| Magnetization dynamics in various ferromagnetic thin film samples is investigated using time-resolved magneto-optical Kerr effect microscopy. The results have sub-picosecond time resolution and sub-micrometer spatial resolution, and are successfully compared with micromagnetic simulations. Related theoretical models and calculations are also discussed.; There are two types of spatial inhomogeneity that influence the observed magnetization dynamics. In the first project, the magnetic sample is a uniform, continuous thin film, but the excitation magnetic pulse is highly localized within several micrometers (in one dimension). Different types of spin waves can be excited depending on the relation between the bias magnetic field and the wave vector, and they can be quantitatively described by micromagnetic simulations. A semi-analytical k-space integration method is also proposed; this method is effectively as accurate as those obtained by micromagnetic simulations, but can save a great deal of time and computer resources. It is also found that a single magnetic damping constant is enough to interpret all the observed phenomena, although their apparent damping may look very different.; The second type of spatial inhomogeneity means that the sample itself is patterned into very small elements with the diameters less than 200 nm, while the excitation pulse is relatively uniform. The nanomagnets (nanodisks in this case) are either in quasi-single domain state or in vortex state at equilibrium, and they can switch from one to another by changing the applied bias field. As a result, the magnetization dynamics and associated frequencies will change dramatically when the state transitions (vortex annihilation and vortex nucleation) occur. The real shape of the nanodisks characterized by a "domed" top, rather than the normal "flat" top cylinder model, needs to be considered in micromagnetic simulations to accurately reproduce the critical fields for both vortex annihilation and nucleation. The physical nature of these phenomena is also discussed based on the simulated results. |
