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 degrees) for FeO6 octahedra, and large reduction of Fe-O-Fe coupling angle (153 degrees -> 133 degrees) 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 chemistry perspective, we have shown that the ferromagnetism in BiFeO3 cluster is originates from Fe-O-Fe angle changes. Now, by changing Fe-O-Fe angle in BiFeO3 crystal, we have shown that there is a magnetic phase transition (antiferromagnetic to ferromagnetic) near angle FeOFe = 133 degrees. Therefore, we conclusively shown that the key structural parameter responsible for ferromagnetism in nano-BiFeO3 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.