Surface oxidation of Permalloy thin films

The chemical and magnetic structures of oxides on the surface of Permalloy ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ films were investigated as functions of annealing time with x-ray and polarized neutron reflectometry. For annealing times of less than one hour, the oxide consisted of a 1.5-nm-thick layer of $\mathrm{NiO}$ on an Fe oxide layer that was in contact with Permalloy. The Fe oxide thickness increases with annealing time with a parabolic rate constant of ${10}^{-18}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$ (for an annealing temperature of $373\mathrm{K}$). The growth of the oxide layer is limited by the rate at which oxygen appears below the $\mathrm{NiO}$ layer. No portion of the oxide region was found to be ferromagnetically ordered for films annealed less than one hour. The growth of the Fe oxide region is well correlated with the measured increase of the second-order magnetic susceptibility for similarly prepared samples.