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The methodology for adapting a standard micromagnetic code to run on graphics processing units (GPUs) and exploit the potential for parallel calculations of this platform is discussed. GPMagnet, a general purpose finite-difference GPU-based micromagnetic tool, is used as an example. Speed-up factors of two orders of magnitude can be achieved with GPMagnet with respect to a serial code. This allows for running extensive simulations, nearly inaccessible with a standard micromagnetic solver, at reasonable computational times.

Micromagnetic simulations are used to describe the domain-wall dynamics along thin ferromagnetic strips driven by short pulses of magnetic field or electrical current with sinusoidal shape. For perfect strips without pinning centers, the net displacement of the domain wall is proportional both to the amplitude and the duration of the field pulse. A similar behavior is observed under current pulses if some nonadiabatic corrections are taken into account. On the contrary, the net displacement is null in the perfect adiabatic case. The domain-wall dynamics driven by these pulses is also characterized for strips which contain a single constriction, which acts as pinning site for the wall. The results reveal that an initially pinned domain wall can be eventually expelled far away from the constriction but if the maximum displacement does not surpass a given threshold the domain-wall experiences an attractive force which pushes it again toward the initial pinning site. Finally, the analysis of the domain wall jumps between two pinning sites is carried out both at zero and at room temperature for several separations between them. The simulations point out that the jumps can be achieved by means of short field or current pulses in the subnanosecond regime, an observation which could find application for a fast and easily controlled writing mechanism for future magnetic random access memory devices based on a pinned domain wall.

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A theoretical analysis on domain wall dynamics along thin ferromagnetic strips with high perpendicular magnetocrystalline anisotropy driven by both magnetic fields and spin-polarized currents is reported. The domain wall depinning from a constriction is characterized both at zero and at room temperature for different values of the nonadiabatic parameter. The results indicate that engineering of pinning sites in thin strips of high perpendicular anisotropy provides an efficient pathway to achieve both high stability against thermal fluctuations and low current-induced domain wall depinning and, therefore, it can find application on designing memory devices driven by static currents.

The current-driven domain wall motion along two exchange-coupled ferromagnetic layers with perpendicular anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single ferromagnetic layer. Our results, where only the lower ferromagnetic layer is subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin Hall effect, indicate that the domain walls can be synchronously driven in the presence of a strong interlayer exchange coupling, and that the velocity is significantly enhanced due to the antiferromagnetic exchange coupling as compared with the single-layer case. On the contrary, when the coupling is of ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic characterization of the current-driven motion in these multilayers, both in the absence and in the presence of longitudinal fields, and the results are explained based on a one-dimensional model. The interfacial DzyaloshinskiiMoriya interaction, only necessary in this lower layer, gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled walls by a dragging mechanism, without significant tilting of the domain wall plane. Finally, the domain wall dynamics along curved strips is also evaluated.These results indicate that the antiferromagnetic coupling between the ferromagnetic layers mitigates the tilting of the walls, which suggest these systems to achieve efficient and highlypacked displacement of trains of walls for spintronics devices. A study, taking into account defects and thermal fluctuations, allows to analyze the validity range of these claims.2

Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic alloys and, by extension, other antiferromagnetically coupled systems as some multilayers, are here presented. The analysis has been made by means of micromagnetic simulations that consider these systems as constituted by two subsystems coupled in terms of an additional exchange interlacing them. Both subsystems differ in their respective gyromagnetic ratios and temperature dependence. Other interactions, as for example anisotropic exchange or spin-orbit torques, can be accounted for differently within each subsystem according to the physical structure. Micromagnetic simulations are also endorsed by means of a collective coordinates model which, in contrast with some previous approaches to these antiferromagnetically coupled systems, based on effective parameters, also considers them as formed by two coupled subsystems with experimentally definite parameters. Both simulations and the collective model reinforce the angular moment compensation argument as accountable for the linear increase with current of domain wall velocities in these alloys at a certain temperature or composition. Importantly, the proposed approach by means of two coupled subsystems permits to infer relevant results in the development of future experimental setups that are unattainable by means of effective models.