Accelerated 3D numerical scheme for the evaluation of hysteresis in ferromagnetic grains

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Micromagnetic simulations on GPU, a case study : vortex core switching by high-frequency magnetic fields

Since magnetic vortex cores have two ground states, they are candidates for digital memory bits in future magnetic random access memory (MRAM) devices. Vortex core switching can be induced by exciting the gyrotropic eigenmode, e.g., by applying cyclic magnetic fields with typically a sub-gigahertz frequency. However, recent studies reveal that other modes exist that can be excited at higher frequencies, but still lead to switching with relatively small field amplitudes. Here, we perform a full scan of the frequency/amplitude parameter space to explore such excitation modes. The enormous amount of simulations can only be performed in an acceptable time span when the micromagnetic (CPU) simulations are drastically accelerated. To this aim, we developed MUMAX, a GPU-based software tool that speeds up micromagnetic simulations with about two orders of magnitude compared to standard CPU micromagnetic tools. By exploiting MUMAX's numerical power we were able to explore new switching opportunities at moderate field amplitudes in the frequency range between 5 and 12 GHz

Influence of disorder on vortex domain wall mobility in magnetic nanowires

A large amount of future spintronic devices is based on the control of the static and dynamic properties of magnetic domain walls in magnetic nanowires. For these applications, understanding the domain wall mobility under the action of spin polarized currents is of paramount importance. Numerous studies describe the spin-current driven domain wall motion in nanowires with ideal material properties, while only some authors take into account the influence of the nanowire edge roughness [1]. In this contribution we numerically investigate the influence of distributed disorder on the vortex domain wall mobility in Permalloy nanowires. To this aim, we use the GPU based micromagnetic software package MuMax[2] to simulate the propagation of vortex domain walls in nanowires with cross sectional dimensions of 400x10 nm². We apply spin polarized currents acting on the domain wall by means of the Spin Transfer Torque (STT) mechanism, considering a system with perfect adiabaticity (β=0) and with non-adiabatic STT contributions (β=α and β=2α, α is the Gilbert damping). As in [3], the disorder is simulated as a random distribution of 3.125x3.125nm² sized voids. For each current value, average domain wall velocities are computed considering 25 different realisations of the disorder. We find that even very small disorder concentrations have a huge impact on the domain wall mobility. In the non-adiabatic case (β=2α), the domain wall velocity is largely suppressed below the Walker breakdown since the disorder is able to pin the vortex structure hindering the formation of the transverse domain wall, characteristic to the movement in this current region. In the adiabatic case (β=0), the intrinsic depinning threshold is largely reduced. Even very small disorder densities disable the domain wall to internally balance the Landau-Lifshitz-Gilbert torques with the STT torques, resulting in a non-zero domain wall speed. At low currents, the disorder pins the domain wall structure