Ferromagnetic resonance study of free hole contribution to magnetization   and magnetic anisotropy in modulation-doped Ga$_{1 - x}$Mn$_{x}$As/Ga$_{1 -   y}$Al$_{y}$As:Be
  Ferromagnetic resonance (FMR) is used to study magnetic anisotropy of GaMnAs in a series of Ga$_{1 - x}$Mn$_{x}$As/Ga$_{1 - y}$Al$_{y}$As heterostructures modulation-doped by Be. The FMR experiments provide a direct measure of cubic and uniaxial magnetic anisotropy fields, and their dependence on the doping level. It is found that the increase in doping -- in addition to rising the Curie temperature of the Ga$_{1 - x}$Mn$_{x}$As layers -- also leads to a very significant increase of their uniaxial anisotropy field. The FMR measurements further show that the effective $g$-factor of Ga$_{1 - x}$Mn$_{x}$As is also strongly affected by the doping. This in turn provides a direct measure of the contribution from the free hole magnetization to the magnetization of Ga$_{1 - x}$Mn$_{x}$As system as a whole. 
X. Liu, W. L. Lim, M. Dobrowolska, J. K. Furdyna, T. Wojtowicz