Low-Temperature Surface Phase Transitions in Multiferroic BiFeO₃ Nanocrystals Probed via Electron Paramagnetic Resonance

Abstract

The low-temperature phase transitions observed in magnetoelectric bismuth ferrite (BiFeO3, BFO) have recently been a topic of interest to several researchers. This communication focuses on connecting recent structural revelations, such as the existence of a “skin” layer in BFO, the lattice contraction, and subsequent expansions in the skin layers with X-band electron paramagnetic resonance (EPR) spectra. A closer look at Lande’s g-factor and the EPR asymmetry parameters reveal vital information about the origin of the phase transitions at 140, 200, and 280 K. Correlating the EPR results with existing theoretical calculations indicates that oxygen vacancies (VO) accumulate at the skin layer, causing lattice contraction. This contraction causes local changes in the spin magnetic moment and translates to an anomaly in the resonant lines. The discussions imply that the phase transition at 140 K is due to spin reorientation caused by changes of interatomic distances and angles between the FeFe3+2+–VO••–FeFe3+2+ sites. Transition at 200 K is observed to occur due to elastic distortion of the oxygen octahedra. The transition at 280 K is thought to be due to the freezing of spins in the lattice.