Intervalence charge transfer and charge transport in the spinel ferrite ferromagnetic semiconductor Ru-doped CoFe2O4

Intervalence charge transfer (IVCT) refers to the transfer of electrons between two metal (M) sites with different oxidation states, through a bridging ligand: Mn+1 + M-/m -> M-n + M/m+1 . It is considered that the IVCT is related to the hopping probability of electrons (or electron mobility) in solids. Controlling the conductivity of ferromagnetic semiconductors (FMSs) is critical for device applications, and thus, the manipulation of conductivity through IVCT may be an approach of band engineering in FMSs. In Ru-doped CoFe2O4 (CFO), which shows ferrimagnetism and semiconducting transport properties, the reduction in electric resistivity is attributed to both carrier doping caused by the Ru substitution of Co and increase in carrier mobility due to hybridization between the wide Ru 4d and Fe 3d orbitals. The latter indicates the so-called IVCT mechanism, i.e., charge transfer between the mixed valence Fe2+/Fe(3+ )states, facilitated by the bridging Ru 4d orbital, expressed as Fe2+ + Ru4+ <-> Fe3+ + Ru3+. To elucidate the emergence of the IVCT state, we conducted x-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RPES) measurements on undoped CFO and Ru-doped Co0.5Ru0.5Fe2O4 (CRFO) thin films. The XAS and RPES spectra indicated the presence of mixed valence Fe2+/Fe3+ states and hybridization between the Fe 3d and Ru 4d states. These results provide experimental evidence for the IVCT state in CRFO, demonstrating a mechanism that controls electron mobility through hybridization between the 3d transition metal cations with intervening 4d states.