Enhanced Magnetization and Modulated Orbital Hybridization in Epitaxially Constrained BiFeO₃ Thin Films with Rhombohedral Symmetry

BiFeO 3 (BFO) is currently considered to be the most promising candidate material for device applications of room-temperature multiferroics. However, there exist some controversial arguments on the origin of the enhanced magnetization and polarization observed in the epitaxially constrained BFO thin film heterostructures. More specifically, the issue can be addressed by the following question: Can the epitaxial strain enhance the magnetization and the ferroelectric polarization in BFO? To clarify this controversial issue, we have systematically examined the magnetization characteristics of the rhombohedral BFO films epitaxially grown on (111)-oriented SrTiO 3 (STO) in terms of the in-plane misfit strain between the BFO layer and the STO substrate. The increase in the saturation magnetization with decreasing film thickness was found to be closely related with the misfit strain. By carefully examining synchrotron X-ray absorption spectra, we have further correlated the enhanced magnetization in a highly strained film with the reduced degree of hybridization between Fe 3d and O 2p orbitals and with the splitting of the triplet t 2g orbital into a 1g and e g π orbitals arising from the trigonal D 3 d distortion.