Rotation of exchange anisotropy in biased Co/CoO bilayers

Magnetic field induced irreversible changes of the exchange anisotropy in Co/CoO bilayers were investigated. Cobalt films were grown by dc magnetron sputtering and then partially oxidized. They were then field cooled in 7000 Oe to 4.2 K to induce the exchange bias. A variable magnitude magnetic field was applied in the film plane at various angles with respect to the exchange bias direction. The effects of this variable magnetic field on the exchange coupling were studied by the reversible anisotropic magnetoresistance technique. Three qualitatively different behaviors of the exchange anisotropy direction were observed. The particular behavior was determined by the magnitude of the applied magnetic field and the rotation angle. A simple phenomenological model of the exchange anisotropy was developed to explain the experimental results, which allows for explicit quantitative separation of the unidirectional and the rotatable parts of the exchange anisotropy.