FE-simulation of fast switching behavior of granular nanoelements

Transient magnetization states during switching are investigated numerically in thin granular square shaped nanoelements (100/spl times/100 nm/sup 2/) with uniaxial (Co-hcp), cubic anisotropy (Co-fcc) and zero anisotropy (Ni/sub 80/Fe/sub 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 2.0 J/sub s//(/spl mu//sub 0//spl middot/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 0.02 J/sub s//(/spl mu//sub 0//spl middot/ns) and 0.1 ns and 245 kA/m (/spl alpha/=0.02) and 329 kA/m (/spl alpha/=1.0) for 2 J/sub s//(/spl mu//sub 0//spl middot/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 0.02 J/sub s//(/spl mu//sub 0//spl middot/ns) and 0.1 ns for 2 J/sub s//(/spl mu//sub 0//spl middot/ns)). The smallest switching fields are obtained in NiFe with zero anisotropy, i.e., <20 kA/m for 0.02 J/sub s//(/spl mu//sub 0//spl middot/ns) and <140 kA/m for 2 J/sub s//(/spl mu//sub 0//spl middot/ns). Precessional oscillation effects occurred in the (Ni/sub 80/Fe/sub 20/) square element for a small damping constant (/spl 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 (Ni/sub 80/Fe/sub 20/).