  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.
