UC San Diego

Magnetic materials are vital components of many existing and future applications, from data storage and spin-logic devices, to Terahertz sensors and artificial synapses from neuromorphing computing. Driven by the need to faster responses and high-density storage, the focus of this work is the modeling of thermal and optical excitation of magnetic materials by an external laser source.

Many models focus on the use of fields or current as the primary driving force behind the change in magnetization and the models, without taking in account the optical contribution of the light, which has been shown to produce changes in magnetizing on a faster timescale than the ones observed with the use of either current or field.

Moreover, granular media are usually modelled using simplistic finite difference approach or numerically intensive finite-elements approach to model every grain. These approach leads to either an unrealistic description of the media (finite-difference) or to an over-sampling of the nodes of the problem, increasing significantly the computational time required to run the simulations.

This dissertation improves upon the state of the art of micromagnetic modelling by introducing a Voronoi tessellation model to simulate realistic granular structures at elevated temperature for high anisotropy materials. This approach considers the geometry of the grains for compute the far-field contribution. This approach has been proven effective in modeling realistic media for heat assisted magnetic recording and perpendicular media in general. The model presented it also introduces in the dynamic of the magnetizing the optical contribution and helps to describes complex phenomena like the ultrafast-demagnetization and the helicity dependent optical reversal of magnetic material subjected to an external optical source.

While the model provides a qualitative interpretation of the experiments, additional data is required to evaluate the quantitative contribution of the optical excitation and the correctness of the thermal fluctuations.

This dissertation is structured as follow. In Chapter 1 and 2 introduce key concepts of magnetism and the basic Micromagnetic model that is going to be used as the basis for the numerical simulations.

In Chapter 3 I introduce a micromagnetic code based on Voronoi tessellation and the non-uniform fast Fourier transform (NUFFT) method. The code is capable of efficiently and accurately simulating magnetization dynamics in large and structurally complex granular systems, such as multilayer granular media used for perpendicular magnetic recording, bit patterned media, granular nanowires, and read heads. In these systems the granular microstructure and distributions in grain and interface properties play an important role in device performance. The presented Voronoi simulator allows comprehensive studies to be performed as it accounts for the detailed granular microstructure and distributions that characterize true systems. Simulation time is greatly reduced by a non-uniform fast Fourier transform algorithm and implementation on graphics processing units (GPUs). Simulations of conventional magnetic recording, heat-assisted magnetization reversal, domain wall dynamics in granular nanowires, and particulate tape recording are presented.

Chapter 4 explores the generation of electrical field signals in the terahertz frequency (THz) range using antiferromagnets (AFM). Using micromagnetic model simulation, we investigated a potential mechanism for laser-induced THz signals in the AFM phase of FeRh/Pt bilayer films. In the simulations, the FeRh film is modelled as two Fe-sublattices coupled via intra-lattice exchange field and subjected to a sub-picosecond thermal pulse. Our simulation exposes a partial canting between the magnetizations of two Fe-sublattices, within the first picosecond after the excitation. This short-lived state relaxes abruptly into the initial AFM phase, injecting a spin current into the Pt layer via spin pumping, which will eventually be converted into charge current oscillating at THz frequency.

Chapters 5 and 6 discuss the phenomenon of all-optical switching of the magnetization in magnetic nanostructures. While all-optical switching of the magnetization in magnetic nanostructures by femtosecond circularly polarized laser pulses without an external magnetic field has been demonstrated in several systems, a theoretical framework that convincingly explain the phenomenon is still missing. In Chapter 5 we propose a theory where the ferromagnetic macrospin ground state is optically excited by the circularly polarized light to a spin reversed state, which is then “Coulomb collapsed” to the magnetization reversed ground state. The optical excitation lasts for the duration of the laser pulse and the system relaxes at a fast rate due to the electron-electron interaction. In Chapter 6 we present a computational model based on this theory. We construct a three-state model for the magnetization dynamics, the Landau- Lifshitz-Lambda (LLL) model, as an ensemble of such states to account for the temperature effects. After the optical excitation lapses, the LLL model reduces to the Landau-Lifshitz-Bloch formulation, allowing to consider the magnetization relaxation dynamics at elevated temperatures. We apply the theory to simulate AOS in FePt films subject to multiple femtosecond circular polarized laser pulses. The simulation results demonstrate characteristic AOS features and agree with recent experiments.

Chapter 7 identifies problems in the performance of the established stochastic model in micromagnetics in modeling the thermal fluctuations of longitudinal and transverse components of the magnetization at elevated temperature. A correct estimation of the thermal fluctuation is paramount to develop multiscale atomistic-micromagnetic models. The chapter presents a consistent solution for the diffusion coefficients that satisfy the corresponding Fokker-Planck equation and provide the correct equilibrium magnetization at elevated temperature.