Friday,
November 16, 2007 - 4:00 P.M., NSH 184
Spin or magnetization dynamics in ferromagnetic semiconductor GaMnAs have been extensively studied by frequency-domain techniques, such as ferromagnetic resonance (FMR) and time-resolved Magneto-Optical Kerr effect (MOKE) measurements. Using the FMR approach, multi-mode spin wave spectra are observed as the direction of the magnetic field H is varied within the plane of the layer, as well as relative to the plane. The dependence of spin wave modes on the orientation of H is analyzed in terms of two fundamental models: the surface inhomogeneity model and the volume inhomogeneity model. The analysis of spin wave resonance spectra in terms of dynamic surface spin pinning (derived from the surface anisotropy) allows us to determine the value of the exchange stiffness constant D. Experiments were carried on a series of Ga1-xMnxAs films with different Mn concentrations and film thicknesses, in order to establish the dependences of exchange stiffness constants D on these parameters. As a different approach to the problem of spin dynamics and to the manipulation of magnetic phase transitions in GaMnAs, ultrafast photoinduced collective magnetization precession and have also been studied in this material by time-resolved Kerr rotation measurements. Here the observed temperature-dependent coherent oscillations provide a measure of ultrafast changes in the in-plane orientation of magnetization in a given ferromagnetic domain due to a laser induced transient changes of the magnetic anisotropy of GaMnAs. Moreover, the existence of oscillations up to a lower temperature below Tc is attributed to a second order magnetic phase transition. Finally, the results obtained from the field-dependence of the magnon frequencies and the oscillation-amplitude ratios reveal a relatively large negative contribution to the energy due to surface anisotropy, leading to spin-wave excitations that are a mixture of bulk and surface modes, consistent with the observations obtained in our FMR measurements.