Limitations of the macrospin approximation of materials with inhomogeneous perpendicular anisotropy

2020 
Experimental investigations of the perpendicular anisotropy in thin films that are interpreted using a macrospin model often require the inclusion of a higher-order anisotropy contribution. However, recent ferromagnetic resonance experiments on [Co/Ni] N multilayers indicate that the macrospin model cannot explain the full angular dependence in this system. Using micromagnetic simulations of a system with lateral variations of the second-order perpendicular uniaxial anisotropy, we show that while the macrospin model is able to capture the average properties of the system along high symmetry orientations by including a higher-order anisotropy, the model fails to reproduce the full angle dependence. Our studies provide another indication that higher-order anisotropies may not be intrinsic to these systems but instead may be caused by inhomogeneities.
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