Probing local magnetic states in the van der Waals ferromagnet Fe₄GeTe₂ by a vector-field magnetic force microscope

In this study, we systematically investigate the magnetic domain structure of Fe₄GeTe₂ single crystals, employing a cryogenic vector-field magnetic force microscope to probe its temperature and magnetic field dependencies. The material undergoes a spin-reversal transition at around 110 K, leading to a gradual magnetization reorientation from in-plane to out-of-plane as temperature decreases. Our observations reveal a complex domain structure featuring striped shapes enclosed by wavy closed loops, exhibiting limited sensitivity to temperature variations without an external magnetic field. When subject to an out-of-plane magnetic field, the domain structure transforms into micrometric elongated striped shapes, gradually evolving into a more irregular pattern. Conversely, in in-plane measurements, the system displays an inhomogeneous distribution of micrometric bubble-like domains, progressively interconnecting into striped shapes. These distinctive responses are attributed to the interplay between small uniaxial and shape anisotropies within the material. Our findings contribute to a deeper understanding of the magnetic domain dynamics in van der Waals ferromagnetic materials, laying the groundwork for further investigations and potential applications in electronic devices.