Dynamics, damping and defects in ferromagnetic thin films. R. D. McMichael National Institute of Standards and Technology, Gaithersburg, MD USA One interesting consequence of the link between the magnetic moment and the spin angular momentum of electrons is that the magnetization precesses around the effective field in a way that is analogous to the precession of a top in the gravitational field. Typical precession frequencies are in the GHz range. Damping processes allow the magnetization to come to equilibrium in a few nanoseconds, a time that is becoming slow relative to increasing data rates in disk drives, magnetic memory chips and sensors. Ferromagnetic resonance experiments use the width of the peak in susceptibility at the magnetization precession frequency to measure magnetization damping. These measurements are generally clouded by the presence of material inhomogeneities that broaden the ferromagnetic resonance lines. In this talk, I will give an introduction to ferromagnetic resonance and three models of the added line width due to inhomogeneities: one that ignores magnetic interactions and assumes that the resonance is local, one that handles weak inhomogeneities while accounting for interactions, and a new model that links the two older models. This new model uses the eigenvalues and eigenmodes of the spin wave Hamiltonian to calculate the susceptibility spectrum, and shows that the localization transition depends on both the film thickness and the perturbation strength. Experimental examples of the effects of different kinds of defects will be given.