Enhanced magnetic and conductive properties of Ba and Co co-doped BiFeO3 ceramics
Introduction
Multiferroic materials show both ferroelectric order and ferromagnetic order in the same phase. They have been attracting the attention of researchers due to their unique behaviors of coupling between the ferroelectricity, ferromagnetism and ferroelasticity and also because of their potential applications of devices in spintronics, information storage, sensing and actuator [1], [2], [3]. BiFeO3 (BFO) has becoming an interesting candidate of application due to its high ferroelectric Curie temperature (Tc~1103 K) and antiferromagnetic Néel temperature (TN~643 K) [4]. The ferroelectric order in BFO is introduced due to the distortion of the Bi3+ 6s2 lone pair, while its G-type antiferromagnetism is due to the local spin ordering of Fe3+ which forms a cycloidal spiral spin structure with a spin periodicity of 620 Å. Currently, bulk BFO is not suitable for device applications because it has some inherent problems, such as the appearance of impurity phases, a high leakage current, and a wide difference in ferroic transition temperatures [4], [5]. Moreover the possible nonzero remnant magnetization (Mr) permitted by the canted G-type antiferromagnetic order is cancelled by the space-modulated spin structure on the 620 Å wavelength [5], [6], [7]. In order to overcome these problems, many attempts have been made recently, such as doping trivalent ions (La3+, Nd3+,Yb3+, Sm3+ Eu3+ and Gd3+) [8], [9], [10], [11], [12], [13], [14], [15], or divalent ions (Ca2+, Pb2+, Sr2+, Ba2+) [16], [17], [18] at the A site of BFO to substitute part of Bi3+or ions of Nb5+ Mn4+ Ti4+ or Cr3+ [19], [20], [21] at the B site to substitute part of Fe3+.
Bulk BFO, at room temperature, has a rhombohedrally distorted perovskite structure with R3c space group. The large orbital radius of A-site Bi 6s lone pair is the primary cause of the unit cell distortion in BFO, which results in its stabilization of low-symmetry structure and appearance of large polarization along the pseudocubic 〈1 1 1〉 direction. It has been reported that substitution at Bi site easily gives rise to a phase transition and makes an enhancement of the dielectric or magnetic properties. Such as a phase transition from rhombohedral to orthorhombic and tetragonal was observed in 10% to 30% La-doped LaxBi1−xFeO3, respectively [22]. Such doping helps in suppressing the appearance of secondary phase and in improving the electrical and magnetic properties. In the case of Nd doping, a continual transformation of crystal structure from the rhombohedral to triclinic and then pseudo-tetragonal structure was observed in Bi1−xNdxFeO3 (x=0–0.2) system, and an enhanced remnant polarization (~9 μC/cm2) and remnant magnetization (~0.227 emu/g) were obtained [23]. The substitution of bismuth by gadolinium could induce a R3c→Pnma phase structural transition at the content above 10%, which leads to the suppression of spiral modulated spin structure and develops ferromagnetic properties in BiFeO3-based materials [24]. It has recently been demonstrated by Makhdoom et al. that the substitution of Bi by Ba results in a phase transition from rhombohedral to pseudo cubic, and the leakage current density of sample with 10% Ba dopant is found to be four orders of magnitude less than that of the pure BiFeO3 [25]. Also, it was reported that Ba dopant could shift the ferroelectric and magnetic transitions of the BFO compounds towards higher temperature, as well as enhanced remnant magnetization and polarization [16], [17], [18], [26]. So, in addition to modulate the physical properties, Bi-site substitution also helps to stabilize the perovskite phase of BFO.
For the Fe-site substitution, in addition to the appearance of magnetic improvements, substitution of 4+ ions into BFO requires charge compensation, which can be achieved via some of the following mechanisms: (I) filling of oxygen vacancies, (II) decrease of cation valence (i.e. formation of Fe2+), and (III) creation of cation vacancies. The substitution of 2+ ions exhibits the converse results. So doping with high valence element like Mn4+ could enhance the dielectric polarization by reducing defects. Also, as reported, substituting transition mental ions (i.e. Nb5+ Ti5+ Co3+ or Cr3+) at Fe-site in BiFeO3 could help to improve the magnetic properties and at the same time simplify the synthesis procedure [27], [28], [29]. In addition, substituting transition mental ions Co3+ at the Fe site could help to improve the magnetic properties without charge compensation. [29].
However, the saturation level of forming a solid solution for Fe-site substitution usually is rather low; consequently impurity phases are easily formed in these systems. For example, in our recent studies, we found that an impurity phase can be observed clearly in BiFe0.95Co0.05O3 ceramic, indicating a lower level of Co-doping saturation in BFO. Moreover, in BiFe1−xCuxO3 ceramics, even lower level of dopant Cu (3%) could induce an appearance of impurity phases. In order to overcome these obstacles, co-doping at both A and B sites of BFO is worthy trying, since Bi-site doping could stabilize the perovskite structure and increase the tolerance factor of ABO3 structure. For example, it was reported recently that La-Co co-doped BFO exhibits improved electrical and magnetic properties and the content of Co can reach to 10% in thin-film system [30] and to 3% in bulk ceramics [31] without any impure phase. Similarly, for La–Nb and La–Ti co-doped BFO samples, phase change to orthorhombic structure occurs at higher level of substitution [32], [33].
The primary objective of this fundamental study is to obtain some knowledge of influence of A-site dopant on B-site substitution and their co-action on BFO's electric and magnetic properties. In this work, Bi1−xBaxFe1−yCoyO3 (BaxCoyBFO, x=0, 0.1, 0.2 y=0, 0.01, 0.03, 0.06) ceramics were synthesized, and their electric and magnetic properties and the interaction between A and B site with different levels of ions doping were investigate. We find that A-site Ba doping could help to get pure phase of perovskite material with more content of Fe-site dopants, and then overall composition and crystal structure of BaxCoyBFO can be tailored over a wide range of dopant level.
Section snippets
Experimental
BiFeO3, Co-doped BiFe1−yCoyO3 (y=0.01 and 0.03, i.e. Co0.01BFO and Co0.03BFO) and Ba and Co co-doped Bi1−xBaxFe1−yCoyO3 (x=0.1, 0.2, y=0.01, 0.03 and 0.06, i.e. Ba0.1Co0.01BFO, Ba0.1Co0.03BFO, Ba0.2Co0.01BFO, Ba0.2Co0.03BFO and Ba0.2Co0.06BFO) were prepared by modified solid-state reaction method with high purity (99.99%) starting powders of Bi2O3, Fe2O3 and BaO. These materials were carefully weighed and stoichiometrically mixed in an agate mortar for 3 h using alcohol as a medium and then
Results and discussion
Fig. 1(a)–(d) shows XRD patterns of the sintered Bi1−xBaxFe1−yCoyO3 (x=0, 0.1, 0.2 y=0.01, 0.03, 0.06) samples. The diffraction peak positions for Ba–Co co-doped BiFeO3 show gradual variations in peaks when compared with BiFeO3. All of the main diffraction peaks of pure BFO and Co-doped samples are matched with those of rhombohedral structure with a space group of R3c. As shown in Fig. 1(a) and (b), small amount of impurity phases still exist in CoyBFO and Ba0.1Co0.06BFO ceramics. The impurity
Conclusion
In conclusion, it was found that A-site doping in Bi1−xBaxFe1−yCoyO3 with divalent Ba2+ ions could help making pure phase materials with more content of Cobalt doping at B site. The magnetic properties are greatly enhanced by co-doping and the maximal remanent magnetization of 0.93 emu/g and minimum coercive field of 774 Oe have been obtained for the Ba0.2Co0.06BFO specimen. Meanwhile, it also got the biggest value of dielectric constant and leakage current density because of containing more
Acknowledgements
This work was funded by the Research Grant Council of Hong Kong (Project No. HKU702409P), the National Natural Science Foundation of China (11004148, 11104202), the Natural Science Foundation of Tianjin (11JCZDJC21800, 11JCYBJC02700), the Research Foundation of Tianjin Education Council (20090308), and SRF for ROCS, SEM.
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