Keywords:Ferromagnetic semiconductor, Spinel ferrite, X-ray absorption spectroscopy
To fabricate spintronic devices such as magneto-resistive random-access memory and spin transistors, ferromagnetic semiconductors with the Curie temperatures (TC > 300 K) are needed. Spinel ferrite M2+Fe2O4 (M = Mg, Mn, Co, etc) shows semiconducting conductivity at room temperature because the valence state of Fe ion is 3+ and there is no carrier hopping on the B sites. Substituting an ion having the valence higher than 3+ for the cation site M leads to a mixed valence state of Fe2+/Fe3+ on the B site in M2+Fe2O4, resulting in the increase of the electric conductivity as in the case of Fe3O4. Actually, Ru doping on cobalt ferrite CoFe2O4 (CFO) thin films dramatically improves the conductivity. It is expected that this improvement of the conductivity originates from inter-valence charge transfer between the Fe 3d and Ru 4d orbitals; Fe3+ + Ru3+ à Fe2+ + Ru4+. In this study, we have conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) on CFO and Co0.5Ru0.5Fe2O4 (CRFO) thin films to investigate the change of the electronic states due to the Ru-doping on CFO.