  Some of the highest transition temperatures achieved for Mn-doped GaAs have
been in delta-doped heterostructures with well-separated planes of Mn. But in
the absence of magnetic anisotropy, the Mermin-Wagner theorem implies that a
single plane of magnetic ions cannot be ferromagnetic. Using a Heisenberg
model, we show that the same mechanism that produces magnetic frustration and
suppresses the transition temperature in bulk Mn-doped GaAs, due to the
difference between the light and heavy band masses, can stabilize ferromagnetic
order for a single layer of Mn in a GaAs quantum well. This comes at the price
of quantum fluctuations that suppress the ordered moment from that of a fully
saturated ferromagnet. By comparing the predictions of Heisenberg and
Kohn-Luttinger models, we conclude that the Heisenberg description of a
Mn-doped GaAs quantum well breaks down when the Mn concentration becomes large,
but works quite well in the weak-coupling limit of small Mn concentrations.
This comparison allows us to estimate the size of the quantum fluctuations in
the quantum well.

Materials Science;Strongly Correlated Electrons