Advanced modelling of domain wall dynamics for spintronic devices

Abstract

[EN]The study of magnetism at the nanoscale has important applications in everyday life. As an example, the vast majority of all data is currently stored in magnetic hard drives, while magnetic sensors are ubiquitous in automotive applications and in the internet of things (IoT) technology. The interaction between the spins of conducting electrons and those of the localized magnetic moments of a ferromagnet is at the base of a new field of studies called Spintronics, whose technological perspectives are to overcome the existing semiconductor technology in terms of power saving, endurance and reliability. Domain wall propagation is the mechanism through which a magnetic system changes its state when its equilibrium is perturbed via an external action and its dynamics is well described using the micromagnetic formalism. Micromagnetic nu- merical simulations are a proficient tool that links experimental observations and theoretical predictions, leading the way in the theoretical understanding of magne- tization dynamics and domain wall motion. In chapter 1 we lay down the fundamental physical concepts of the micromag- netic description of magnetism and present the principal analytical tools used through- out the the rest of the work. Chapter 2 is dedicated to the description of the nu- merical solver used in this work: a custom micromagnetic code based on C++ and CUDA programming languages, developed within the group. Subsequently, we fo- cus on two different problems, making use of the descriptive and predictive power of micromagnetic simulations respectively. In chapter 3, we investigate the effect of disorder on field driven domain wall dynamics in CoFeB thin films. Such structures are the building blocks of MRAM memories and the understanding of field driven domain wall dynamics is a key step towards the optimization of device functionality. Exploiting the ability of mi- cromagnetic simulations to reproduce certain disorder features realistically, we get insight into magnetization dynamics taking place at a scale below instruments reso- lution, uncovering important connections between domain wall dynamics and ma- terial disorder features. Disorder triggers internal domain wall dynamics that gen- erates a faster energy dissipation and a faster domain wall propagation in the high- field regime. In chapter 4, we propose a new spintronic device based on the emission of spin waves by means of the controlled rotation of a domain wall in a ferromagnetic wire. The transmission of information via the periodic oscillatory perturbation of mag- netization, called spin wave, offers new perspectives in the design of low power sensors and emitters. We design a system with realistic material characteristics and investigate how the self oscillatory state of a domain wall, induced by the injection of a charge current, can emit a spin wave signal at frequency of tens of GHz that directly depends on the injected current intensity.