-known ferroelectric material with a perovskite structure, is a good candidate for this purpose. This is because that the B-site Ti4+ can be easily substituted by other transition metal ions including the magnetic ions, such as Fe3+ and others [6–10]. Tetragonal BaTiO3 with ferroelectric properties has found broad applications in the electronics and electrical industry. BaTiO3 has a Curie temperature at about 120 ◦ C, under which it exhibits good ferroelectricity [11]. At temperatures higher than the Curie temperature, BaTiO3 will lose ferroelectricity and experience a phase transformation from tetragonal to cubic and then to hexagonal crystal structure at a much higher temperature. In order to introduce ferromagnetism to BaTiO3 , magnetic
∗ Corresponding author. Tel.: +86 13052003288; fax: +86 21 64332508. E-mail address: [email protected] (G.-P. Du). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.031
ions Fe3+ have been used to substitute Ti4+ , and room temperature ferromagnetism has been observed in Fe-doped BaTiO3 [9,10]. The doping of Fe3+ to BaTiO3 can stabilize the hightemperature phase, the hexagonal phase, to room temperature [12], and therefore the Fe-doped BaTiO3 ceramics are usually polymorphic, consisting of both tetragonal and hexagonal phases. It has been shown that both higher Fe3+ doping concentrations and higher processing temperatures promote the formation of hexagonal phase in BaTiO3 [6]. Since the hexagonal phase is not ferroelectric, the formation of the hexagonal phase consequently weakens the ferroelectricity of BaTiO3 to a large extent. Fe3+ as an acceptor dopant (substituted for Ti4+ ) generally results in the formation of oxygen vacancies in BaTiO3 . It is believed [8,13] that the oxygen vacancies are responsible for inducing the formation of the hexagonal phase in Fe-doped BaTiO3 ceramics. In this work, we intended to introduce donor dopants into the Fe-doped BaTiO3 for the purpose of a charge compensation with the Fe3+ acceptor dopants, and it was hoped that such a charge compensation will effectively suppress the formation of the hexgaonal phase in Fe-doped BaTiO3 . No systematic investigation with regards to the above doping effects for the Fe-doped BaTiO3 materials has been known in the literature. Here, we chose Fe3+ doping concentration at 10 at%, or the Ba(Ti0.9 Fe0.1 )O3 system, and Nb5+ as the donor dopant substituting for Ti4+ of Ba(Ti0.9 Fe0.1 )O3 . The effects of Nb5+ doping on the phase transition and ferromagnetism of the Ba(Ti0.9 Fe0.1 )O3 ceramics were systematically investigated.
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Fig. 1. The XRD patterns of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%.
2. Experimental High-purity BaCO3 , TiO2 , Fe2 O3 , and Nb2 O5 powders were weighed and mixed according to the formulae Ba(Ti0.9−x Fe0.1 Nbx )O3 , with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. The mixed powders were thoroughly ground and calcined in air at 1200 ◦ C. Two cycles of calcination (each cycle for 10 h) were conducted, and the powders were ground once more between the two cycles of calcination. The calcined powders were pressed into pellets, which were then sintered in air at 1250 ◦ C for 2 h, and then cooled down to room temperature with the furnace. The phase structures were analyzed with the X-ray powder diffraction (XRD, Bruker D8 Focus) technique. The ferroelectric properties were tested with the TF Analyzer 2000, while the ferromagnetic properties were measured with a vibrating sample magnetometer (VSM, LakeShore Model 7404).
3. Results and discussion Fig. 1 shows the XRD patterns of the Nb-doped Ba(Ti0.9 Fe0.1 )O3 ceramics for Nb5+ doping concentrations at x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. As expected, the Ba(Ti0.9 Fe0.1 )O3 ceramics consisted of both tetragonal and hexagonal phase structures, and the hexagonal phase component was dominant. With the increase of x, the XRD diffraction peaks for the tetragonal phase became stronger in a consistent manner, and they were stronger than those for the hexagonal phase component when the Nb5+ doping concentration was at x = 5 at% (Fig. 1). At x = 8 at%, no hexagonal phase was observed in the XRD patterns (Fig. 1). By using pure tetragonal and hexagonal BaTiO3 powder mixtures, the phase composition of the Ba(Ti0.9 Fe0.1 )O3 ceramics can be estimated. The dependence of the phase composition on the Nb5+ doping concentration is shown in Fig. 2. In this work, we also doped higher concentrations (such as
Fig. 2. The phase composition of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%.
Fig. 3. The M(H) hysteresis loops of the Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 0 at%, 1 at%, 3 at%, 5 at%, and 8 at%. The upper inset: is for x = 8 at% measured at 100 K. The lower inset shows the dependence of MS and HC on x.
x = 10 at% and higher) of Nb5+ into the Ba(Ti0.9 Fe0.1 )O3 ceramics, and it was found that the XRD patterns are similar to that at 8 at% of Nb5+ doping. This implies that 8 at% of Nb5+ doping is enough to suppress the formation of the hexagonal phase. From these results, it can be found that Nb5+ doping can effectively suppress the formation of the hexagonal phase in the Ba(Ti0.9 Fe0.1 )O3 ceramics. This indicates that the phase structures of the Fe-doped BaTiO3 ceramics can be readily controlled by donor doping. Fe3+ dopant results in oxygen vacancies in BaTiO3 , and it is believed [8] that when the concentration of oxygen vacancies in Fedoped BaTiO3 is sufficiently high, hexagonal phase can be formed and stabilized to room temperature. Formation of hexagonal phase has also been found in other acceptor-doped (such as Mn, Cu, Cr, etc.) BaTiO3 ceramics [13]. The reason for the suppression of hexagonal phase formation by Nb5+ doping in Ba(Ti0.9 Fe0.1 )O3 ceramics (Fig. 1) should be due to the effect of charge compensation for Fe3+ dopants, which in turn lowered the concentration of oxygen vacancies to such a low level that the hexagonal phase can not be formed. One can see that the doping concentration of Nb5+ at 8 at% is lower than the Fe3+ concentration which is 10 at%, and therefore there still exist some oxygen vacancies in the Ba(Ti0.9 Fe0.1 )O3 ceramics. This may imply that there exists a critical concentration of oxygen vacancies in BaTiO3 , beyond which the formation of the hexagonal phase will occur. Further investigation will be needed to support this point. It is reasonable to believe that tetragonal BaTiO3 crystal can tolerate a certain concentration of oxygen vacancies. Presence of oxygen vacancies causes lattice distortion in BaTiO3 . When the concentration of oxygen vacancies in tetragonal BaTiO3 crystal exceeds a critical value, the lattice distortion caused by the oxygen vacancies is so large that the tetragonal structure of BaTiO3 can not be preserved, i.e. a phase transition from tetragonal to hexagonal occurs. Fig. 3 shows the room temperature magnetization (M) vs. applied magnetic field (H) hysteresis loops of the Ba(Ti0.9 Fe0.1 )O3 ceramics with different Nb5+ doping concentration x. Except for the sample with x = 8 at%, all samples showed room temperature ferromagnetism. As shown in the upper inset of Fig. 3, the sample with x = 8 at%, however, exhibited ferromagnetism at 100 K. The lower inset of Fig. 3 shows the dependence of the saturation
G.-P. Du et al. / Journal of Alloys and Compounds 492 (2010) L79–L81
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within the crystal lattice of BaTiO3 are the variables and can be optimized for obtaining desired properties. 4. Conclusions
Fig. 4. The M(T) curves for the tetragonal Ba(Ti0.9−x Fe0.1 Nbx )O3 ceramics with x = 8 and 10 at%.
magnetization (MS ) and magnetic coercivities (HC ) on x at room temperature. MS increased with x to reach a maximum value at about 0.12 emu/g when x = 3 at%, then followed by a decrease. HC , on the other hand, varied little in the range between 3200 Gauss and 3300 Gauss for x = 0–3 at%, and then quickly decreased to 2200 Gauss for x = 5 at%. At x = 8 at% of Nb5+ doping when the Ba(Ti0.9 Fe0.1 )O3 ceramics became tetragonal (Fig. 1), it can be seen in Fig .3 that the Curie temperature is below room temperature. In order to investigate the influence of the Nb5+ doping concentration on the Curie temperature of the tetragonal Fe-doped BaTiO3 ceramics, we conducted M(T) measurements for x = 8 and 10 at% of Nb5+ doping. Fig. 4 shows the M(T) curves for the tetragonal Fe-doped BaTiO3 ceramic samples with x = 8 and 10 at%. As shown in Fig. 4, the Curie temperature of the sample with x = 8 at% of Nb5+ doping is about 130 K, while for x = 10 at% the Curie temperature is below 100 K. In tetragonal Fe-doped BaTiO3 ceramics, we supposed that the exchange interaction among magnetic Fe3+ ions is responsible for the presence of ferromagnetism, and this exchange interaction is established through the neighboring octahedrons. Each Fe3+ ion is located at the center of an octahedron. The exchange interaction will be weakened as more Nb5+ ions are doped and occupying more octahedrons. As a result, the Curie temperature will therefore decrease accordingly. For the interest of practical applications, however, it is desirable for a material to exhibit ferromagnetism at room or higher temperatures. This is our aim of next research effort. The types of donor doping ions, the processing conditions, and the donor doping sites
In summary, we demonstrated that the phase structures of the Fe-doped BaTiO3 ceramics can be effectively controlled by donor doping based on the consideration of charge compensation between donor and acceptor dopants. A model material system with 10 at% of Fe3+ doping, or the Ba(Ti0.9 Fe0.1 )O3 ceramic system, was synthesized. Nb5+ , as a donor dopant, was employed to substitute Ti4+ of the Ba(Ti0.9 Fe0.1 )O3 ceramics. Nb5+ doping consistently suppressed the formation of the hexagonal phase in the Ba(Ti0.9 Fe0.1 )O3 ceramics, and no hexagonal phase was observed when the doping concentration was increased to 8 at%. The ferromagnetism of Fe-doped BaTiO3 ceramics were strongly influenced by Nb5+ donor doping. Room temperature ferromagnetism was exhibited in all samples except for the ones with x = 8 at%, which had a Curie temperature about 130 K. Acknowledgements This work was financially supported by the Leading Academic Discipline Project (NO. DZL804), the Program for Innovative Research Team (NO. DXL902), and the Frontier Research Project (NO. DYL701) of the Shanghai Normal University, and the Innovation Program of Shanghai Municipal Education Commission (NO. 09YZ151). References [1] A.A. Belik, M. Azuma, T. Saito, Y. Shimakawa, M. Takano, Chem. Mater. 17 (2005) 269. [2] H.F. Zhang, S.W. Or, H.L.W. Chan, Mater. Res. Bull. 44 (2009) 1339. [3] C.W. Nan, N. Cai, L. Liu, J. Zhai, Y. Ye, Y. Lin, J. Appl. Phys. 94 (2003) 5930. [4] M.P. Singh, W. Prellier, L. Mechin, C. Simon, B. Raveau, J. Appl. Phys. 99 (2006), 024105. [5] M.A. Zurbuchen, T. Wu, S. Saha, J. Mitcell, S.K. Streiffer, Appl. Phys. Lett. 87 (2005), 232908. [6] I.E. Grey, C. Li, L.M.D. Cranswick, R.S. Roth, T.A. Vanderah, J. Solid State Chem. 135 (1998) 312. [7] E. Mashkina, C. McCammon, F. Seifert, J. Solid State Chem. 177 (2004) 262. [8] R. Bottcher, H.T. Langhammer, T. Muller, H.P. Abicht, J. Phys. :Condens. Matter 20 (2008), 505209. [9] S. Ray, P. Mahadevan, S. Mandal, S.R. Krishnakumar, C.S. Kuroda, T. Sasaki, T. Taniyama, M. Itoh, Phys. Rev. B 77 (2008) 104416. [10] F.T. Lin, D.M. Jiang, X.M. Ma, W.Z. Shi, J. Magn. Magn. Mater. 320 (2008) 691. [11] K.C. Kao, Dielectric Phenomena in Solids, Elsevier Academic Press, London, 2004. [12] R.M. Glaister, H.F. Kay, Proc. Phys. Soc. 76 (1960) 763. [13] H.T. Langhammer, T. Muller, A. Polity, K.H. Felgner, H.P. Abicht, Mater. Lett. 26 (1996) 205.