Ab initio investigations of magnetic properties of ultrathin transition ...

Abstract
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 substrate
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 doublerow-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 leftand
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