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Soutenance de thèse

Lundi 30 Juin à 10h45,
Salle des séminaires, Bât. A

Oratrice : Sandrine Da Col
"Parois magnétiques dans les nanofils cylindriques / Magnetic domain walls in cylindrical nanowires"

Abstract

The underlying physics of magnetic domain wall motion is currently arousing a strong interest, enhanced by the possibilities of applications into magnetic memories.
Domain walls are mostly studied in ultrathin nanostrips made by lithography.
In contrast, a cylindrical geometry would imply different micromagnetic configurations due to the existence of a third dimension beyond the characteristic lengths of magnetism (a few tens of nm). Hence, domain walls dynamical behaviors would stand out and the use of cylindrical systems could notably be a possible solution to the speed limitation observed hitherto in nanostrips.
Their elaboration process includes the fabrication of nanoporous template followed by the electrolytic filling of the pores. It leads to nanowires with high aspect ratio, narrow distribution in diameter, self-organized in dense arrays, which are three requirements for the realization of a 3D racetrack memory, i.e. the Holy Grail of magnetic memories.
In spite of their undeniable interest, so far only very few domain walls studies have been conducted on such cylindrical systems .
This thesis therefore intends to contribute to the subject. Part of the thesis was devoted to the development of steps of the fabrication process : reduction of membrane porosity, longitudinal modulation of the pore diameter, electrodeposition of a magnetic alloy. These geometrical and structural adjustments of the nanowires have been used to study several facets of domain walls in nanowires. In the first place, an experimental way to reduce the magnetostatic interactions that could disturb domain wall propagation in dense arrays of nanowires was proposed.
Its efficency was demonstrated through hysteresis loops of dense arrays, on the domain wall nucleation that occurs at nanowires extremities, triggering magnetization reversal.
Other inter- and intra-wire pinning mechanisms were then evidenced by analyzing initial magnetization curves measured after the controlled nucleation of domain walls.
The observations made on individual nanowires by magnetic force microscopy (MFM) reveal propagation fields of a few milliteslas, which opens the way to dynamical studies in such systems. At last, the observation of domain wall internal structure by X-ray magnetic circular dichroism in photoemission electron microscopy (XMCD-PEEM) evidenced the two types of domain walls theoretically and numerically predicted in the literature, for which very different mobilities are expected.

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