Ab initioinvestigation of the magnetism of tetragonal Mn: Bulk, surface, ultrathin films, and multilayers

The magnetic properties of the tetragonally distorted γ and δ phases of Mn stabilized by epitaxial growth on metallic surfaces are still subject of a lively debate, but so far no consistent and generally accepted picture has emerged. We have performed detailed and comprehensive investigations of the geometric and magnetic properties of tetragonal Mn in the bulk, at a (100) surface, in ultrathin Mn∕Fe(100) films, and in Mn∕Fe multilayers using ab initio spin-density functional techniques. The cubic structures of both γ (face-centered cubic) and δ (body-centered cubic) Mn are unstable against tetragonal distortion. Whereas for δ-Mn a structure contracted along the c axis and with c(2×2) in-plane (100) antiferromagnetism (AFM) is the unique ground state, for γ-Mn a contracted tetragonal $(c∕a=0.945)$ phase with a layered (100) AFM, and an expanded $(c∕a=1.048)$ phase with in-plane (100) AFM are energetically almost degenerate. In addition we find that the antiferromagnetic phases of both δ and γ Mn are susceptible to long-period helical modulations. At the (100) surface, the Mn moments are strongly enhanced, and the strong antiferromagnetic coupling between the high surface moments favors in-plane AFM in the surface layer even on top of the tetragonally compressed near-fcc phase stabilizing layered antiferromagnetism in the bulk. A similar result is found for ultrathin Mn∕Fe(100) films with up to six monolayers. A strong ferromagnetic Mn∕Fe coupling at the interface favors layered antiferromagnetism in the deeper layers, but the in-plane antiferromagnetic structure in the top layer is stable in any case. For the thinnest Mn films we have also examined noncollinear magnetic structures and found evidence for a perpendicular coupling between the Mn surface layer and the deeper layers of film and substrate. The strong ferromagnetic Mn∕Fe interface coupling also determines the properties of Fe∕Mn multilayers. The ferromagnetic interface coupling is not perturbed by Fe∕Mn intermixing and stabilizes a layered antiferromagnetism in the Mn spacer. We discuss our results in the light of the available experimental data and of previous theoretical calculations.