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Inter-valence charge transfer (IVCT) is electron transfer between two metal$M$ sites differing in oxidation states through a bridging ligand: $M^{n+1} +M'^{m} \rightarrow M^{n} + M'^{m+1}$. It is considered that IVCT is related tothe hopping probability of electron (or the electron mobility) in solids. Sincecontrolling the conductivity of ferromagnetic semiconductors (FMSs) is a keysubject for the development of spintronic device applications, the manipulationof the conductivity through IVCT may become a new approach of band engineeringin FMSs. In Ru-doped cobalt ferrite CoFe$_2$O$_4$ (CFO) that showsferrimagnetism and semiconducting transport properties, the reduction of theelectric resistivity is attributed to both the carrier doping caused by the Rusubstitution for Co and the increase of the carrier mobility due tohybridization between the wide Ru $4d$ and the Fe $3d$ orbitals. The latter isthe so-called IVCT mechanism that is charge transfer between the mixed valenceFe$^{2+}$/Fe$^{3+}$ states facilitated by bridging Ru $4d$ orbital: Fe$^{2+}$ +Ru$^{4+}$ $\leftrightarrow$ Fe$^{3+}$ + Ru$^{3+}$. To elucidate the emergenceof the IVCT state, we have conducted x-ray absorption spectroscopy (XAS) andresonant photoemission spectroscopy (RPES) measurements on non-doped CFO andCo$_{0.5}$Ru$_{0.5}$Fe$_2$O$_4$ (CRFO) thin films. The observations of the XASand RPES spectra indicate that the presence of the mixed valenceFe$^{2+}$/Fe$^{3+}$ state and the hybridization between the Fe $3d$ and Ru $4d$states in the valence band. These results provide experimental evidence for theIVCT state in CRFO, demonstrating a novel mechanism that controls the electronmobility through hybridization between the $3d$ transition-metal cations withintervening $4d$ states.

Inter-valence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe$_2$O$_4$