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 (BiFeO 3 , 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 (V O ) 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 Fe Fe 3+ 2+ −V O •• − Fe Fe 3+ 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.

Coherent Oscillations in a SrRuO3/BiFeO3 Superlattice

“…Because BFO is insulating while SRO is metallic, the transport property should be dominated by the SRO layer. We suppose that interlayer coupling between the two materials resulted in the observed anomaly, whose origin is the spin state change in the BFO layer [ 22 ]. Slight resistivity upturn below 20 K may be attributed to the localization effect due to disorder [ 23 ].…”

“…The magnetization confirms that the superlattice sample has a ferromagnetic order similar to SRO but with a reduced transition temperature of 125 K. The signature of the magnetic order can also be recognized in transport measurement by a kink that is marked with an arrow in Figure 1c. It is evident that there exists an additional anomaly in the resistance data around 260 K. It has been reported that subtle changes in the spin state of BFO, such as a spin reorientation transition, may arise between 10 K and 300 K [20][21][22]. Because BFO is insulating while SRO is metallic, the transport property should be dominated by the SRO layer.…”

The size effect of BiFeO3 nanocrystals on the spatial spin modulated structure

“…It should be noted that at present, there is no consensus in the literature on what exactly happens to bismuth ferrite at low temperatures: transformation into a spin glass, 43 spin-orientation transition, 44 or surface transition. 45 Using the model from ref. 46, we determined the anharmonicity parameter values for all the samples:…”