2D COS and PCMW2D analysis of the magnetic transitions in Raman spectra of BiFeO₃

BiFeO₃ is one of the most attracting materials because it is the only known room-temperature multiferroic material. It has been studied by Raman spectroscopy extensively. We perform two-dimensional correlation spectroscopy (2D COS) and perturbation-correlation moving window two-dimensional correlation spectroscopy (PCMW2D) analysis on the temperature-dependent Raman spectra of a BiFeO₃ single crystal. The two low temperature spin reorientation transitions around 135 K and 190 K are confirmed through the significant change in phonon peaks from PCMW2D correlation spectra. We also identify the origin of the mode at ~ 1255 cm⁻¹, which has been controversial until now. In our 2D COS analysis, the intensity of this mode has a strong correlation with the intensity of 145 and 176 cm⁻¹ phonons which correspond to the vibration modes of the Bi-O and Fe-O chemical bonds. We propose that the mode at ~ 1255 cm⁻¹ is the two-phonon scattering of the 551 cm⁻¹ mode assisted by the 145 −176 cm⁻¹ phonons.