Effects of Co-substitutes on multiferroic properties of Bi5FeTi3O15 ceramics

Solid State Communications 152 (2012) 483–487

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Effects of Co-substitutes on multiferroic properties of Bi5 FeTi3 O15 ceramics Xiangyu Mao a , Hui Sun a , Wei Wang a , Yalin Lu b , Xiaobing Chen a,∗ a

College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, People’s Republic of China

b

Leser and Optics Research Center, Department of Physics, US Air Force Academy, CO 80840, USA

article

info

Article history: Received 10 July 2011 Received in revised form 4 December 2011 Accepted 2 January 2012 by M. Wang Available online 8 January 2012 Keywords: A. Multiferroics A. Ferroelectrics A. Ferromagnetism

abstract The samples Bi5 FeTi3 O15 (BFTO) and Bi5 Fe0.5 Co0.5 Ti3 O15 (BFCT) were prepared by incorporating BiFeO3 (BFO) and BiFe0.5 Co0.5 O3 (BFCO) into the host Bi4 Ti3 O12 (BTO) using the solid state reaction technique. The Raman shift data indicate that the as-prepared materials are of the four-layer Aurivillius phase with an orthorhombic symmetry. At room temperature (RT) and under a driving electric field of 230 kV/cm, the half substitution of Fe by Co ions is found to result in the increase in the remanent polarization (2Pr ) by about 35% and simultaneously the decrease in 2Ec by about 41%, respectively. The magnetic moment at least tripled its value by substituting half Fe ions by Co ions. The 2Mr of BFCT is about three thousand times the value of BFTO. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Bismuth layer structured ferroelectrics (BLSFs) are built with regular layers of (Bi2 O2 )2+ and (An−1 Bn O3n+1 )2− perovskite slabs, where A represents Bi3+ , Pb2+ , Sr2+ , Ca2+ , Ba2+ , etc., at the 12-coordinated site; B represents Fe3+ , Mn3+ , Ti4+ , V5+ , Nb5+ , Ta5+ , W6+ , etc., at the 6-coordinated site; and n is an integer corresponding to the number of oxygen octahedra of the B ions in the pseudo-perovskite block. The A-site cation is located in the large cavity at the center of eight corner-sharing BO6 octahedra. (An−1 Bn O3n+1 ) denotes the perovskite-like slabs derived by termination of the three-dimensional ABO3 perovskite structure along the (100) axis, which are interleaved with (Bi2 O2 )2+ fluoritetype layers [1,2]. The bismuth titanate (Bi4 Ti3 O12 , BTO), as a typical bismuth layered perovskite material, is composed of three perovskite slabs sandwiched by two (Bi2 O2 )2+ layers. The BTO has been demonstrated to show a high Curie temperature of 675 °C and a large spontaneous polarization. Such strong ferroelectricity (FE) is generally considered to arise from the lone pair electrons of Bi3+ ions [3]. The (Bi2 O2 )2+ layer is believed to play a role of space–charge compensation and insulation, which could reduce leakage current considerably [4]. In recent years, multiferroic materials have received great attention both from experimental and theoretical respects [5,6]. For multiferroic materials, the ferroelectricity and the ferromagnetism are simultaneously observed in the same phase [7,8]. However, the multiferroic systems are rare in nature, and are widely investigated for their potential applications. Many efforts have been

∗

Corresponding author. Tel.: +86 514 8799 7803; fax: +86 514 8797 5467. E-mail addresses: [email protected] (Y. Lu), [email protected] (X. Chen).

0038-1098/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2012.01.001

devoted to prepare new materials with multiferroic properties or to discover multiferroic properties in the known compounds. Although the gigantic magnetoelectric (ME) effect in piezoelectric–magnetostrictive composite structures has been well investigated, the single-phase compounds have caused particular attention, since the multiferroic compounds allow to tune the ME effect in the quantum level. To date, the most frequently investigated multiferroic compounds are BiFeO3 (BFO) [6] and BiMnO3 [9] which exhibit the FE order and spin order at relatively high temperatures. When the equal compositional ratio of BFO and BTO is combined together, the resulting compound is Bi5 FeTi3 O15 (BFTO). The BFTO is a four-layered perovskite unit of nominal composition (Bi3 Ti3 FeO13 )2− sandwiched between two (Bi2 O2 )2+ layers along the c axis. According to the previous reports, the FE Curie temperature of BFTO is ∼750 °C accompanied with the structural transition from A21 am symmetry to I4/mmm symmetry [10–14]. By inserting BiFeO3 (BFO) into BTO, we prepared BFTO ceramic and its electrical and magnetic properties are reported as well [15]. Recently, Bi5 Fe0.5 Co0.5 Ti3 O15 (BFCT) ceramics has been synthesized successfully by incorporating a magnetic unit BFO and BiCoO3 (BCO) into a ferroelectric block BTO [16]. Since magnetic and ME properties are governed by coupling between magnetic cations such as Fe3+ and Co3+ , it is of great interest to modulate the number of perovskitelike layers inside the structure, and furthermore the intra and interlayer magnetic interactions are expected to be strongly related to the interlayer distance and the inserted magnetic ions. In this paper, the detailed experimental results of the BFTO and BFCT ceramics are presented such as microstructure based on Raman spectra, FE, and magnetic properties. By inserting magnetic BFO and BiCo0.5 Fe0.5 O3 (BFCO) into BTO ceramic, BFTO and BFCT samples are successfully synthesized by solid state

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Fig. 1. SEM micrographs of fresh fracture surfaces of (a) BFTO and (b) BFCT samples.

reaction method. Particularly, the effect of Co substitution on FE and FM properties is discussed in detail. Furthermore, the Raman spectra of BFTO and BFCT are investigated at various temperatures. 2. Experiment The samples of BFTO and BFCT were prepared by incorporating BFO and BFCO into the host BTO using the solid state reaction technique [15,16]. The micrographs of fracture surfaces were observed using a field emission scanning electron microscope (FESEM) Hitachi, S-4800. The high-resolution transmission electron microscopy (HRTEM) observations are recorded using JEOL 2010F transmission electron microscope operated at 200 kV. Raman scattering spectra are measured in a backward microconfiguration, using the 514.5 nm line from an Ar+ laser (∼300 mW) focused on a spot with a diameter of 1 ∼ 2 mm on the sample surface. The scattered light is dispersed by a triple spectrometer (Jobin Yvon T64000) and collected with a liquid– nitrogen-cooled charge coupled device (CCD) detector. The magnetic properties of the samples are measured by a vibrating samples magnetometer (VSM) (EV-7, ADE Co., USA). The pellets of the samples are polished to the thickness of about 0.18 mm, Ag electrode with an area of about 1.8 mm2 is coated for ferroelectric property measurement. The ferroelectric properties are measured on a Precision LC ferroelectric analyzer (Radiant Technology product, USA). 3. Results and discussions The BFTO and BFCT ceramics are of a single Aurivillius phase with a four-layered perovskite structure [15,16]. Fig. 1(a) and (b) show FE-SEM images taken from fresh fracture surfaces of BFTO and BFCT samples, respectively. Both the BFTO and BFCT ceramics are composed of well compact grains with less residual porosity. The grain of the BFTO sample is formed in the shape of thin platelets, which is typical for Aurivillius bismuth layered compounds due to the preferential growth of the abcrystalline plain, as shown in Fig. 1(a) [17,18]. The average grain size of the BFCT sample, as shown in Fig. 1(b), is smaller than that of the BFTO with a shape greatly different from the platelet, which could be attributed to the substitution of Fe by Co. The P–E hysteresis loops of the BFTO and BFCT [16] samples at RT are shown in Fig. 2. The ferroelectric hysteresis loops are measured under a driving electric field of 230 kV/cm, and the remanent polarization (2Pr ) of BFTO and BFCT samples are obtained to be about 9.7 µC/cm2 and 13 µC/cm2 , respectively. The coercive field (2Ec ) of the BFTO and BFCT samples is found to be about 198 kV/cm and 140 kV/cm, respectively. The half substitution of Fe by Co ions is found to result in the increase of the 2Pr by about 35% and simultaneously the decrease of 2Ec

Fig. 2. At RT, the ferroelectric loops of BFTO and BCFT samples.

by about 41%, respectively. Since the difference of ions radius between Fe and Co ions is less than 1%, [19] the lattice distortion is considered not to be the dominating factor for the improvement of FE. It may be the decreasing concentration of oxygen vacancy that is responsible for the improvement of FE. As previously reported, for bismuth-based ferroelectrics such as BTO, the doping of La and V were effective to enhance the ferroelectric properties because of the reduced oxygen vacancy by the stabilized oxygen octahedron [20–22]. Additionally, it was reported as well that Co doping can reduce the leakage current in BFO, [23] which may be related to the decreasing oxygen vacancy. The magnetic hysteresis loops of BFTO and BCFT samples are displayed in Fig. 3, which are measured by VSM at RT. BFTO shows a very narrow M–H loop with nearly linear relationship, which is a characteristic of a paramagnetic state. The remanent magnetization (2Mr ) of BFTO is found to be as small as 2.7 × 10−3 memu/g (see inset of Fig. 3, taken from Ref. [15]). However, the BFCT sample exhibits a typical ferromagnetic M–H loop, with 2Mr of 7.8 memu/g and coercive field (2Hc ) of 410 Oe, respectively. At RT, the remanent magnetization of BFCT is about three thousand times of that of BFTO. The ferromagnetism in BFCT arises from the long range ordering arrangement of magnetic Fe and Co iron [24]. BFO and BCO, which are inserted into BTO by solid state reaction, were reported to be a G-type antiferromagnet (AFM) spin structure below TN ∼ 480 °C and a C -type AFM spin structure below TN ∼ 200 °C, respectively [6,25]. On the other hand, BFTO was reported to be of the AFM order below TN ∼ −190 °C [14]. It is very interesting that the enhanced FM characteristic appears due to half Co substitution. Since Co3+ ions have smaller magnetic moment (4.90 µB ) than that

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Fig. 3. At RT, magnetic hysteresis of BFTO and BCFT samples, inset: the enlarged central part of the M–H curve for BFTO. Fig. 5. At RT, Raman spectra of the (a) BFTO, (b) BFCT, and (c) SBTi ceramics.

Fig. 4. HRTEM electron of the BFCT sample.

of Fe3+ ions (5.29 µB ), it is not enough to consider the appearance of enhanced FM characteristic only from the viewpoint of the magnetic moment. In BFCT, the ratio of Fe3+ and Co3+ is equal to 1:1, the Fe–O octahedra is of great possibility to be adjacent to Co–O octahedra, which results in the direct Fe–O–Co magnetic coupling interaction possible and favor the formation of FM state. It has been confirmed in previous reports that Co doping in the Fesite in BFO could improve the FM characteristic [26]. In addition, it can be noted from the HRTEM picture shown in Fig. 4 that the distortion (see the area enclosed by dot line) appears in pseudoperovskite slabs below the Bi–O layer. Therefore, it is evident that the FeO6 and CoO6 octahedra have been introduced between BiO layer and TiO6 octahedron by inserting BFO and BCO into BTO. By this method, Fe and Co ions tend to locate in the same pseudo-perovskite slab, which increases the possibility for the Fe–O–Co coupling and results in the enhanced FM characteristic. For comparison, the BFCT sample prepared by conventional solid state reaction from Bi2 O3 , Fe2 O3 , Co2 O3 , and TiO2 exhibits reduced FM characteristics with 2Mr only equal to 2.3 memu/g, where the Fe and Co ions tend to distribute randomly among four pseudo-perovskite slabs and result in the of the Fe–O–Co coupling. Summary, the synthesizing method by inserting BFO and BCO into BTO has been proved to be an effective way to improve the FM property of bismuth layer structured ferroelectrics. In order to clarify the mechanism underlying the optimization of the FM and FE properties caused by Co modification, the

Raman spectra for BFTO and BFCT samples are investigated in the Raman frequency range of 8–1000 cm−1 . Fig. 5 displays the Raman spectra of BFTO, BFCT and SrBi4 Ti4 O15 (SBTi) (it was taken from our previous work) [27] samples at RT. Since all of the three samples belong to a four-layer Aurivillius phase, the whole characteristics for the observed Raman spectra of BFTO, BFCT and SBTi ceramics are very similar to each other. For Raman spectrum of bismuth layer structured crystals, it is generally recognized that their phonon modes can be classified into two categories: low frequency modes below 200 cm−1 and high frequency modes above 200 cm−1 . The low frequency modes below 200 cm−1 are related to larger atomic masses; for instance, the mode in ∼60 cm−1 reflects the vibration of Bi3+ ions in (Bi2 O2 )2+ layers and the mode in 90–160 cm−1 arises from the vibration of Bi3+ ions at A-site ions [28,29]. The high frequency modes above 200 cm−1 with A1g character are known to result from the torsional bending and the stretching modes of BO6 octahedral [28]. The low frequency mode from Bi2 O2 layers of BFTO and BFCT are located at ∼59.7 cm−1 and ∼59.4 cm−1 , respectively, which indicate that the insertion of the BFO and BCO perovskite blocks does not affect the Bi2 O2 layers. The motion of Bi ions at A-site ions in pseudo-perovskite slabs is observed at 117 cm−1 for BFTO and 114 cm−1 for BFCT, respectively. The mode shift can likely be attributed to the difference in the lattice parameter caused by the difference in the ionic radius between Fe and Co. The high frequency mode from torsional bending of TiO6 octahedra appears at around ∼270 cm−1 for SBTi, while the corresponding modes are observed at about 264 cm−1 for BFTO and 267 cm−1 for BFCT, respectively, which implies that Co doping may stabilize oxygen octahedron. Another high frequency mode is found at ∼325 cm−1 for BFTO and ∼327 cm−1 for BFCT, respectively, which corresponds to ferroelectric phase-transition as reported previously. The mode corresponding to TiO6 octahedra appears at 557 cm−1 for SBTi, while this mode is separated into a profile comprising double peaks. The double peak of BFTO appears at 541 cm−1 and 559 m−1 , while those for BFCT locate at 536 and 563 cm−1 . For BFTO, the 541 cm−1 mode corresponds to TiO6 octahedra and the 559 cm−1 mode originates from FeO6 octahedra, because the larger atomic mass of Fe causes the FeO6 octahedra mode to shift to lower Raman frequency. For BFCT, the intensity for the lower mode (536 cm−1 ) is found to increase apparently, which is probably caused by the overlapping of the torsional mode related to FeO6 and CoO6 octahedra. For BFTO and BFCT, the torsional bending mode from the FeO6 and CoO6 octahedra is observed at ∼698 and ∼678 cm−1 ,

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Fig. 6. Temperature-dependent Raman spectra of the (a) BFTO and (b) BFCT ranging from 25 to 400 °C, the inset of Fig. 6(a) and (b) shows the enlarged part of Raman shift from 470 to 670 cm−1 and from 800 cm−1 to 900 cm−1 , respectively.

however the same mode is absent in the Raman spectrum of SBTi, which indicates that Fe and Co exist in the form of FeO6 octahedra [29] and CoO6 octahedra [30]. In addition, as a result of Co substitution, the intensity of 731 cm−1 mode according to TiO6 octahedra increases considerably in comparison with that of BFTO and SBTi. For BFTO, there exist FeO6 octahedra in addition to TiO6 octahedra. For a BFTO sample, the stretching mode of 859 cm−1 is lower than that of an SBTi sample (865 cm−1 ). Because Fe ions are heavier than Ti ions, the shift of this mode to a lower frequency indicates the FeO6 octahedra inserting to the BTO successfully. In the BFCT, there is another mode of 829 cm−1 while it is not found in BFTO and SBTi. We have reason to believe that this mode was related to CoO6 octahedra. From the Raman spectra results, we conclude that BiFeO3 and BiCoO3 have been successfully inserted to the BTO, forming the four-layered perovskite BFTO and BFCT single phase. The temperature-dependent Raman spectra recorded over the range from RT to 400 °C for BFTO and BFCT are presented in Fig. 6. The overall spectral characteristics do not change notably as the temperature increases from 25 to 400 °C. However, all Raman modes shift to a low-wavenumber side due to thermal expansion. Such temperature dependence indicates that BFTO and BFCT maintain its crystal structure among the temperature range from RT to 400 °C. It is noted that the Raman mode (324 cm−1 for BFTO and 327 cm−1 for BFCT) corresponding to ferroelectric phase-transition decreases gradually with increasing temperature, in agreement with previous reports [14,30,31]. As shown in the inset of Fig. 6(a), the torsional bending mode of FeO6 and TiO6 octahedra undergoes a variation from double peak at 25 °C to single peak at 200 °C, and double peak again above 340 °C. The underlying physical mechanism for such variation tendency still needs more investigation. For the bending mode (831 cm−1 ) of BFCT as displayed in the inset of Fig. 6(b), it is interesting to observe that an apparent variation with temperature is in consistent with the ferromagnetic to paramagnetic transition (TcM ∼ 345 °C) [16]. The 831 cm−1 mode is found to vanish gradually as the temperature increases to higher than 340 °C, which corresponds to the Curie temperature TcM ∼ 345 °C. But the underlying physical mechanism of this scenario still remains an open question and needs to be further clarified. 4. Conclusions The BFTO and BFCT ceramics are of a single Aurivillius phase with a four-layer perovskite structure by inserting BiFeO3 and BiFe0.5 Co0.5 O3 into Bi4 Ti3 O12 . At RT, the remanent polarization (2Pr ) of BFTO and BFCT samples are obtained to be about

9.7 µC/cm2 and 13 µC/cm2 , respectively. The coercive field (2Ec ) of both the samples is found to be about 198 kV/cm and 140 kV/cm, respectively. The doping of Co was effective to enhance the ferroelectric properties because of the reduced oxygen vacancy resulting from the stabilized oxygen octahedron. The BFCT sample exhibits a typical ferromagnetic M–H loop, with 2Mr of 7.8 memu/g and coercive field (2Hc ) of 410 Oe, respectively, in which the remanent magnetization is about three thousand times of that of BFTO. Acknowledgments This work was supported by the Chinese National Natural Science Foundation (Grant No. 51072177) and the Jiangsu Province Department of Education (Grant No. 08KJB140011). References [1] B. Aurivillius, Ark. Kemi 1 (1949) 463. [2] B. Aurivillius, Ark. Kemi 2 (1950) 519. [3] B.H. Park, S.J. Hyun, S.D. Bu, T.W. Noh, J. Lee, H.-D. Kim, T.H. Kim, W. Jo, Appl. Phys. Lett. 74 (1999) 1907. [4] S.K. Kim, M. Miyayama, H. Yanagida, Mater. Res. Bull. 31 (1996) 121. [5] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [6] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Vieland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [7] W. Prellier, M.P. Singh, P. Murugave, J. Phys.: Condens. Matter 17 (2005) R803. [8] R. Ramesh, N.A. Spaldin, Nature Matter 6 (2007) 21. [9] T. Kimura, S. Kawamoto, Y. Yamada, M. Azuma, M. Takano, Y.Y. Tokura, Phys. Rev. B 67 (2003) 180401(R). [10] S.T. Zhang, Y.F. Chen, Z.G. Liu, N.B. Min, J. Wang, G.X. Cheng, J. Appl. Phys. 97 (2005) 104106. [11] A. Snedden, C.H. Hervoches, P. Lightfoot, Phys. Rev. B 67 (2003) 092102. [12] S.-L. Ahn, Y. Noguchi, M. Miyayama, T. Kudo, Mater. Res. Bull. 35 (2000) 825. [13] A. Srinivas, S.V. Suryanarayana, G.S. Kumar, M.M. Kumar, J. Phys.: Condens. Matter 11 (1999) 3335. [14] R.S. Singh, T. Bhimasankaram, G.S. Kumar, S.V. Suryananrayana, Solid State Commun. 91 (1994) 567. [15] X.Y. Mao, W. Wan, X.B. Chen, Solid State Commun. 147 (2008) 186. [16] X.Y. Mao, W. Wang, X.B. Chen, Y.L. Lu, Appl. Phys. Lett. 95 (2009) 82901. [17] J. Hao, X. Wang, R. Chen, Z. Gui, L. Li, J. Am. Ceram. Soc. 87 (2004) 1404. [18] Z. Shen, J. Liu, J. Grins, M. Nygren, P. Wang, Y. Kan, H. Yan, U. Sutter, Adv. Mater. 17 (2005) 676. [19] R.D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theory. Gen. Crystallogr. 32 (1976) 751. [20] J.F. Meng, P.S. Dobal, R.S. Katiyar, G.T. Zou, J. Raman Spectrosc. 29 (1998) 1003. [21] Y.Y. Wua, D.M Zhang, J. Yu, Y.B. Wang, Mater. Chem. Phys. 113 (2009) 422. [22] X.Y. Mao, J.H. He, J. Zhu, X.B. Chen, J. Appl. Phys. 100 (2006) 044104. [23] H. Naganuma, J. Miura, S. Okamura, Appl. Phys. Lett. 93 (2008) 052901. [24] H.G. Seale, I.O. Troyanchuk, A.P. Sazonov, K.L. Stefanopoulos, D.M. Toebbens, Phys. B 403 (2008) 2924. [25] A.A. Belik, M. Azuma, T. Saito, Y. Shimakawa, M. Takano, Chem. Mater. 17 (2005) 269. [26] Y.G. Wang, G. Xu, L.L. Yang, Z.H. Ren, X. Wei, W.J. Weng, P.Y. Du, G. Shen, G.R. Han, Mater. Lett. 62 (2008) 3806.

X. Mao et al. / Solid State Communications 152 (2012) 483–487 [27] J. Zhu, X.B. Chen, Z.P. Zhang, J.C. Shen, Acta. Mater. 53 (2005) 3155. [28] S. Kojima, R. Imaizumi, S. Hamazaki, M. Takashige, Japan. J. Appl. Phys. Part 1 33 (1994) 5559. [29] S.T. Zhang, Y.F. Chen, Z.G. Liu, N.B. Ming, J. Appl. Phys. 97 (2005) 104106.

487

[30] J.W. Zhu, X.J. Sun, Y.P. Wang, X. Wang, X.J. Yang, L.D. Lu, J. Rare Earths 25 (2007) 601. [31] A. Srinivas, D.W. Kim, K.S. Hong, S.V. Suryanarayana, Appl. Phys. Lett. 83 (2003) 2217.