Correlation between Structure and Ferromagnetism in Nano-BiFeO₃

Abstract

Structural and electronic properties of BiFeO 3 clusters were studied using first-principles electronic structure calculations. The main aim of this work is to unveil the role of structural modifications in developing ferromagnetism in nano-BiFeO 3 . We have found that the ferromagnetic ground state is energetically more favorable than the antiferromagnetic ground state for this cluster. In a relaxed cluster, there are large distortions for BiO 6 octahedra, comparatively small distortion but large rotation (20°) for FeO 6 octahedra, and large reduction of Fe–O–Fe coupling angle (153° → 133°) compared to bulk BiFeO 3 . A large charge transfer from Bi-s states is predicted for cluster which may be responsible for the observed structural changes. These results are in consistent with recent experimental observation of Bi sublattice melting for small BiFeO 3 nanoparticles (<18 nm). Also, a large charge transfer from Bi-s states explains why small BiFeO 3 nanoparticles cannot sustain ferroelectricity. Applying a crystal chemistry perspective, we have shown that the ferromagnetism in BiFeO 3 cluster is originates from Fe–O–Fe angle changes. Now, by changing Fe–O–Fe angle in BiFeO 3 crystal, we have shown that there is a magnetic phase transition (antiferromagnetic to ferromagnetic) near ∠FeOFe = 133°. Therefore, we conclusively shown that the key structural parameter responsible for ferromagnetism in nano-BiFeO 3 is the Fe–O–Fe coupling angle. These results are also important for device fabrication which often uses nano-BFO/BFO-films where it is possible to change the Fe–O–Fe coupling angle easily by applying some strains.