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 (T M ). Moderate but clear enhancement in the real part of the dielectric constant is observed in the vicinity of the T M in all the samples. However a relaxation-like behavior in the imaginary part is observed only in nanoplates or big cuboids across the T M . The real part of the dielectric constant fits well to the modified-Barrett equation above T M (in the spin-canted region), whereas clear deviations are observed below T M (in the pure antiferromagnetic region). The frequency of the phonon mode, deduced from the Barrett fitting, matches with the E u Raman mode of hematite. Temperature-dependent Raman spectra reveal anomalies in all major phononic modes in the vicinity of the T M . 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.