Ab initio investigations of magnetic properties of ultrathin transition metal films on 4d substrates

摘要

In this thesis, we investigate the magnetic properties of 3d transition-metal monolayers on 4d transition-metal substrates by means of state of the art first-principles quantum theory. In contrast to previous investigations on noble metal substrates, the strong hybridization between 3d metals and the substrate is an additional parameter determining the properties. In order to reveal the underlying physics of these systems we study trends by performing systematic investigations across the transition-metal series. Case studies are presented for which Rh has been chosen as exemplary 4d substrate. We consider two substrates orientations, a square lattice provided by Rh(001) and a hexagonal lattice provided by Rh(111). We find, all 3d transition-metal (V, Cr, Mn, Fe, Co and Ni) monolayers deposited on the Rh substrate are magnetic and exhibit large local moments which follow Hund’s rule with a maximum magnetic moment for Mn of about 3.7 µB depending on the substrate orientation. The largest induced magnetic moment of about 0.46 µB is found for Rh atoms adjacent to the Co(001)-film. On Rh(001) we predict a ferromagnetic (FM) ground state for V, Co and Ni, while Cr, Mn and Fe monolayers favor a c(2 × 2) antiferromagnetic (AFM) state, a checkerboard arrangement of up and down magnetic moments. The magnetic anisotropy energies of these ultrathin magnetic films are calculated for the FM and the AFM states. With the exception of V and Cr, the easy axis of the magnetization is predicted to be in the film plane. With the exception of Fe, analogous results are obtained for the 3d-metal monolayers on Rh(111). For Fe on Rh(111) a novel magnetic ground state is predicted, a double-row-wise antiferromagnetic state along the [112] direction, a sequence of ferromagnetic double-rows of atoms, whose magnetic moments couple antiferromagetically from double row to double row. The magnetic structure can be understood as superposition of a left- and right-rotating flat spin spiral. In a second set of case studies the properties of an Fe monolayer deposited on varies hexagonally terminated hcp (0001) and fcc (111) surfaces of 4d-transition metals (Tc, Ru, Rh, to Pd) are presented. The magnetic state of Fe changes gradually from noncollinear 120° Néel state for Fe films on Tc, and Ru, to the double-row-wise antiferromagnetic state on Rh, to the ferromagnetic one on Pd and Ag. The noncollinear state is a result of antiferromagnetic intersite exchange interactions in combination with the triangular lattice provided by the hexagonal surface termination of the (111) surfaces. A similar systematic trend is observed for a Co monolayer on these substrate, but shifted towards ferromagnetism equivalent to one element in the periodic table. Also the magnetic properties of Co chains on stepped Rh(111) surfaces is investigated. It is shown that the easy axis of the magnetization changes from out-of-plane in case of a Co monolayer to in-plane for the atomic chain. The trends are explained on the basis of the Heisenberg model with exchange parameters whose sign and value change systematically as function of the band filling across the transition-metal series. The Heisenberg model was extended by a Stoner-like term to include the induced magnetization of the 4d substrate. The results are based on the density functional theory in the vector-spin-density formulation employing the spin-polarized local density and generalized gradient approximation. The self-consistent relativistic total energy and force calculations have been carried out with the full-potential linearized augmented plane wave (FLAPW) method in the film geometry. The concept of total-energy calculations with incommensurable spin-spirals of wave vectors along the high-symmetry lines in the two-dimensional Brillouin zone was applied to search for the magnetic ground states.