History-dependent Magnetic and Morphological Phases in the 2D van der Waals Magnets Fe5GeTe2 and Fe3GeTe2

Recently, the combination of 2-dimensional (2D) magnetism with the field of spintronics, i.e. the manipulation of magnetic states with electric currents, has started to gain much traction in modern solid-state physics due to many prospective applications. Observations of magnetic skyrmions in the 2D itinerant ferromagnet Fe3GeTe2 have been reported, opening further possibilities for technological implementation. However, the stability of the different magnetic states and morphological phases in FexGeTe2 remains an unresolved issue. In this work, we utilize real-space imaging to determine thickness-dependent magnetic phase diagrams of exfoliated Fe3GeTe2 and Fe5GeTe2 films. Our findings show besides complex, history-dependent magnetization states also that changes in the crystalline structure significantly alter the magnetic behavior, tuning the available in- and out-of-plane components. Ultimately, the choice of material and a proper nucleation mechanism result in the stabilization of a variety of (meta-) stable magnetic configurations, including skyrmions, at zero field, and at a single temperature. These findings open novel perspectives for designing van der Waal heterostructure-based devices incorporating topological spin textures.