Chemical effects at metal/oxide interfaces studied by x-ray-absorption spectroscopy

A chemical and magnetic characterization of ferromagnet/antiferromagnet interfaces is essential to understand the microscopic origins of exchange anisotropy and other magnetic phenomena. We have used high-resolution L-edge x-ray absorption spectroscopy (XAS), which is element specific and sensitive to chemical environment and spin orientation, to investigate the interface of antiferromagnetic oxides with ferromagnetic metals. Clear quantitative evidence of oxidation/reduction reactions at the as-grown metal/oxide interface is presented. In situ– and ex situ–grown samples of the form oxide $(5\mathrm{}–30\hspace{0.5em}\AA )/\mathrm{metal}$ $(1\mathrm{}–10\hspace{0.5em}\AA ),$ where oxide is either NiO or CoO and metal is either Fe, Co, or Ni, were studied by high-resolution XAS. For all samples, a metal(oxide) layer adjacent to an oxide(metal) layer was partially oxidized(reduced). Quantitative analysis of the spectra showed that one to two atomic layers on either side of the interface were oxidized/reduced. An elemental series of samples showed that the amount of oxidation/reduction was in accord with the difference in oxidation potentials of the adjacent cations, e.g., oxide layers were more strongly reduced by an iron metal layer than by cobalt or nickel metal layers. Annealing to temperatures, typically used to bias devices, was shown to significantly increase the amount of oxidation/reduction. The oxidation behavior of iron was shown to depend on the amount of oxygen available. Our results are believed to provide important information for the improved understanding of exchange anisotropy.