Motion of magnetic domain walls in engineered cylindrical nanowires

Orateur : Soutenance de Thèse de Alexis WARTELLE

The thesis is concerned with the dynamics of ferromagnetic domain walls in cylindrical nanowires. These were electrodeposited by colleagues of mine into na noporous alumina templates with a tailored pore geometry. The materials are soft FeNi or CoNi alloys ; the diameters range from 150 nm to 250-300 nm, with a typical length of 30 µm. My work first comprised experimental developments of sample holders and high-frequency electronics towards field-induced domain wall motion. The latter I investigated with X-ray Magnetic Circular Dichroism coupled to transmission PhotoEmission Electron Microscopy (XMCD-PEEM), which allows to monitor the internal domain wall configuration before and after displacement. Although mobility measurements could not be carried out, my experiments have evidenced transformations between domain wall types that had not been predicted in simulations. Since one type of domain wall, the Bloch point wall, contains a topological defect (the Bloch point itself), this unexpected behaviour questions the sometimes argued protection attributed to topologically non-trivial textures. Aside from such electrodeposited samples, I have also studied an upright core-shell nanowire grown by colleagues with Focused-Electron-Beam-Induced Deposition. This nanostructure featured a cobalt core and a platinum shell. Its magnetic configuration was investigated with transmission XMCD-PEEM as well. Contrary to the aforementioned horizontally-lying wires, the core-shell sample was vertical with no diameter modulations. On the other hand, the geometry featured bends engineered to favour domain wall pinning. In this novel imaging configuration, the challenge was to recover as much of the nanowire’s magnetic state as possible. I was able to demonstrate the presence of at least one domain wall.

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