Noncollinear magnetic structures of FeMn ultrathin films on Cu(001)

We investigate, by means of ab initio spinor-density-functional calculations, the magnetic structures of one and two monolayers of MnFe ordered alloys on the Cu(001) surface. For comparison, we also examine the FeMn compound with L10 structure, as bulk reference limit of the FeMn films. In the limit of the ultrathin films considered, the 3Q antiferromagnetic ground state of the FeMn bulk is unstable. A strongly non-collinear planar ferro- or ferri-magnetic spin structure is established, instead, as ground state. It is characterised by mainly parallel alignments of the Fe spins and Mn-Fe spins with orientations close to perpendicular. Depending on the Mn/Fe chemical order and coordination, the Fe-Mn exchange coupling is either antiferromagnetic or ferromagnetic biased. The trends of the spin structure and Fe-Mn exchange coupling with different chemical orders and thicknesses of the films are discussed based on the calculated Fe and Mn local density of states. In particular, the behaviour of the Fe-Mn exchange coupling is found to be controlled mainly by the position of the Fermi energy within the Fe-3d minority-spin local density of states.