Spin orientation of hematite (α-Fe2O3) nanoparticles during the Morin transition

We have synthesized 40-nm-sized hematite particles by the forced hydrolysis of acid Fe³⁺ solution to investigate the change in spin direction during the Morin transition. Mössbauer spectra were obtained at various temperatures ranging from 4.3 to 300 K. The Morin temperature (T_M) was found to be 220 K, which is lower than 263 K of bulk hematite. Fe³⁺ spins in the 40-nm-sized hematite particle flip from 90° to 28° with respect to the c-axis of hexagonal structure during the Morin transition, which corresponds to the [110] direction of rhombohedral structure. This is not in agreement with the spin flip from 90° to 0° observed in bulk hematite or sub- and micron-sized particles at TM.