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

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. Mössbauer 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.