Signatures of spin-phonon coupling in hematite crystallites through dielectric and Raman spectroscopy

Abstract

We report complex dielectric and Raman spectroscopy measurements in hematite crystallites, which are either hexagon-shaped plates or cuboids. Temperature variation of magnetization data in all these samples exhibits sharp Morin transition (TM). Moderate but clear enhancement in the real part of the dielectric constant $(\epsilon')$ is observed in the vicinity of the TM in all the samples. However a relaxation-like behavior in the imaginary part $(\epsilon'')$ is observed only in nanoplates or big cuboids across the TM. The real part of the dielectric constant fits well to the modified-Barrett equation above TM (in the spin-canted region), whereas clear deviations are observed below TM (in the pure antiferromagnetic region). The frequency of the phonon mode, deduced from the Barrett fitting, matches with the Eu Raman mode of hematite. Temperature-dependent Raman spectra reveal anomalies in all major phononic modes in the vicinity of the TM. Anharmonic fitting of the temperature-dependent Raman and modified-Barrett fitting of the dielectric data brings forward modifications in spin-phonon coupling in the vicinity of the Morin transition temperature in hematite.