Structural, magnetic and dielectric properties of Sr and V doped BiFeO₃ multiferroics

Highlights

• Sr and V doped BiFeO₃ multiferroics were synthesized by solid state reaction.

• Ceramics crystallized in rhombohedrally distorted perovskite structure.

• Remnant magnetization and coercive field were improved with V doping.

Abstract

Bi_0.85Sr_0.15FeO₃ (BSFO), Bi_0.85Sr_0.15Fe_0.97V_0.03O₃ (BSFVO1) and Bi_0.85Sr_0.15Fe_0.95V_0.05O₃ (BSFVO2) ceramics were synthesized by solid state reaction method. X-ray diffraction studies and Rietveld refinement results indicate that all the samples crystallized in rhombohedrally distorted perovskite structure. The remnant magnetization and coercive field of BSFVO2 were greatly enhanced in comparison with BSFO. The enhancement of remnant magnetization was attributed to collapse of the spiral spin structure caused by change in bond length and bond angles of BSFO on V substitution. The enhanced value of coercive field might be attributed to decreased grain size with V substitution. BSFO sample shows dispersion in dielectric constant (έ) and dielectric loss (tan δ) values in lower frequency region. With V doping this dispersion is reduced resulting in frequency independent region. Dielectric anomaly peak due to charge defects in BSFO sample is also suppressed significantly on V substitution. BSFVO2 sample shows almost temperature stable behavior in έ and tan δ in the studied temperature range. Temperature dependence of index ‘s’ of power law suggests that overlapping large polaron tunneling model is applicable for describing the conduction mechanism in BSFO sample while small polaron tunneling model is appropriate for BSFVO1 and BSFVO2 samples in the studied temperature range.

Introduction

Existence of spontaneous magnetic ordering and ferroelectric polarization in a single phase material has been a subject of growing interest for both the dielectric and magnetic scientific communities. Coupling between magnetic and electric ordering leads to magnetoelectric (ME) effect. The ME effect provides an additional degree of freedom in designing of functional sensors, current devices, transducers and multistate memory devices. Besides application potentials, the fundamental physics of multiferroic materials is rich and fascinating [1], [2], [3], [4]. However, there are very few naturally occurring materials that can exhibit both spontaneous magnetization and electric polarization. This is possibly due to the fact that the transition metal ‘d’ electron essential in the presence of magnetic moment, also reduces lattice distortion. Lattice distortion is essential in the presence of ferroelectric behavior. Also, known single-phase magnetic ferroelectrics usually have low magnetic ordering temperatures, thus constricting the possibilities for their applications [4]. From this point of view, the most interesting results are expected for the BiFeO₃-based perovskite materials. Stoichiometric BiFeO₃ (BFO) crystallizes in a rhombohedrally distorted perovskite structure. It has been considered as one of the fascinating multiferroics because of its ferroelectric transition (at about 1100 K) and antiferromagnetic Neel temperatures (at about 640 K) are well above room temperature. The ferroelectric mechanism in BFO is caused by the steriochemically active 6s² lone pair of Bi³⁺ while the weak ferromagnetism is due to residual moment from the canted Fe³⁺ spin structure. The magnetic moments of Fe³⁺ cations in BFO couple ferromagnetically within the pseudocubic (111) planes and antiferromagnetically between the adjacent planes showing the G-type antiferromagnetic order [5], [6], [7]. However, it has been shown that a long-range incommensurate cycloidal spiral magnetic structure with a large period of 62 nm is present in BFO. This cycloidal structure results in the disappearance of weak ferromagnetism and the linear ME effect due to averaging over the period [5], [7]. In addition, the bulk BFO is characterized by serious current leakage problems due to the existence of large number of charge centers caused by oxygen ion vacancies and Bi₂O₃ evaporation during sintering process which makes it difficult to achieve high resistivity. These problems limit the use of BFO for fabrication of multifunctional devices [8], [9], [10]. Several research groups have reported that the multiferroic properties of BFO can be improved with various ion substitution at A or/and B site [11], [12], [13], [14], [15], [16], [17], [18]. Recently, divalent cations (e. g., Ca, Sr, Pb, Ba) substitution at A-site of BFO ceramics have been reported to enhance the magnetization of BFO ceramic [14], [19], [20]. Interestingly, the magnetic moment with divalent alkaline earth metal substitution is comparable to that with rare earth substituted BFO. Also partial substitution of Fe³⁺ at the B-site with higher valence cations like Nb⁵⁺ and Ti⁴⁺ are reported to decrease the leakage current density significantly [21], [22]. V⁵⁺ is also a higher valence cation than Fe³⁺. Therefore, B site doping with V⁵⁺ is expected to increase the electrical resistivity of BFO by compensating the charge defects that cause high conduction. This would allow the ferroelectricity and magnetoelectric coupling to be determined at room temperature. A significant improvement of ferroelectric properties in La and V co-doped BFO ceramic have been observed by Yu et al. [23], [24]. But investigations on co-substitution of alkaline earth metal Sr and V in bulk BFO have not been reported so far. Therefore, aim of the present work is to investigate the structural, magnetic and dielectric properties of Bi_0.85Sr_0.15Fe_1–xV_xO₃ multiferroic ceramics.