Thermal hysteresis of spin reorientation at Morin transition in alkoxide derived hematite nanoparticles

TL;DR

This study investigates the thermal hysteresis observed during the Morin transition in highly crystalline hematite nanoparticles, revealing size and crystallinity effects on phase stability and transition behavior.

Contribution

It introduces a novel sol-gel synthesis method for hematite nanoparticles and demonstrates how size and crystallinity influence the Morin transition and hysteresis.

Findings

Thermal hysteresis observed at the Morin transition temperature.

Small size and high crystallinity hinder phase nucleation.

Hyperfine parameters show a jump at T_M with hysteresis.

Abstract

We present results on structural and magnetic properties of highly crystalline alpha-Fe2O3 nanoparticles of average size ~200 nm, synthesized from a novel sol-gel method using metal alkoxide precursor. These particles are multi-domain, showing the weak ferromagnetic-antiferromagnetic (WF-AF) transition (i.e., the Morin transition) at T_M = 256(2) K. M\"ossbauer measurements revealed a jump in hyperfine parameters at T ~ T_M, which also displays thermal hysteresis upon cooling or heating the sample. The analysis of hyperfine parameters as a function of temperature allowed us to discard temperature gradients as well as the coexistence of WF/AF phases as possible origins of this hysteretic behaviour. Instead, the hysteresis can be qualitatively explained by the small size and high-crystallinity of the particles, which hinder the nucleation of the WF or AF phases yielding metastable states beyond TM.