Three-dimensional spin structure on a two-dimensional lattice: Mn/Cu(111)

Based on first-principles vector spin-density total-energy calculations of the magnetic and electronic structure of Cr and Mn transition-metal monolay ers on the triangular lattice of a (111) oriented Cu surface, we propose fo r Mn a three-dimensional noncollinear spin structure on a two-dimensional t riangular lattice as magnetic ground state. This new spin structure is a mu ltiple spin-density wave of three row-wise antiferromagnetic spin states an d comes about due to magnetic interactions beyond the nearest neighbors and due to higher order spin interactions (i.e., four spin). The magnetic grou nd state of Cr is a coplanar noncollinear periodic 120 degrees Neel structu re.