Magnetic interactions in BiFe0.5Mn0.5O3 films and BiFeO3/BiMnO3 superlattices

BiFeO3 (BFO) is the most widely studied multiferroic material, due to its above room temperature antiferromagnetism (TN ~ 640 K) and ferroelectricity (TCFE ~ 1100 K)1. BFO has G-type antiferromagnetic (AF) spin structure with spin canting and superimposed cycloidal modulation2, and various magnetic anomalies observed at low temperature3,4. Ion substitution is a widely adopted strategy, suppressing the leakage current and improving the ferroelectricity5,6, destroying the cycloidal modulation and enhancing the ferromagnetism7,8, towards the realization of room temperature control of ferromagnetism with an electric field9. With magnetic ion substitution in BFO, more complicated magnetic interactions might be included, leading to the observation of cluster spin glass10. The clear understanding of exchange interactions between Fe and substituting ions in BFO is important for the comprehensive studies on magnetic properties, e.g., the rather confusing magnetic properties in Mn doped BiFeO3, though Mn has been demonstrated to be an effective substituent for ferroelectricity11. Mn ions of concentration up to 50% have been doped into BFO by epitaxial growth on (001) SrTiO3 (STO) substrates, structure and magnetic properties of BiFe0.5Mn0.5O3 (BFMO) have been studied12,13,14. Strongly enhanced ferromagnetism has been reported by Choi et al.12, but only negligible weak ferromagnetism has been reported by Bi et al.14. Furthermore, the superexchange interaction between Fe3+ and Mn3+ is rather complicated. Ferromagnetic (FM) interaction has been observed between Fe and Mn at the interface of La0.7Sr0.3MnO3 and BFO due to orbital reconstruction15, while AF interaction exists at the interface of La0.5Ca0.5MnO3 and BFO without orbital reconstruction16. Both BFO and BMO have distorted perovskite structure with similar pseudocubic lattice constants (3.96 Ǻ for BFO and 3.95 Ǻ for BMO)17,18, which is very close to that of STO (3.905 A)12. Therefore, in addition to the previously reported solid solution of BFMO, superlattices with alternative BFO and BMO layers are likely to be epitaxially grown on (001) STO substrates, and the inhomogeneous distribution of Fe and Mn will be suppressed14. There is only superexchange interaction between Fe and Mn ions at the interfaces along c direction, which is expected to facilitate the study of the Fe-O-Mn interaction. In this paper, BFMO films and BFO/BMO superlattices (simply denoted as BFO/BMO) were grown on (001) STO substrates. Spin glass behavior were observed in both samples. The Fe-O-Mn interaction has been confirmed to be AF by X-ray magnetic circular dichroism (XMCD) measurements. Spin glass in BFMO can be categorized as cluster spin glass, while spin glass in BFO/BMO results from competing AF and FM interactions at interfaces.