Physical Review B 60(12), 12950 (1999)
Uncoupled grains in polycrystalline antiferromagnetic films can behave like superparamagnetic particles in that the order in each grain is stable on short time scales but unstable on long time scales due to thermal excitation over energy barriers. This instability persists when the grains are coupled to a ferromagnetic film in an exchange-bias multilayer. When the order in the grains is stable for all orientations of the ferromagnetic magnetization, the grains contribute to the unidirectional anisotropy that gives rise to the observed exchange-bias loop shift. Otherwise, they contribute to the high field rotational hysteresis found in rotational torque experiments, an isotropic field shift found in ferromagnetic resonance measurements, and other effects in experiments that are sensitive to different time scales. A simple model for the energy barriers predicts the temperature dependence of these effects.
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