Controlling the electronic structure of Co1−xFe2+xO4thin films through iron doping

Abstract

The electronic, magnetic and transport properties of iron-doped cobalt ferrite (Co${}_{1\ensuremath{-}x}{\mathrm{Fe}}_{2+x}$O${}_{4}$) thin films grown epitaxially on MgO (001) substrates are investigated by soft x-ray absorption and photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, superconducting quantum interference device magnetometry, and resistivity measurements. The crystal structure for Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$ is determined to be nearly inverse spinel, with the degree of inversion increasing for increased doping until it becomes fully inverse spinel for Fe${}_{3}$O${}_{4}$. The doped iron cations have a valency of 2$+$ and reside solely on octahedral sites, which allows for conduction owing to hopping between Fe${}^{2+}$ and Fe${}^{3+}$ octahedral cations. The addition of Fe${}^{2+}$ cations increases the electron density of states near the Fermi energy, shifting the Fermi level from 0.75 to 0 eV with respect to the top of the valence band, as the doping increases from $x$ $=$ 0.01 to 1. This change in electronic structure results in a change in resistivity by over two orders of magnitude. In contrast, the magnetic properties of CoFe${}_{2}$O${}_{4}$ thin films, characterized by a significantly reduced saturation magnetization compared to the bulk and large magnetic anisotropies, are affected less significantly by doping in the range from 0 to 0.63. These results show that Co${}_{1\ensuremath{-}x}$Fe${}_{2+x}$O${}_{4}$ has tunable electronic properties while maintaining magnetic properties similar to CoFe${}_{2}$O${}_{4}$.