Reconstruction and magnetic structure of ultrathin γ-Fe films on Cu(111)

We report ab initio investigations of the magnetic structure and reconstruction of ultrathin films of ?-Fe on Cu(111) substrates. For this system striking differences in the magnetic properties of films produced by thermal deposition and pulse-laser deposition have been reported [J. Shen et al. J. Phys.: Condens. Matter 15, R1 (2003)]. We find that unlike for ?-Fe films on Cu(001), the geometrical and magnetic structures of Fe/Cu(111) films cannot be explained in terms of simple ferromagnetic or layered antiferromagnetic structures stacked along the surface normal. Instead, we find that the (bi)layer antiferromagnetic structures oriented along [001] representing the magnetic ground state of Fe/Cu(001) films with four or more monolayers also determine the magnetic structure of Fe/Cu(111), even in the monolayer limit. In both cases, the stability of the bilayer antiferromagnetic structure is related to the stability of this type of magnetic ordering for nearly cubic or tetragonally distorted bulk ?-Fe. However, a ferromagnetic state of films with 2-4 ML (monolayers) is only about 6 meV/Fe atom higher in energy. Ferromagnetic ordering is coupled to strong monoclinic shear distortions in the bulk as well as in thin Fe/Cu(001) films. In Fe/Cu(111), however, the close packing in the layer leads to a quite large elastic energy for the large lateral displacements coupled to the ferromagnetic ordering so that the antiferromagnetic structure is marginally more stable from 1 to 4 ML. The coupling between magnetic structure and surface reconstruction could open a way for an experimental verification of our predictions.