We investigate the influence of lateral variations of the second-order perpendicular anisotropy in thin films on the effective anisotropies required to represent this structure using a macrospin approximation. Second-order and fourth-order effective anisotropies are required for the macrospin approximation. In the case of quasi-static calculations, the fourth-order effective anisotropy is closely linked to deviations of the average magnetization angle from the field direction and lateral variations of the magnetization direction in the structure leading to dependence on the field strength and the lateral length scale of anisotropy variations of the effective anisotropies. We find that the field and lateral length scale dependence of the effective anisotropies extracted from simulations of the magnetization dynamics are profoundly different from those of the quasi-static simulations. This is caused by resonance localization that depends on the orientation of the external magnetic field.

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