  Intrinsic antiferromagnetism in van der Waals (vdW) monolayer (ML) crystals
enriches the understanding regarding two-dimensional (2D) magnetic orders and
holds special virtues over ferromagnetism in spintronic applications. However,
the studies on intrinsic antiferromagnetism are sparse, owing to the lack of
net magnetisation. In this study, by combining spin-polarised scanning
tunnelling microscopy and first-principles calculations, we investigate the
magnetism of vdW ML CrTe2, which has been successfully grown through molecular
beam epitaxy. Surprisingly, we observe a stable antiferromagnetic (AFM) order
at the atomic scale in the ML crystal, whose bulk is a strong ferromagnet, and
correlate its imaged zigzag spin texture with the atomic lattice structure. The
AFM order exhibits an intriguing noncollinear spin-flop transition under
magnetic fields, consistent with its calculated moderate magnetic anisotropy.
The findings of this study demonstrate the intricacy of 2D vdW magnetic
materials and pave the way for their in-depth studies.
