Control of conductivity in Fe-rich cobalt-ferrite thin films with perpendicular magnetic anisotropy

Abstract

We fabricated two types of cobalt-ferrite (001) thin films, insulative Fe-rich cobalt-ferrite ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4+\ensuremath{\delta}}$ (I-CFO) and conductive Fe-rich cobalt-ferrite ${\mathrm{Co}}_{y}{\mathrm{Fe}}_{3\ensuremath{-}y}{\mathrm{O}}_{4}$ (C-CFO), with perpendicular magnetic anisotropy (PMA) on MgO (001) substrates. Although the stoichiometric cobalt ferrite is known as an insulating material, it is found that the conductivity of Fe-rich cobalt ferrites can be controlled by changing the source materials and deposition conditions in the pulsed laser deposition technique. The I-CFO and C-CFO films exhibit PMA through the in-plane lattice distortion. We investigated the Fe-ion-specific valence states in both I-CFO and C-CFO films by M\"ossbauer spectroscopy and x-ray magnetic circular dichroism and found that the difference in conductivity corresponds to the abundance ratio of the ${\mathrm{Fe}}^{2+}$ state at the octahedral $B$-site $({O}_{\mathrm{h}})$ in the inverse spinel structure. Furthermore, first-principles calculations reproduce the changes in the density of states at the Fermi level depending on the cation vacancies at the $B$-site, which explains the difference in conductivity between I-CFO and C-CFO.