  Recent discoveries of intrinsic two-dimensional (2D) ferromagnetism in
insulating/semiconducting van der Waals (vdW) crystals open up new
possibilities for studying fundamental 2D magnetism and devices employing
localized spins. However, a vdW material that exhibits 2D itinerant magnetism
remains elusive. In fact, the synthesis of such single-crystal ferromagnetic
metals with strong perpendicular anisotropy at the atomically thin limit has
been a long-standing challenge. Here, we demonstrate that monolayer Fe3GeTe2 is
a robust 2D itinerant ferromagnet with strong out-of-plane anisotropy.
Layer-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism
for thicknesses less than 4 nm (five layers), accompanying a fast drop of the
Curie temperature from 207 K down to 130 K in the monolayer. For Fe3GeTe2
flakes thicker than ~15 nm, a peculiar magnetic behavior emerges within an
intermediate temperature range, which we show is due to the formation of
labyrinthine domain patterns. Our work introduces a novel atomically thin
ferromagnetic metal that could be useful for the study of controllable 2D
itinerant Ising ferromagnetism and for engineering spintronic vdW
heterostructures.
