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978-3-8439-1032-3, Reihe Physik
Martin P. Magiera
Magnetic friction in driven Heisenberg-like systems
183 Seiten, Dissertation Universität Duisburg-Essen (2013), Softcover, A5
Magnetic friction – a contribution to energy dissipation which originates solely from magnetic interactions. Although tribology, the science of interacting surfaces in relative motion, and magnetism represent very important and active research fields, only few studies take into account magnetic contributions to friction. A systematic study of energy dissipation which occurs because of the relative motion of two magnetic systems is still lacking. The difficulty in studying such processes is that the systems are always – per definition – in a non-equilibrium state. The theoretical description of such a non-equilibrium state is a very difficult task, thus in most cases computer simulations are required.
In this thesis the friction force which occurs when a magnetic tip is moved relative to a ferromagnetic substrate is studied using atomic computer simulations. The interaction between substrate and tip is of solely magnetic nature. After the introduction of an atomistic model, fundamental questions are answered. A general explanation for the occurrence of a linear dependence of the friction force on the relative velocity between tip and substrate is presented. Moreover, the deviations from the behavior in the static limit are explained. Spin waves, the weak excitations in a ferromagnet, provide the right explanation for the velocity dependence of the friction force occurring in a spin chain. They give rise to a non-linear dependence of the friction force on the relative velocity, when the velocity exceeds a threshold.
Magnetic vortices turn out to be responsible for a substantial increase of the friction force in two dimensional systems. A detailed study of the occurrence of vortex states will be provided, as well as the influence of the moving tip on the vortex size. The stability of the vortex state with respect to thermal fluctuations is studied, as well as the temperature dependence of the friction force. Finally a correlation between the friction force and the specific heat of the system is detected in two-dimensional systems, and recovered in three-dimensional systems.