# Dielectric and Raman Spectroscopy in Hematite crystallites across the Morin Transition

## TL;DR

This study investigates dielectric and Raman properties of hematite crystallites across the Morin transition, revealing changes in dielectric constant, relaxation behavior, and phonon modes linked to spin reorientation from antiferromagnetic to weak ferromagnetic states.

## Contribution

It provides new insights into the dielectric and phononic behavior of hematite across the Morin transition, highlighting the role of spin-phonon coupling and polaronic conduction mechanisms.

## Key findings

- Dielectric constant increases in the weak ferromagnetic region.
- Relaxation behavior observed only in nano plates and large cuboids at low frequencies.
- Anomalies in Raman modes indicate spin-phonon coupling varies across the transition.

## Abstract

We report complex dielectric and Raman spectroscopy measurements in four samples of $\alpha$- Fe$_2$O$_3$, consisting of crystallites which are either hexagonal shaped plates or cuboids. All four samples exhibit the spin reorientation transition from a pure antiferromagnetic (AFM) to a weak-ferromagnetic (WFM) state at the Morin Transition temperature (T$_M$) intrinsic to $\alpha$- Fe$_2$O$_3$. These samples, pressed and sintered in identical conditions for the dielectric measurements, reveal moderate but clear enhancement in the real part of the dielectric constant ($\epsilon'$) in the WFM region. However, a relaxation-like behavior in the imaginary part of $\epsilon''$ is observed only in nano plates or big cuboids. Further still, this relaxation patten is observed only in lower frequency region, lasting upto a few kHz and follows Arrhenius law within this limited range. The activation energy deduced from the fitting is suggestive of polaronic conduction. Temperature dependent Raman spectra reveal anomalies in all major phononic modes and also in 2Eu mode in the vicinity of the Morin transition. A peak like behavior in Raman Shifts, in conjuncture with anharmonic fitting reveals that the nature of spin phonon coupling is different in pure AFM and WFM regions and it is tied to the mild variations, as observed in the dielectric constant of$\alpha$- Fe$_2$O$_3$ near the T$_M$.