  It is well known that the magnetic anisotropy in a compressively-strained
Mn-doped GaAs film changes from perpendicular to parallel with increasing hole
concentration $p$. We study this reorientation transition at T=0 in a quantum
well with Mn impurities confined to a single plane. With increasing $p$, the
angle $\theta $ that minimizes the energy $E$ increases continuously from 0
(perpendicular anisotropy) to $\pi /2$ (parallel anisotropy) within some range
of $p$. The shape of $\Emin (p)$ suggests that the quantum well becomes phase
separated with regions containing low hole concentrations and perpendicular
moments interspersed with other regions containing high hole concentrations and
parallel moments. However, due to the Coulomb energy cost associated with phase
separation, the true magnetic state in the transition region is canted with $0
< \theta < \pi /2$.
