Temperature dependence of exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

We describe a simple model for the dependence of tthe exchange bias and rel ated effects that result from coupling a ferromagnetic thin film to a polyc rystalline antiferromagnetic film. In this model, an important source of te mperature dependence comes from thermal instabilities of the antiferromagne tic state in the antiferromagnetic grains, much as occurs in superparamagne tic grains. At low enough temperatures, the antiferromagnetic state in each grain is stable as the ferromagnetic magnetization is rotated and the mode l predicts the unidirectional anisotropy that gives rise to the observed ex change-bias loop shift. At higher temperatures, the antiferromagnetic state remains stable on short time scales, but on longer time scales, becomes un stable due to thermal excitations over energy barriers. For these temperatu res, the model predicts the high field rotational hystersis found in rotati onal torque experiments and the isotropic field shift found in ferromagneti c resonance measurements. [S0163-1829(99)01842-1].