The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 with magnetic ordering up to room temperature is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of centrosymmetry. These skyrmions can be observed even in the absence of external fields. To understand the complex magnetic order in Fe5GeTe2, we compare macroscopic magnetometry characterization results with microscopic density functional theory and spin-model calculations. Our results show that even up to room temperature, topological spin structures can be stabilized in centrosymmetric van der Waals magnets.

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Thermally induced skyrmion dynamics as well as skyrmion pinning effects in thin films have attracted significant interest. While pinning poses challenges in deterministic skyrmion devices and slows down skyrmion diffusion, for applications in non-conventional computing, both pinning of an appropriate strength and skyrmion diffusion speed are key. Here, we employ periodic field excitations to realize an increase of the skyrmion diffusion by more than two orders of magnitude. Amplifying the excitation, we report a drastic reduction of the effective skyrmion pinning and observe a transition from pinning-dominated diffusive hopping to dynamics approaching free diffusion. By tailoring the field oscillation frequency and amplitude, we demonstrate continuous tuning of the effective pinning and skyrmion dynamics, which is a key asset and enabler for nonconventional computing applications. We find that the periodic excitations additionally allow us to stabilize skyrmions at different sizes for field values that are inaccessible in static systems, opening up new approaches to ultra-fast skyrmion motion by transiently exciting moving skyrmions.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

The role of the crystal lattice, temperature and magnetic field for the spin structure formation in the 2D van der Waals magnet Fe5GeTe2 is a key open question. Using Lorentz transmission electron microscopy, we experimentally observe topological spin structures up to room temperature in the metastable pre-cooling and stable post-cooling phase of Fe5GeTe2. Over wide temperature and field ranges, skyrmionic magnetic bubbles form without preferred chirality, which is indicative of a centrosymmetric crystal structure. In the pre-cooling phase, these bubbles are observable even without the application of an external field, while in the post-cooling phase, a transformation from bubble domains to stripe

No abstract