Magnetic configurations in Co-based nanowires explored by electron holography and micromagnetic calculations

David Fernando Reyes Vasquez 1
1 CEMES-I3EM - Interférométrie, In situ et Instrumentation pour la Microscopie Electronique
CEMES - Centre d'élaboration de matériaux et d'études structurales
Abstract : Magnetic nanowires have raised significant interest in the last 15 years due to their potential use for spintronics. Technical achievements require a detailed description of the local magnetic states inside the nanowires at the remnant state. In this thesis, I performed quantitative and qualitative studies of the remnant magnetic states on magnetic nanowires by Electron Holography (EH) experiments and micromagnetic simulations. A detailed investigation was carried out on two types of nanowires: multilayered Co/Cu and diameter-modulated FeCoCu nanowires. Both systems were grown by template-based synthesis using electrodeposition process. The combination of local magnetic, structural and chemical characterizations obtained in a TEM with micromagnetic simulations brought a complete description of the systems. In the multilayered Co/Cu nanowires, I analysed how different factors such as the Co and Cu thicknesses or the Co crystal structure define the remnant magnetic configuration into isolated nanowires. After applying saturation fields along directions either parallel or perpendicular to the NW axis, I studied multilayered Co/Cu nanowires with the following relative Co/Cu thickness layers: 25nm/15nm, 25nm/45nm, 50nm/50nm, and 100nm/100nm. Three main remnant configurations were found: (i) antiparallel coupling between Co layers, (ii) mono-domain-like state and (iii) vortex state. In the Co(25 nm)/Cu(15 nm) nanowires, depending on the direction of the saturation field, the Co layers can present either an antiparallel coupling (perpendicular saturation field) or vortex coupling (parallel saturation field) with their core aligned parallel to the wire axis. However, 10% of the nanowires studied present a mono-domain-like state that remains for both parallel and perpendicular saturation fields. In the Co(50 nm)/Cu(50 nm) and Co(25 nm)/Cu(45 nm) nanowires, a larger Cu thickness separating the ferromagnetic layers reduces the magnetic interaction between neighbouring Co layers. The remnant state is hence formed by the combination of monodomain Co layers oriented perpendicularly to the wire axis and some tilted vortex states. Finally for the Co(100 nm)/Cu(100 nm) nanowires a monodomain-like state is found no matters the direction of the saturation field. All these magnetic configurations were determined and simulated using micromagnetic calculations until a quantitative agreement with experimental results has been obtained. I was able to explain the appearance and stability of these configurations according to the main magnetic parameters such as exchange, value and direction of the anisotropy and magnetization. The comparison between simulations and experimental results were used to precisely determine the value of these parameters. In the diameter-modulated cylindrical FeCoCu nanowires, a detailed description of the geometry-induced effect on the local spin configuration was performed. EH experiments seem to reveal that the wires present a remnant single-domain magnetic state with the spins longitudinally aligned. However, we found through micromagnetic simulations that such apparent single-domain state is strongly affected by the local variation of the diameter. The study of the leakage field and the demagnetizing field inside the nanowire highlighted the leading role of magnetic charges in modulated areas. The magnetization presents a more complicated structure than a simple alignment along the wire axis. Finally my results have led to a new interpretation of previous MFM experiments.
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David Fernando Reyes Vasquez. Magnetic configurations in Co-based nanowires explored by electron holography and micromagnetic calculations. Condensed Matter [cond-mat]. Université de Toulouse (Paul Sabatier), 2016. English. ⟨NNT : 2016TOU30356⟩. ⟨tel-01701926v2⟩

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