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Introduction
In recent years, great attention has been paid to single-phase BiFeO 3 (BFO) multiferroic materials crystallized in the ABO 3-type perovskite structure with the R3c space group at room temperature, which display the co-existence of spontaneous electric and magnetic ordering in the same phase (Hill 2000; Fiebig et al2002). The characteristics of BFO multiferroic materials with attractive functionalities caused by the interaction between electric polarization and magnetization promise some potential applications in advanced magnetoelectric devices (Eerenstein et al2006). However, as promising as BFO multiferroic materials are, some drawbacks need to be overcome prior to real commercial applications. One problem is that the high leakage current in BFO materials, mainly originating from various impurity phases (Bi 2Fe 4O9, Bi 36Fe 24O57 and Bi 25FeO 40) as well as oxygen vacancies, results in poor ferroelectric behaviour and reliability of magnetoelectric devices (Munoz et al1985; Qi et al2005; Kumar and Yadav 2006). The other question is that, even though BFO is a G-type-antiferromagnetic (AFM) ordering, a weak FM component is observed owing to canting of Fe 3+ spins at room temperature (Smolenskii and Chupis 1982; Hur et al2004). As a result, it is necessary to further improve both ferroelectric and magnetic properties of BFO materials for the researchers. At present, it is well- established that an appropriate doping in BFO materials via partial ionic substitution to Bi 1−xAxFeO 3 (A is a lanthanide ion, rare earth metal) or to BiFe 1−xMxO3 (M is another 3d transition metal) is a promising way for upgrading multiferroics properties (Lee et al2006; Takeshi Kawae et al2008).
Due to the isovalence of Gd 3+ ions with respect to Bi 3+ ions together with adjacent ionic radius (Gd 3+:1 ⋅11 Å, Bi 3+:1 ⋅17 Å), Gd ion doping is not expected to suppress the oxygen vacancies in BFO materials. However, the suppression of oxygen vacancies is only a cause for the decrease of the leakage current in BFO materials, the aforementioned impurity phases are also responsible for the reduction of leakage of current in BFO materials (Palkar et al2004; Zhang et al2006). Gd...