Abstract: Two-dimensional (2D) magnetic van der Waals (vdW) materials exhibit intriguing physics and are potential candidates for spintronic devices owing to their atomically-layered heterostructures and intrinsic spin interaction. Thus, it is of importance to understand the magnetic domain behavior in vdW materials under external stimuli in order to control them by tuning the governing energy terms.  Fe₃GeTe₂ is one of the promising magnetic vdW materials as it is an itinerant ferromagnet in the 2D-limit.

In this talk, we will report the direct observation of magnetic domain behavior of Fe₃GeTe₂ in response to temperature, magnetic and electrical fields by using in-situ cryo Lorentz transmission electron microscopy. Two distinct magnetic domains, namely Néel stripe domains and skyrmions, are formed after following the zero-field and field cooling protocols respectively.  We will present how the magnetic field influences significantly the local spin texture of magnetic domains in Fe₃GeTe₂. In addition, we demonstrate a loss of skyrmion lattice order and an evolution of the skyrmion liquid phase during cooling as a result of significant change in the skyrmion size distribution. The lattice order is restored during heating, leading to a thermal hysteresis. Finally, we will also discuss the effect of the oxidation behavior of Fe3GeT2 on the resulting magnetic domains. Our work provides an insight into understanding and controlling the magnetic domains in vdW materials for their future applications.