Ferromagnetic resonance in GaMnAs

We report a systematic study of ferromagnetic resonance (FMR) on thin films of the ferromagnetic semiconductor GaMnAs grown by low temperature molecular beam epitaxy. Pronounced shifts of the FMR line are observed towards fields above and below the g=2.00 resonance position for the magnetic field orientations perpendicular and parallel to the plane of the layer. At higher temperatures these shifts decrease rapidly as the Curie temperature is approached, converging on the g=2.00 resonance position that is characteristic of the Mn++ ion, in agreement with the effects of demagnetization on FMR in thin film geometry. At low temperatures, however, all specimens show a striking deviation from the simple demagnetization-induced picture. We ascribe this to anisotropy fields, which increase in importance as the temperature decreases. (C) 2002 American Institute of Physics.