Magnetization dynamics on the nanoscale : From first principles to atomistic spin dynamics

Abstract: In this thesis first-principles methods, based on density functional theory, have been used to characterize a wide range of magnetic materials. Special emphasis has been put on pairwise magnetic interactions, such as Heisenberg exchange and Dzyaloshinskii-Moriya interactions, and also on in the Gilbert damping parameter. These parameters play a crucial role in determining the magnetization dynamics of the considered materials.Magnetic interaction parameters, has been calculated for several materials based on Co/Ni/Co heterostructures deposited on non-magnetic heavy metals where. The aim was to clarify how the composition of the underlayers affect the magnetic properties, in particular the Dzyaloshinskii-Moriya interactions. The DMI was found to be strongly dependent on the material of the underlayer, which is consistent with previous theoretical works. Such behaviour can be traced back to the change of the spin-orbit coupling with the material of the underlayer, as well as with the hybridization of the d- states of the magnetic system with the d- state of the non-magnetic substrate.First-principles calculations of the Gilbert damping parameter has been performed for several magnetic materials. Among them the full Heusler families, Co2FeZ, Co2MnZ with Z=(Al, Si, Ga, Ge). It was found that the first-principles methods, reproduce quite well the experimental trends, even though the obtained values are consistently smaller than the experimental measurements. A clear correlation between the Gilbert damping and the density of states at the Fermi energy was found, which is in agreement with previous works. In general as the density of states at the Fermi energy decreases, the damping decreases also.The parameters from first principles methods, have been used in conjunction with atomistic spin dynamics simulations, in order to study ultra-narrow domain walls. The domain wall motion of a monolayer of Fe on W(110) has been studied for a situation when the domain wall is driven via thermally generated spin waves from a thermal gradient. It was found that the ultra-narrow domain walls have an unexpected behaviour compared to wide domain walls in the continuum limit. This behaviour have been explained by the fact that for ultra-narrow domain walls the reflection of spin waves is not negligible.Furthermore, the dynamics of topologically protected structures, such as topological excitations in a kagome lattice and edge dislocations in FeGe has been studied. For the FeGe case, the description of the thermally driven dynamics of the edge dislocations, was found to be a possible explanation for the experimentally observed time dependence of the spiral wavelength. In the kagome lattice, it was also found that due to its topological properties, topological excitations can be created in it.

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