  We use an aluminium single electron transistor with a magnetic gate to
directly quantify the chemical potential anisotropy of GaMnAs materials.
Uniaxial and cubic contributions to the chemical potential anisotropy are
determined from field rotation experiments. In performing magnetic field sweeps
we observe additional isotropic magnetic field dependence of the chemical
potential which shows a non-monotonic behavior. The observed effects are
explained by calculations based on the $\mathbf{k}\cdot\mathbf{p}$ kinetic
exchange model of ferromagnetism in GaMnAs. Our device inverts the conventional
approach for constructing spin transistors: instead of spin-transport
controlled by ordinary gates we spin-gate ordinary charge transport.
