  Identifying intrinsic noncollinear magnetic order in monolayer van der Waals
(vdW) crystals is highly desirable for understanding the delicate magnetic
interactions at reduced spatial constraints and miniaturized spintronic
applications, but remains elusive in experiments. Here, we achieved
spin-resolved imaging of helimagnetism at atomic scale in monolayer NiI2
crystals, that were grown on graphene-covered SiC(0001) substrate, using
spin-polarized scanning tunneling microscopy. Our experiments identify the
existence of a spin spiral state with canted plane in monolayer NiI2. The spin
modulation Q vector of the spin spiral is determined as (0.2203, 0, 0), which
is different from its bulk value or its in-plane projection, but agrees well
with our first principles calculations. The spin spiral surprisingly indicates
collective spin switching behavior under magnetic field, whose origin is
ascribed to the incommensurability between the spin spiral and the crystal
lattice. Our work unambiguously identifies the helimagnetic state in monolayer
NiI2, paving the way for illuminating its expected type-II multiferroic order
and developing spintronic devices based on vdW magnets.
