Coercivity in exchange-bias bilayers

A model is presented for coercivity in polycrystalline exchange-bias bilayers. It includes two contributions for their enhanced coercivity, inhomogeneous reversal, and irreversible transitions in the antiferromagnetic grains. The model can be characterized in terms of a small number of dimensionless parameters, and its behavior has been determined through simulations of magnetic reversal for a range of values of these parameters. In these simulations, the first contribution to the coercivity arises from energy losses in the ferromagnet due to irreversible transitions over small, local energy barriers in the ferromagnetic film due to the inhomogeneous couplings to the antiferromagnet. This inhomogeneous reversal contributes to the coercivity at all temperatures. The second contribution to the coercivity arises from energy losses in the antiferromagnet due to irreversible transitions of the antiferromagnetic order in the grains. In the present model, the antiferromagnetic order only becomes unstable at nonzero temperature, so that this contribution to the coercivity only occurs at nonzero temperatures. In addition to the coercivity, the computed hysteresis loops are found to be asymmetric, and the loop shift is shown to differ from the grain-averaged unidirectional anisotropy.