| keyword(s) : films ultra-minces – Semi-conducteurs magnétiques dilués – Pt/Co/Pt – GaMnAs – Anisotropie perpendiculaire – Nano-structures magnétiques – Paroi de domaines – Renversement d'aimantation sous champ magnétique – Déplacement de paroi de domaines sous courant polarisé en spin – Micro-magnétisme – Microscopie magnéto-optique Kerr polaire |
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| From the field induced magnetisation reversal in a nano-disk to the current induced domain wall motion in a nano-track, a study by polar Kerr microscopy |
| The goal of this PhD was to study magnetisation dynamics in two perpendicularly magnetised nano-systems; metallic Pt/Co(5 Å)/Pt and the diluted ferromagnetic semiconductor GaMnAs. Magnetisation reversal, under the action of a magnetic field in nano-dots or the spin polarised current induced domain wall motion in a track, has been studied using polar Kerr magneto-optics. Nano-structuration was realised by classical eching to obtain GaMnAs nano-dots and tracks and by an elegant process in the ultrathin cobalt layer case: helium ion irradiation which yields nano-dots in a planar and paramagnetic environment. For these two systems, with different ferromagnetism and anisotropy origins, the lateral confinement implies a transition from a multi-mode reversal to a single-mode reversal. When the nano-dot diameter is close to the exchange length, as is the case in the smallest GaMnAs nano-dot the Néel-Brown reversal allows to interpret experimental results. However, it is shown that the single magnetisation reversal of a 130 nm diameter Pt/Co/Pt nano-disk can not be coherent and individual investigations on these nano-disks as well as a micromagnetic model show that the magnetisation reversal is governed by the nucleation of a droplet at the periphery. In contrast with the metallic case, spin-polarised current induced domain wall motion is easy in the GaMnAs. Measurements performed at a constant effective temperature show that it is mandatory to consider a non-adiabatic spin transfer mechanism. |
| english keyword(s) : Ultra-thin films – Diluted magnetic semiconductors – Pt/Co/Pt – GaMnAs – Perpendicular anisotropy – Magnetic nano-structures – Domain wall – Field induced magnetisation reversal – Spin polarised current induced domain wall motion – micro-magnetism – Polar magneto-optical Kerr microscopy |
