Manipulation of topological spin configuration via tailoring thickness in van der Waals ferromagnetic Fe5(-x)GeTe(2)

Topological spin textures with versatile configurations exhibit fascinating physical behavior as high-efficiency information units. Recent observations of nontrivial spin textures in two-dimensional (2D) van der Waals (vdW) ferromagnets have shed light on their functionality as a paradigm for spintronic devices. Here, the spin configuration of single-crystal 2D vdW Fe5-xGeTe2 is investigated by Lorentz transmission electron microscopy, where the conversion from Bloch-type bubbles to Neel-type skyrmions can be observed by tuning sample thickness. Remarkably, high-density Neel-type skyrmions can be obtained via field-cooling manipulation. We further demonstrate the underlying mechanism by varying the sample thickness and Dzyaloshinskii-Moriya interaction (DMI) parameters in a series of micromagnetic simulations, manifesting the presence of DMI at the Fe5-xGeTe2 surface by first-principles calculation. Our findings disclose the rich variety of topological spin textures in 2D vdW Fe5-xGeTe2, possessing great promise for future nonvolatile memories based on spin topology.