Simulations of magnetic reversal in polycrystalline exchange-bias bilayers exhibit two contributions to the enhanced coercivity found in exchange-bias systems, one due to inhomogeneous reversal and and the other to irreversible transitions in the antiferromagnetic grains. In the model considered, at zero temperature there are no irreversible transitions in the antiferromagnetic grains. Here, the coercivity is entirely due to inhomogeneous effects with energy lost in the ferromagnet due to a large number of small, local jumps in the magnetization. At finite temperatures, the order in the antiferromagnetic grains becomes unstable; the resulting transitions make an additional contribution to the coercivity. Additionally, the computed hysteresis loops are found to be asymmetric, and the loop shift is shown to differ from the grain-averaged unidirectional anisotropy. The model can be characterized in terms of a small number of dimensionless parameters, and its behavior has been determined for a range of values of these parameters.
Physical Review B (Condensed Matter and Materials Physics)
coercivity, exchange bias, hysteresis, magnetization reversal, superparamagnetism, thin film magnetism