The University of Tokyo, Japan
Marie Yoshikiyo has completed her bachelor and masters from Department of Chemistry, School of Science, The University of Tokyo. She is now a Ph.D. student at The University of Tokyo under the supervision of Prof. Shin-ichi Ohkoshi. (2013-present: JSPS Research Fellowship for Young Scientists).
ε-Fe2O3, which is a rare phase of iron oxide Fe2O3, was first obtained as a pure phase by our research group in 2004. It exhibits a huge coercive field of 20 kOe at room temperature, and due to the strong magnetic anisotropy, this material shows millimeter wave absorption at a very high frequency of 182 GHz.1 In this work, we report the theoretical studies on the electronic structures of ε-Fe2O3 to understand its huge coercivity. Synthesis of ε-Fe2O3 nanoparticles uses a combination technique of reverse-micelle and sol-gel techniques. The crystal structure is orthorhombic with four different Fe sites, A, B, C, and D sites, where A, and B sites are distorted octahedral, C site is a regular octahedral, and D site is a tetrahedral site. Using this crystal structure, we studied the electronic structure using first-principles calculations and molecular orbital calculations to understand the origin of the huge coercive field.2 The density of states showed that ε-Fe2O3 is a charge-transfer type insulator with positive sublattice magnetizations at B and C sites and negative sublattice magnetizations at A and D sites, consistent with our previous study based on molecular field theory.3 The charge density map showed a strong hybridization between Fe3d and O2p orbitals. Molecular orbital calculations indicated that this hybridization originates from the distorted coordination geometry of the Fe sites. This hybridization induces charge-transfer from O2p to Fe3d, which is assumed to create a non-zero orbital angular momentum, affecting the magnetic anisotropy of ε-Fe2O3.