Atomistic spin dynamics – from first-principles calculations to large scale simulations

Orateur : Anders BERGMAN (Uppsala University, Sweden)

With the evolution of experimental methods towards higher and higher resolution, both spatial and temporal, the interest in the dynamics of materials on an atomistic level is steadily increasing. This is especially true for magnetic materials were methods like spin polarized scanning tunneling microscopy allow for resolution of individual magnetic moments and magneto optical pump-probe experiments can study magnetization dynamics on a sub-picosecond time scale. With this progress comes an increased interest in theoretical methods to complement, and at best forego, experiments. Here we will present the method of first-principles atomistic spin dynamics (ASD) simulations where electronic structure calculations are used to parameterize reliable spin Hamiltonians from which the magnetization dynamics can be simulated by solving the Landau-Lifshitz-Gilbert equations of motion. In addition we will present recent applications of this method to a variety of systems. We have found an unexpected behavior of the magnetization switching in small supported Co clusters. For this system, contrary to expected, it is actually possible to decrease the magnetization reversal times for the clusters by decreasing the applied magnetic field. We have been able to correlate this switching behaviour with the low symmetry of the magnetocrystalline anisotropy present in the clusters. We will also show results from our study on the dynamics of topologically stable, particle-like magnetic excitations, Skyrmions. Spin dynamics simulations show how these Skyrmions can form in thin films when applying magnetic fields and how these magnetic structures decay in the presence of high temperature. Furthermore we will show how individual Skyrmions can be manipulated with spin polarized currents. Finally we will discuss planned projects and possible directions for future research. This includes studies on how excited spin waves can be used as a mean to manipulate magnetic structures and also schemes on how to combine atomistic spin dynamics with molecular dynamics methods, which can open up for studies of a large number of materials where the lattice and magnetization are strongly coupled.

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