Magnetic microscopy of domains and domain walls in ferromagnetic nanotubes

Abstract : This thesis explores magnetic configurations, namely magnetic domains and domain walls (DWs) in single ferromagnetic metallic nanotubes (diameters 50–400nm) by means of magnetic microscopies and numerical modelling. The work benefited from international collaboration with TU Darmstadt (synthesis), synchrotrons Elettra and Soleil as well as CNRS CEMES (magnetic imaging). Using electrochemical methods and nanoporous templates, we could fabricate Ni, NiCo, CoNiB, and NiFeB nanotubes as well as Ni wire-tube elements. For the imaging, we relied mainly on X-ray Magnetic Circular Dichroism coupled with PhotoEmission Electron Microscopy (XMCD-PEEM). We show the first experimental microscopy images of magnetic domains in metallic nanotubes. In long (30µm) CoNiB tubes, we observed many azimuthal (flux-closure) magnetic domains separated by very narrow DWs. This is in contrast with literature and recent experiments where only axial domains appeared for similar geometry. By annealing, changing the chemical composition or just decreasing the nanotube diameter we could obtain also the axial domains. Therefore, tubes are versatile as magnetic domains can be prepared almost à la carte. We demonstrated switching of both axial and azimuthal domains with a magnetic field. We imaged also multilayered tubes – an equivalent of multilayered flat films that form a basic brick of current spintronics. We obtained two magnetic layers (exchange-) decoupled by an oxide spacer. Such a first-of-its-kind structure and its imaging paves the way towards 3D spintronics and magnetism based on vertical arrays of tubes.
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Submitted on : Monday, March 26, 2018 - 5:09:06 PM
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Michal Staňo. Magnetic microscopy of domains and domain walls in ferromagnetic nanotubes. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes, 2017. English. ⟨NNT : 2017GREAY063⟩. ⟨tel-01743840⟩

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