The role of disorder in the domain wall dynamics of magnetic nanostrips

We study the role of the disorder in the dynamics of the domain walls (DW) in nanostrips with in-plane magnetization. In contrast with previous works where the disorder is due to edge roughness, we consider the role of a random distribution of voids, thus simulating local changes of the magnetization saturation value. By making use of the high-speed computational capability of GPUs, and an ad hoc micromagnetic code, we compute the speed of DWs under both applied fields (up to 15 mT), and spin-polarized currents (up to 30 A/μm2), for four different void densities. Field and currents are applied for 20 ns. We also consider both adiabatic and non-adiabatic spin-torque effects (ξ parameter equal 0 and 0.04, respectively). For all the cases, we repeat the simulation for 50 realizations of the void distributions. No thermal effects are considered. While some results can be understood in the line of the models reported in the literature, some others are much more peculiar. For instance, we expect a lower value of the maximum DW speed. This actually occurs in the field driven case, but with a less dramatic drop at the Walker breakdown, due to the difficulty to nucleate an antivortex DW. When nucleated, it gets easily pinned, thus preventing its retrograde motion typical for disorder-free strips. In the case of current drive with non-adiabatic spin-transfer torque, the Walker breakdown current increases strongly with the void density. This results in an increased value of the maximum speed available. Another important consequence of the disorder is that at low fields/currents the depinning transition regions appear to be more rounded, resembling creep behavior. This can have important consequences in the interpretation of experimental data