Correlation between Structure and Ferromagnetism in Nano-BiFeO3

Structural and electronic properties of BiFeO3 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-BiFeO3. 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 BiO6 octahedra, comparatively small distortion but large rotation (20°) for FeO6 octahedra, and large reduction of Fe–O–Fe coupling angle (153° → 133°) compared to bulk BiFeO3. 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 BiFeO3 nanoparticles (<18 nm). Also, a large charge transfer from Bi-s states explains why small BiFeO3 nanoparticles cannot sustain ferroelectricity. Applying a crystal...