Continuum micromagnetic modeling of antiferromagnetically exchange-coupled multilayers

The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterized by antiferromagnetic interface coupling. The soft and hard phases have uniaxial anisotropy with a common direction, along which the external field is applied. The model assumes a nonuniform rotation of the magnetization, and it also considers an interface coupling that is reduced with respect to the strong-limit case. It is found that the deviation of the magnetization from the saturated antiparallel state can occur at two distinct nucleation fields, which mainly involve only one of the two phases. Moreover, in the case of a reduced interface coupling, the saturated parallel state becomes accessible and thus the nucleation from this state is taken into account. The critical equations have been deduced, allowing us to identify the conditions for which the nucleation regime changes from reversible to irreversible as a function of the intrinsic and extrinsic parameters. The results of the model, applied to a typical soft/hard system with planar anisotropy, have been summarized in suitable phase diagrams, as a function of the layer thicknesses and of the strength of the interface coupling. The analysis, supported by additional static and dynamic micromagnetic simulations, shows the occurrence of a rich variety of magnetization curves. As a secondary result we have found that, in the parallel nucleation process, the influence of the interface coupling extends inside the two phases to distances appreciably larger than the corresponding Bloch wall widths.