Structure and magnetism of γ-Fe overlayers on face-centered-cubic Co(001) substrates

Abstract

The structure and magnetism of ultrathin \ensuremath{\gamma}-Fe films on face-centered-cubic (fcc) Co(001) substrates stabilized by epitaxial growth on Cu(001) has been investigated using the ab initio local-spin-density technique including generalized gradient corrections. Two cases have been considered: Co films that are sufficiently thick such that the lateral lattice constant of fcc Co can relax to its equilibrium value, and ultrathin Co films where the epitaxial constraint imposes the lateral lattice constant of the Cu substrate. For films with up to five monolayers of Fe all possible magnetic structures have been investigated, and for six monolayers of Fe, selected magnetic configurations have been studied. For the energetically most favorable cases a full multilayer relaxation has been performed. This yields a magnetic phase diagram with a ferromagnetic ground state for one or two monolayers of Fe, and a bilayer antiferromagnetic ground state for the thicker films. The ferromagnetism of films with three and four monolayers of Fe observed experimentally can be explained by considering the influence of the epitaxial constraint at the Co/Cu interface and/or interfacial mixing at the Fe/Co boundary. The magnetic and structural phase diagram of the films is discussed in relation to the magnetic phase of tetragonally strained bulk \ensuremath{\gamma}-Fe.