No abstract

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 highresolution Ledge x-ray absorption spectroscopy ͑XAS͒, 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 (5-30 Å)/metal (1-10 Å), 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 metal͑oxide͒ layer adjacent to an oxide͑metal͒ layer was partially oxidized͑reduced͒. 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.

Using x-ray magnetic circular dichroism we have detected the very interfacial spins that are responsible for the horizontal loop shift in three different exchange bias sandwiches, chosen because of their potential for device applications. The "pinned" uncompensated interfacial spins constitute only a fraction of a monolayer and do not rotate in an external magnetic field since they are tightly locked to the antiferromagnetic lattice. A simple extension of the Meiklejohn and Bean model is proposed to quantitatively account for the exchange bias fields in the three studied systems from the experimentally determined number of pinned moments and their sizes.