FE-simulation of fast switching behavior of granular nanoelements

Transient magnetization states during switching are investigated numerically in thin granular square shaped nanoelements; (100 x 100 nm(2)) with uniaxial (Co-hep), cubic anisotropy (Co-fcc) and zero anisotropy (Ni-80 Fe-20) with 10 nm grain size and a thickness of 10 nm and taking into account a random orientation of the grains using a hybrid finite-element/boundary-element model. In-plane switching dynamics are calculated for external fields with a constant sweep rate of 0.02 and 2.0J(s)/(muo (.) ns). The switching time and critical field strongly depends on the Gilbert damping parameter and sweep rate. Largest switching times and fields were found for the Co thin film element with uniaxial anisotropy, such as 5 ns and 120 kA/m for 0.02J(s)/(muo (.) ns) and 0.1 ns and 245 kA/m (alpha = 0.02) and 329 kA/m (a = 1.0) for 2J(s)/(muo (.) ns), respectively. Depending on the sweep rate the fastest in-plane switching occurs in the granular cubic Co- and permalloy-thin film elements (<2 ns for 0.02J(s)/(muo (.) ns) and 0.1 ns for 2J(s)/(muo (.) ns)). The smallest switching fields are obtained in NiFe with zero anisotropy, i.e., <20 kA/m for 0.02J(s)/(muo (.) ns) and <140 kA/m for 2J(s)/(muo (.) ns). Precessional oscillation effects occurred in the (Ni80Fe20) square element for a small damping constant (alpha = 0.02). The transient magnetization states during reversal vary from nucleation and expansion of reversed domains (Co-hcp) to inhomogeneous rotation inside the nanoelements with cubic (Co-fcc) and zero anisotropy (Ni80Fe20).