Micromagnetic study of magnetic domain wall motionthermal effects and spin torques

Zusammenfassung

Magnetic domain walls represent the boundary between two differently aligned magnetic domains. In ferromagnetic nanostrips, they can be efficiently displaced by electrical current as a consequence of the spin transfer torque mechanism. This effect inspired a number of potential applications for logic and memory devices, which are based on the reliable control and manipulation of domain walls. These applications represent a promising alternative to CMOS based devices, which are reaching their limit in scalability. At the same time, the achievement of low-energy devices, based on domain walls, could have important consequences on the energy consumption of the Information and Communication Technology sector and, consequently, on carbon emissions and climate change. Apart from that, domain wall dynamics is also interesting from a fundamental point of view since domain walls can be displaced by several means such as electrical current, thermal gradients, spin waves etc. Furthermore, advances in material deposition opened the possibility of creating magnetic ultrathin films with a thickness of few angstroms, where the interfacial interactions with the neighbouring layers play a significant role and they can give rise to new interesting effects such as perpendicular magnetic anisotropy or the presence of the Dzyaloshinskii-Moriya interaction. Moreover, these systems are promising for the study of chiral and topological magnetism due to the presence of topologically protected patterns such as Skyrmions or chiral domain walls. In this thesis we analyse two aspects of domain wall motion in ferromagnetic nanostructures by means of micromagnetic simulations. In the first part we analyse the influence of Joule heating and thermal gradients on domain wall dynamics. It is well known that, apart from the spin transfer torque, electrical currents also produce heating as a consequence of Joule effect. Thus, on the one hand it is important to analyse the effect of Joule heating in order to establish the real contribution of the spin transfer torque. On the other hand, Joule heating and thermal gradients can be also used to efficiently displace magnetic domain walls, although the theory behind this effect is still under debate. These studies were performed with a novel micromagnetic software which couples heat and magnetization dynamics. In the second part, we analyse the domain wall dynamics in ultrathin systems with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction. This part includes a collaboration with an experimental group from the University of Leeds (UK) and it is devoted to the fitting of the experimental data for the field and current driven domain wall motion. In particular, we analyse the role of disorder in these systems, which represents a critical issue towards the realization of domain wall devices. Finally, we analyse the dependence of the domain wall depinning field with the damping parameter.