Temperature dependence of exchange bias in polycrystalline ferromagnet-antiferromagnet bilayers

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Abstract
We describe a simple model for the temperature dependence of the exchange bias and related effects that result from coupling a ferromagnetic thin film to a polycrystalline antiferromagnetic film. In this model, an important source of temperature dependence comes from thermal instabilities of the antiferromagnetic state in the antiferromagnetic grains, much as occurs in superparamagnetic grains. At low enough temperatures, the antiferromagnetic state in each grain is stable as the ferromagnetic magnetization is rotated and the model predicts the unidirectional anisotropy that gives rise to the observed exchange-bias loop shift. At higher temperatures, the antiferromagnetic state remains stable on short time scales, but on longer time scales, becomes unstable due to thermal excitations over energy barriers. For these temperatures, the model predicts the high field rotational hysteresis found in rotational torque experiments and the isotropic field shift found in ferromagnetic resonance measurements.