Cylindrical magnetic nanowires are a prototypical system for exploring the physics of three-dimensional magnetization dynamics, and are also proposed as building blocks for three-dimensional storage devices. Among other curvature-induced effects, they can host a unique type of magnetic domain wall (DW) known as Bloch-Point-Wall (BPW), whose topology allows for ultra-fast DW motion, thanks to largely delaying the Walker instability threshold. Recent pioneering work in the host group has shown experimental BPW motion under spin-transfer-torque with velocities of several hundred of m/s, but with stochastic pinning. To achieve a better control of DW motion, this thesis investigates engineered nanowires, composed of Permalloy segments separated by magnetic chemical modulations as controlled pinning sites. Individual nanowires were electrically contacted to enable the injection of electrical pulses in the nanosecond range. X-ray magnetic imaging allowed the three-dimensional magnetic characterization, combined with micromagnetic simulations. This provided a comprehensive picture of curling magnetization at the chemical modulation location, and its coupling to the BPW azimuthal magnetization. Both can be addressed by the Œrsted field associated with an electric current. We have found that depending on its amplitude and duration, it can cause circulation switching, DW transformations or long-distance motion. We have identified the underlying mechanism of these phenomena, which relies on the interplay between volume topological objects (Bloch points) and surface objects (vortex-antivortex pairs). In addition, we provide 50 ps time-resolved experiments of both modulation and BPW circulation switching that allow a direct bridge to the micromagnetic simulations. Finally, ultra-fast DW motion above 1 km/s from one modulation site to the next was achieved, driven by the combined effects of STT, Œrsted field and Joule heating, which cannot be disentangled experimentally.

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https://www.spintec.fr/phd-defense-current-induced-motion-of-magnetic-domain-walls-in-cylindrical-nanowires-with-chemical-modulations/

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