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978-3-8439-1651-6, Reihe Physik
Magnetic Atom Manipulation and Spin-dependent Atomic Friction Investigated by Spin-polarized Scanning Tunneling Microscopy and Monte Carlo Simulations
116 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5
In this thesis, I present a combined experimental and theoretical investigation of the spin-dependent frictional phenomena occurring when a magnetic adatom is moved over a magnetic surface by means of a spin-polarized scanning tunneling microscope (SP-STM) tip.
In the experiments, a single Co adatom was moved over the spin spiral magnetic ground state of the monolayer Mn on W(110). The manipulation imaging technique was applied, in which an area of the substrate is scanned while the Co adatom is trapped in the tip potential and closely follows the tip during the scan. When a magnetic tip is used, the resulting manipulation image shows a magnetic superstructure that is intimately tied to the magnetic structure of the surface. An initial analysis of the manipulation image indicates that the lateral threshold forces to move the adatom depend on the magnetic configuration of the adsorption sites. The measurements raise the question, if magnetic interactions are responsible for the modified behavior of the adatom, and, by extension, if the spin degree of freedom has to be considered in frictional phenomena.
A theoretical framework based on Monte Carlo simulations of a classical Heisenberg model was developed to study the dynamics of the adatom during the manipulation. In the simulations, the substrate and the tip interact with the adatom via Morse potentials and direct exchange interactions. The Metropolis algorithm is applied to relax the position and spin of the adatom in the combined potential of the substrate and the moving tip. The tunnel current is calculated within the simulations in order to reproduce the manipulation images measured experimentally, and the simulated manipulation images show a good agreement with the experiments. Traces of the adatom's position, spin, and energy during the manipulation are analyzed and confirm the spin-dependence of the lateral threshold forces when a magnetic tip is used. Furthermore, the simulations reveal that the friction force, defined in terms of energy dissipation, also shows a distinct dependence on the magnetic interactions between the adatom, the substrate, and the tip.
The presented results show that the spin degree of freedom and short-range magnetic interactions, such as the direct exchange interaction, have to be considered in frictional phenomena on the atomic scale, and may lead to further progress in the development of an advanced microscopic theory of friction.