Coercivity in exchange-bias bilayers - art. no. 064405

A model is presented for coercivity in polycrystalline exchange-bias bilaye rs. It includes two contributions for their enhanced coercivity, inhomogene ous 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 ma gnetic reversal for a range of values of these parameters. In these simulat ions, the first contribution to the coercivity arises from energy losses in the ferromagnet due to irreversible transitions over small, local energy b arriers 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 th e antiferromagnetic order in the grains. In the present model, the antiferr omagnetic order only becomes unstable at nonzero temperature, so that this contribution to the coercivity only occurs at nonzero temperatures. In addi tion to the coercivity, the computed hysteresis loops are found to be asymm etric, and the loop shift is shown to differ from the grain-averaged unidir ectional anisotropy.