The relaxation (damping) of magnetization motion is a result of microscopic field fluctuations on spins by means of interaction with a thermal bath. These fluctuations appear due to either elementary processes, e.g., magnon-electron scattering, or more complicated microscopic processes, such as "slow relaxing" impurities. Each microscopic relaxation mechanism predicts its own temperature and frequency dependencies of FMR line width. The magnetization damping can also depend on defect/impurity concentration and the sample size. Two principal scenarios of magnetization reversal will be also considered. In the first scenario all spins perform coherent motion and an excess of magnetic energy directly goes to a nonmagnetic thermal bath. A general dynamic equation is derived which extends a previous application of the nonlinear oscillator model for the uniaxial case to a system with an arbitrary symmetry. The resulting dynamic equation includes a tensor damping term similar to the Bloch-Bloembergen (BB) form. However, here, in contrast to BB, the magnetization magnitude remains constant for any deviation from equilibrium. Faster reversal compared to phenomenological LLG model is predicted for large reversal angles. In the second reversal scenario, the absolute value of the averaged sample magnetization is decreased by a rapid excitation of spin waves. As the dynamic reversal process begins and the initial uniform magnetization sufficiently deviates from equilibrium, a strong microwave field is created that excites nonlinear spin-wave resonances analogous to Suhl instabilities. We have developed an analytic micromagnetic approach that describes the entire reversal process in an ultra-thin film for about a 90 degree deviation from equilibrium.

Date: Thursday, February 10, 2005
Time: 4:10 pm.