Spin Orientation of Hematite<tex>$(alpha hbox-hboxFe_2hboxO_3)$</tex>Nanoparticles During the Morin Transition

Abstract

We have synthesized 40-nm-sized hematite particles by the forced hydrolysis of acid Fe/sup 3+/ solution to investigate the change in spin direction during the Morin transition. Mo/spl uml/ssbauer spectra were obtained at various temperatures ranging from 4.3 to 300 K. The Morin temperature (T/sub M/) was found to be 220 K, which is lower than 263 K of bulk hematite. Fe/sup 3+/ spins in the 40-nm-sized hematite particle flip from 90/spl deg/ to 28/spl deg/ 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/spl deg/ to 0/spl deg/ observed in bulk hematite or sub- and micron-sized particles at T<T/sub M/.