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Résumé
Skyrmions are topologically protected nanometer-sized magnetic vortices that have sparked interest for future spintronics applications aiming at high-density, high-speed and low-power consuming magnetic memory devices. One type of skyrmions appears in non-centrosymmetric materials with spin-orbit coupling. Bulk materials such as FeGe and CoZnMn have been in the focus of recent research as they host skyrmion lattices near or above room temperature. The interest in thin-film materials has immensely increased as they promise better control of skyrmions. The main challenges are the synthesis of skyrmion-hosting materials with high critical temperatures and the compatibility with Si-based thin-film technologies. Currently most of the studied thin films do not exhibit the relevant properties. It has been argued that strain and defects related to the lattice mismatch inhibit the skyrmion phase.
Here we present novel approaches to synthesize skyrmion-hosting thin films based on FeGe and CoZnMn: 1) the crystallization of an amorphous thin film on a vdW substrate (commercial graphene), 2) van der Waals (vdW) growth on graphene as a substrate to achieve a strain- and defect-free growth of the overlayer.
Sputtered FeGe layers were obtained on graphene on oxidized silicon substrates and annealed via rapid thermal (RTA) and flash lamp annealing (FLA). The goal was to use these annealing techniques to guide the material to the temperature/composition window of the cubic phase. For this, we varied the thin film thickness, Fe:Ge ratio, pre-heating and annealing temperature. The thin film temperature during the FLA was studied through heat transfer simulations. The experiments uncovered the long annealing time via RTA with a capping layer leads to 4 types of morphology: islands, discs, individual crystals and crystals on discs. The sub-second annealing via FLA led to the polycrystalline thin film formation. The obtained materials contain hexagonal and cubic FeGe, Fe-rich phases, pure Ge and Fe oxides analysed by Raman spectroscopy and X-ray diffraction. We provide Raman vibrational modes and the symmetry description from first-principles calculations for FeGex with x=1, 1.5 and 2 to fast and locally characterize thin films and microstructures.
Molecular beam epitaxy was used to grow CoZnMn thin films of different stoichiometries on graphene on oxidized Si. It has been found in bulk crystals that the critical temperature can be systematically engineered beyond room temperature by tuning the composition. We report on the structural, compositional and magnetic properties of Co10-xZn10-yZnx+y (0
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