We report the direct observation of a theoretically predicted magnetic ground state in a monolayer Fe on Rh(111), which is referred to as an up-up-down-down ($\uparrow \uparrow \downarrow \downarrow$) double-row-wise antiferromagnetic spin structure, using spin-polarized scanning tunneling microscopy. This exotic phase, which exists in three orientational domains, is revealed by experiments with magnetic probe tips performed in external magnetic fields. It is shown that a hitherto unconsidered four-spin–three-site beyond-Heisenberg interaction distinctly contributes to the spin coupling of atoms with $S\ge 1$ spins. The observation of the $\uparrow \uparrow \downarrow \downarrow$ order substantiates the presence of higher-order, in particular, three-site interactions, in thin magnetic films of itinerant magnets.

• Received 10 November 2017
• Revised 6 March 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.207202