  Magnetic van der Waals (vdW) materials, including ferromagnets (FM) and
antiferromagnets (AFM), have given access to the investigation of magnetism in
two-dimensional (2D) limit and attracted broad interests recently. However,
most of them are semiconducting or insulating and the vdW itinerant magnets,
especially vdW itinerant AFM, are very rare. Here, we studied the anomalous
Hall effect of a vdW itinerant magnet Fe$_5$GeTe$_2$ (F5GT) with various
thicknesses down to 6.8 nm (two unit cells). Despite the robust ferromagnetic
ground state in thin-layer F5GT, however, we show that the electron doping
implemented by a protonic gate can eventually induce a magnetic phase
transition from FM to AFM. Realization of an antiferromagnetic phase in F5GT
highlights its promising applications in high-temperature antiferromagnetic vdW
devices and heterostructures.
