Ferromagnetic resonance studies of dilute magnetic semiconductors

We describe ferromagnetic resonance (FMR) measurements on ferromagnetic II$_{1-x}$Mn$_{x}$VI semiconductor alloys in thin film form. These include Ga$_{1-x}$Mn$_{x}$As layers grown by low-temperature molecular beam epitaxy on various buffers used to obtain different strain conditions. The analysis of the FMR provides values of cubic and uniaxial magnetic anisotropy fields -- i.e., those associated with the natural (undistorted) zinc-blende structure, and those arising from strain. Similar studies were also carried out on In$_{1-x}$Mn$_{x}$As, providing analogous information. Finally, we applied the FMR technique to Ga$_{1-x}$Mn$_{x}$As/Ga$_{1-y}$Al$_{y}$As heterostructures modulation-doped by Be. Here it was found that the increase in doping -- in addition to raising the Curie temperature of the Ga$_{1-x}$Mn$_{x}$As layers -- also leads to a significant increase of their uniaxial anisotropy field. The FMR data for modulation-doped heterostructures further show that the effective $g$-factor of Ga$_{1-x}$Mn$_{x}$As is strongly affected by the doping, thus providing a direct estimate of the free hole contribution to the magnetization of Ga$_{1-x}$Mn$_{x}$As.