Fast switching behaviour of nanoscopic NiFe- and Co-elements

Three-dimensional micromagnetic simulations were performed to study the magnetisation reversal processes of granular nanoelements using a hybrid finite element/boundary element model. Transient magnetisation states during switching are investigated numerically in granular, thin Ni80Fe20 and Co square shaped nanoelements (100×100 nm2) with 10 nm grain size and a thickness of 10 nm and taking into account a random orientation of the grains. Switching dynamics are calculated for external fields between 80 and 280 kA/m, which were uniformly applied after a rise time of 0.05 and 0.1 ns, respectively, and in comparison for a 10 GHz rotational field. Reversal in the unidirectional field proceeds by the nucleation and propagation of end domains towards the centre of the granular thin film elements. The formation of a vortex magnetisation structure leads to an increase of the switching time in the granular Co element. The switching time strongly depends on the Gilbert damping parameter α. Small values of α(⩽0.1) lead to shorter switching times at small field strength values (h<0.5 Js/μ0). Reversal in rotational fields involves inhomogeneous rotation of the end domains towards the rotational field direction leading to partial flux-closure structures and therefore facilitating the switching by reduced switching times. The micromagnetic study reveals that switching partly occurs already during the rise time of the unidirectionally oriented external field. Shorter switching times are obtained by the application of a half cycle of a 10 GHz rotational field (tsw=0.05 ns). Precessional oscillation effects after switching off the external field which occurred in the Ni80Fe20 square element, were suppressed by the uniaxial anisotropy of the randomly oriented Co grains. Taking into account thermally activated processes the micromagnetic simulations show that the switching time was reduced by less than 10% at T=300 K for Co and Hext.=140 kA/m (h=0.1 Js/μ0).