Abstract
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 (up arrow up arrow down arrow down arrow) 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 >= 1 spins. The observation of the up arrow up arrow down arrow down arrow order substantiates the presence of higher-order, in particular, three-site interactions, in thin magnetic films of itinerant magnets.