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anisotropy (PMA). In ultrathin films, skyrmions can exhibit sub-nanometer scale size [8][9][10][11] and move in response to an applied current with velocities exceeding 100 m s -1 [5] in a controllable [12,13] and reliable [13] way. Therefore, they promise great technological utility for racetracktype memories, [14] logic gates, [15] probabilistic computing, [16] and neuromorphic devices, [17] for which they have to be readily created and manipulated. Homochiral skyrmions can be stabilized by the Dzyaloshinkii-Moriya interaction (DMI) [18,19] in materials with strong spin-orbit coupling and broken inversion symmetry. Since asymmetric multilayer stacks of a ferromagnet and a heavy metal [5][6][7] possess DMI and can also exhibit large current-induced spin-orbit torques that can provide an efficient means to create and manipulate skyrmions, [20][21][22] these systems are now a central focus of current research. Magnetic skyrmions can exist as isolated topological excitations, [23,24] or as ordered arrays (hexagonal lattice) comprising the magnetic ground state, [2,5] depending on material and Magnetic skyrmions promise breakthroughs in future memory and computing devices due to their inherent stability and small size. Their creation and current driven motion have been recently observed at room temperature, but the key mechanisms of their formation are not yet well-understood. Here it is shown that in heavy metal/ferromagnet heterostructures, pulsed currents can drive morphological transitions between labyrinth-like, stripe-like, and skyrmionic states. Using high-resolution X-ray microscopy, the spin texture evolution with temperature and magnetic field is imaged and it is demonstrated that with transient Joule heating, topological charges can be injected into the system, driving it across the stripe-skyrmion boundary. The observations are explained through atomistic spin dynamic and micromagnetic simulations that reveal a crossover to a global skyrmionic ground state above a threshold magnetic field, which is found to decrease with increasing temperature. It is demonstrated how by tuning the phase stability, one can reliably generate skyrmions by short current pulses and stabilize them at zero field, providing new means to create and manipulate spin textures in engineered chiral ferromagnets. Magnetic SkyrmionsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe 5 GeTe 2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe 5 GeTe 2 , we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.