Magnetic circular dichroism in the canted antiferromagnet alpha-Fe2O3: Bulk single crystal and nanocrystals

Hematite, alpha-Fe2O3, is a canted antiferromagnet with the Neel temperature of 960 K. The Morin transition, which occurs when the temperature is lowered to similar to 260 K, when the magnetic moments of the sublattices become strictly antiparallel to each other and the resulting magnetic moment disappears, is a characteristic feature of this compound. Detailed investigations are presented here of the magneto-optical effects in the alpha-Fe2O3 nanoparticles in the transmitted light and in the alpha-Fe2O3 single crystal in the reflected light. Two types of magnetooptic features were revealed in the magnetic circular dichroism (MCD) spectrum. Some of them described by the Gauss line shape were associated with one-ion d-d transitions. Others were described by the S-shaped line with the inflection points corresponding to the absorption maxima. Last features were assigned to the pair excitonmagnon transitions. An exceptionally strong S-shaped line at 2.0-2.5 eV appeared to be a unique feature of alpha-Fe2O3 distinguishing it from other iron oxide compounds. The origin of the high effectiveness of the pair excitonmagnon interaction giving rise to this strong S-shaped feature in the MCD spectrum of alpha-Fe2O3 is discussed.