The dynamic response of a 80 nm x 400 nm x 25 nm Co film with and without polycrystalline grains is calculated. The numerical method combines a finite element scheme for space discretization with a preconditioned backward differentiation method for the time integration of the LLG equation. The use of proper preconditioning techniques for the backward differentiation formulae increases its efficiency by a factor 40. The speed up factor compared to the Adams method is more than three order of magnitude. In the polycrystalline thin film the nucleation of reversed domains occurs throughout the sample at grain boundaries and sharp edges. For the single crystalline film reversal starts at particle ends. Surface roughness does not significantly change the reversal mode but reduces the switching time by 40%.
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