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We present an experimental study of vortex dynamics in magnetic nanocontacts based on pseudo spin valves comprising the ${\mathrm{Co}}_{2}\mathrm{MnGe}$ Heusler compound. The films were grown by molecular beam epitaxy, where precise stoichiometry control and tailored stacking order allowed us to define the bottom ferromagnetic layer as the reference layer, with minimal coupling between the free and reference layers. 20-nm diameter nanocontacts were fabricated using a nanoindentation technique, leading to self-sustained gyration of the vortex generated by spin-transfer torques above a certain current threshold. By combining frequency- and time-domain measurements, we show that different types of spin-transfer induced dynamics related to different modes associated to the magnetic vortex configuration can be observed, such as mode hopping, mode coexistence, and mode extinction appearing in addition to the usual gyration mode.

Vortex nano-oscillators are spintronic devices in which electric currents drive the steady state gyration of magnetic states called vortices with spin-transfer torques. They represent a class of spin-torque nano-oscillators, which have potential applications in rf communications, field generation, and neuro-inspired computing. A specific feature of vortex-based systems is the reversal of the vortex core polarity, which can lead to nonlinear effects such as self-modulated states and chaotic dynamics. Understanding how such states are influenced by external signals is important for both fundamental studies and technological applications.In this thesis, I examined experimentally and theoretically how the dynamics of vortex oscillators in the nanocontact geometry respond to external current modulation. The samples studied were fabricated using a nano-indentation method on a variety of material stacks, such as pseudo spin valves based on transition metal and Heusler alloys. Through time- and frequency-domain analysis, I show that nontrivial modulation effects can appear depending on the oscillation regime, where processes such as fractional synchronization, modulation of the core reversal processes, and transitions between regimes are observed. Heusler-based devices exhibit additional phenomena like mode hopping and possibly coupled vortex dynamics, which results in more complex spectra. Through micromagnetics simulations, I demonstrate that a key parameter is how the vortex orbits change under modulation, which determines whether phase locking is possible. Hysteretic effects due to changes in the domain structure of the device are also brought to light. These results suggest new ways to utilize vortex oscillators for information processing.; Les nano-oscillateurs à vortex sont des systèmes spintroniques dans lequel des courants électriques entrainent la giration par couple de transfert de spin d’une structure magnétique que l’on appelle vortex. Il s’agit d’un type de nano-oscillateur à transfert de spin permettant des applications dans les communications radiofréquences, la génération de champ magnétique et le calcul neuro-inspiré. Une propriété spécifique des systèmes à base de vortex est le renversement de la polarité du cœur de vortex, qui peut mener à des effets non-linéaires tels que le chaos. Comprendre comment de tels états sont influencés par des signaux extérieurs est important à la fois d’un point de vue fondamental et pour des applications technologiques.Dans cette thèse, j’ai examiné expérimentalement et théoriquement comment la dynamique des oscillateurs à vortex dans une géométrie à nanocontact répond à un signal externe. Les échantillons étudiés ont été fabriqué en utilisant une méthode de nano-indentation sur différents empilements, tels qu’une pseudo-vanne de spin à base de permalloy ou d’alliage de Heusler. Par des analyses fréquentielles et temporelles, j’ai montré que des effets de modulation non-triviaux se produisent selon le régime d’oscillation, tels que de la synchronisation fractionnaire, la modulation du renversement de cœur et des transitions entre régimes. Les échantillons à base d’alliage de Heusler font preuve de phénomènes supplémentaires tels que des battements entre modes résultant en un spectre plus complexe. À travers des simulations micromagnétiques, j’ai démontré qu’un paramètre clé est comment l’orbite du vortex change sous la modulation, et qui détermine si le verrouillage de phase est possible. Des effets d’hystéresis dus à un changement de la structure de l’échantillon sont aussi démontrés. Ces résultats suggèrent de nouvelles manières d’utiliser les oscillateurs à vortex pour le traitement du signal et de l’information.

We present an experimental study of spin-torque driven vortex self-oscillations in magnetic nanocontacts. We find that above a certain threshold in applied currents, the vortex gyration around the nanocontact is modulated by relaxation oscillations, which involve periodic reversals of the vortex core. This modulation leads to the appearance of commensurate but also more interestingly here, incommensurate states, which are characterized by devil's staircases in the modulation frequency. We use frequency- and time-domain measurements together with advanced time-series analyses to provide experimental evidence of chaos in incommensurate states of vortex oscillations, in agreement with theoretical predictions., Comment: 5 pages, 4 figures

We have conducted experiments to probe how the dynamics of nanocontact vortex oscillators can be modulated by an external signal. We explore the phase-locking properties in both the commensurate and chaotic regimes, where chaos appears to impede phase-locking while a more standard behavior is seen in the commensurate phase. These different regimes correspond to how the periodicity of the vortex core reversal relates to the frequency of core gyration around the nanocontact; a commensurate phase appears when the reversal rate is an integer fraction of the gyration frequency, while a chaotic state appears when this ratio is irrational. External modulation where the power spectral density exhibits rich features, appears due to the modulation between the external source frequency, gyration frequency, and core reversal frequency. We explain these features with first- or second-order modulation between the three frequencies. Phase-locking is also visible between the external source frequency and internal vortex modes (gyration and core reversal modes)., Comment: 11 pages, 7 figures

Harnessing chaos or intrinsic nonlinear behaviours from dynamical systems is a promising avenue for the development of unconventional information processing technologies. However, the exploitation of such features in spintronic devices has not been attempted despite the many theoretical and experimental evidence of nonlinear behaviour of the magnetization dynamics in nanomagnetic systems. Here, we propose a first step in that direction by unveiling and characterizing the patterns and symbolic dynamics originating from the nonlinear chaotic time-resolved electrical signals generated experimentally by a nanocontact vortex oscillator (NCVO). We use advanced filtering methods to dissociate nonlinear deterministic patterns from thermal fluctuations and show that the emergence of chaos results in the unpredictable alternation of simple oscillatory patterns controlled by the NCVO's core-polarity switching. With phase-space reconstruction techniques, we perform a symbolic analysis of the time series to assess the level of complexity and entropy generated in the chaotic regime. We find that at the centre of its incommensurate region, it can exhibit maximal entropy and complexity. This suggests that NCVOs are promising nonlinear nanoscale source of entropy that could be harnessed for information processing., Comment: 16 pages, 10 figures