Statics and fast dynamics of nanomagnets with vortex structure

Within the framework of the Landau-Lifshitz-Gilbert equation, using permalloy parameters, we study the statics and dynamics of flat circular magnetic nano-structures with an in-plane magnetic vortex configuration, putting particular emphasis on the vorticity of the magnetic state and on the (perpendicular) polarisation of the vortex centre, which may be shifted with respect to the centre of the circle. These binary degrees of freedom can in principle be used to manipulate two independent bits of information. Studying switching processes induced by in-plane and out-of-plane field pulses, we find that it is possible to switch the vorticity of the magnetic dot on a time scale of 40 ps in strong enough and short enough perpendicular external field pulses (Bzext∼0.5 T, duration ∼40 ps). But for realistically small values of the Gilbert damping, only the vorticity can be switched this fast, and it turns out that it is better to dismiss the centre of the circle totally, concentrating on flat 'nano-rings' with an inner radius R1 and an outer radius R2. On these 'nano-rings' the vortex state is more stable, and with respect to the switching of the vorticity, these structures have similar properties as circular dots