Magnetization dynamics and energy dissipation in magnetic thin films

Magnetic thin films are layers of magnetic material ranging from ultrathin films, consisting of only a few atomic layers, up to few micrometers in thickness. The magnetization configurations of magnetic thin films display a wide variety of complex patterns, including bubbles domains, maze patterns and topological solitons or defects. Under the influence of e.g. external magnetic field, the magnetization exhibits rich dynamics, from avalanche-like domain wall jumps in Barkhausen noise to nucleation and annihilation of vortices and antivortices in in-plane anisotropy films. Magnetic thin films are researched extensively due to their unique properties and potential for nano- and microscale applications, such as magnetic memory devices and microelectromechanical systems (MEMS). When the magnetization of a magnet goes through a change due to e.g. domain wall motion in the aforementioned Barkhausen noise, the elementary magnetic moments of the system experience motion called Larmor precession, which slowly winds down as the magnetization relaxes into a new configuration. This relaxation results from couplings between magnetic, electric and phononic degrees of freedom, which transfer energy from the magnetic moments to the lattice, where the energy is then dissipated as heat. These magnetic losses are relevant in applications where there are alternating electromagnetic fields or components moving in such fields, such as transformers, electric motors and magnetic bearings. In this dissertation, we study magnetic losses in thin films using micromagnetic simulations, with emphasis on the magnetization dynamics and dissipation resulting from the motion of a magnetic thin film in an external field or relative to another film. Publication I focuses on the dynamics of topological defects and energy dissipation in a Permalloy thin film experiencing a relaxation from an initially random magnetization state. In publication II, we develop an extension capable of simulating moving thin films to an existing micromagnetic simulation code. Publications III and IV use the extension to investigate domain wall dynamics and the resulting losses in response to motion in thin films with perpendicular magnetic anisotropy. Publication III investigates the damping of high-frequency mechanical oscillation due to magnetic dynamics, while publication IV considers magnetic losses due to domain wall motion in films with disorder.