Public Thesis Defense of Simon De Wergifosse - IMCN
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Nonlinear Dynamics of Magnetic Vortices in Spin-Torque Nano-Oscillators
Friday November 14th, 2025, 3:00pm - Room SUD 11 - Place Croix du Sud, 1348 Louvain-la-Neuve
In this thesis, the strongly nonlinear dynamics of magnetic vortices in spin-torque nano-oscillators are investigated. These devices, which are nanoscale structures able to convert direct currents to alternating voltages, rely on magnetic tunnel junction stacks for which the free layer exhibits a nonuniform vortex magnetization state. When subjected to a sufficient injected current, sustained gyrotropic precessions of the vortex core may occur, generating an oscillatory output signal through magnetoresistance effects. These dynamics are typically captured using two main approaches. Micromagnetic simulations, though delivering high accuracy, demand substantial computational resources. In contrast, the Thiele equation provides analytical solutions by modeling the vortex core as a quasiparticle, yet relying on assumptions and linearization. In this work, we enhance the Thiele equation by improving the magnetostatic and Zeeman terms, the latter arising from the Ampère-Oersted field (AOF) induced by the current. These modifications reveal a clear chirality-dependent splitting of the dynamics, validated through micromagnetic simulations. While our model remains qualitative in the steady-state oscillating regime, we introduce a method to extract the vortex stiffness, linked to the potential magnetic energy, directly from micromagnetic data. This method highlights discrepancies between analytical expressions and simulations, notably due to deformations of the magnetic texture caused by the AOF. To refine our predictions, we then develop a quantitative model incorporating the vortex core position dependence of the gyrotropic and damping terms. Due to cumbersome analytical derivations, these terms are instead calibrated using steady-state simulation results. Our data-driven Thiele equation approach achieves up to eight orders of magnitude faster computation than micromagnetism, while maintaining equivalent accuracy. It precisely reproduces both transient and steady-state dynamics, even for arbitrary input waveforms. Furthermore, our findings reveal a unique dynamical regime for larger-diameter junctions, for which the vortex core is confined between two orbits due to successive double-polarity reversals, a behavior analogous to the leaky integrate-and-fire model of biological neurons. Finally, we examine a system consisting of a pair of oscillators in close proximity, focusing on the AOF influence rather than well-known synchronization through dipolar interactions. We derive an analytical expression for the associated Zeeman energy, confirmed by simulations. Lastly, we underscore the necessity of accounting for this field in theoretical models by displaying shifts in the vortex core equilibrium position and ferromagnetic resonance effects.
Jury members :
Prof. Flavio Abreu Araujo (UCLouvain) (Supervisor)
Prof. Evelyne van Ruymbeke (UCLouvain) (Presidente)
Prof. Benoit Hackens (UCLouvain) (Secretary)
Prof. Luc Piraux (UCLouvain)
Dr. Grégoire de Loubens (SPEC – CEA)
Dr. Philippe Talatchian (SPINTEC – CEA)
Pay attention : the public defense of Simon De Wergifosse will also take place in the form of a videoconference