États chaotiques et effets de modulation dans des oscillateurs à transfert de spin

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 vor-
tex 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 in-
formation processing.