Electronic and magnetic structure of Mn-Ni alloys in two and three dimensions

The electronic and magnetic structure of face-centred tetragonal Mn-Ni comp ounds and of structurally related c(2x2) Mn-Ni alloy films on Ni(001) subst rates has been investigated using ab initio local-spin-density calculations including generalized gradient corrections. For the intermetallic compound a layered antiferromagnetic high-spin ground state with Mn moments of +/-3 .2 mu(B) (LSDA) and +/-3.4 mu(B) (GGCs) and non-magnetic Ni atoms is predic ted, in good agreement with the estimates from magnetic neutron scattering. Calculations of the magnetic anisotropy energy show that the moments are a ligned in the Mn planes, parallel to the edges of the unit cell. An alloy w ith unit cell dimensions of the Mn-Ni planes strained to match the lattice parameter of the Ni(001) substrate has a very similar magnetic structure, a lbeit with slightly reduced moments. A monolayer c(2x2) Mn-Ni alloy shows h igh-spin ferromagnetic order (mu(Mn) = 3.9 mu(B)). Films with two and more monolayers show antiferromagnetic interlayer coupling, with a quite pronoun ced enhancement of the surface moments over those in the deeper layers. The predicted antiferromagnetic ordering of the films emphasizes the similarit y of the atomic and magnetic structures of the two-dimensional films with t hat of the three-dimensional compounds and contradicts recent claims as to a ferromagnetic order of the films not only in the monolayer limit. Possibl e explanations of this discrepancy are discussed.