  Complex magnetic order arises due to the competition of different
interactions between the magnetic moments. Recently, there has been an
increased interest in such states not only to unravel the fundamental physics
involved, but also with regards to applications exploiting their unique
interplay with moving electrons. Whereas it is the Dzyaloshinskii-Moriya
interaction (DMI) that has attracted much attention because of its nature to
induce non-collinear magnetic order including magnetic-field stabilized
skyrmions, it is the frustration of exchange interactions that can drive
magnetic order down to the nano-scale. On top of that, interactions between
multiple spins can stabilize two-dimensional magnetic textures as zero-field
ground states, known as multi-Q states. Here, we introduce a two-dimensional
itinerant magnet with various competing atomic-scale magnetic phases. Using
spin-polarized scanning tunneling microscopy we observe several zero-field
uniaxial or hexagonal nano-scale magnetic states. First-principles calculations
together with an atomistic spin model reveal that these states are stabilized
by the interplay of frustrated exchange and higher-order interactions while the
DMI is weak. Unexpectedly, it is found that not only non-collinear magnetic
states arise, but that higher-order interactions can also lead to collinear
nano-scale multi-Q states.
