  We study the magnetic phases of two coupled two-dimensional electron gases in
order to determine under what circumstances these phases may occur in real
semiconductor quantum wells and what the experimental properties of the
broken-symmetry ground states may be. Within the local-density-approximation to
time-dependent density functional theory (DFT), we find a phase transition
signaled by the vanishing of the intersubband spin-density excitations at low
but accessible (\sim 10^10-10^11 cm^{-2}) electron densities. Through a
self-consistent Hartree-Fock calculation, we associate this transition with an
antiferromagnetic phase and study the phase diagram, thermodynamics, and
collective modes in it. The collective modes are in principle observable in
inelastic light scattering experiments, and we discuss the implications of our
calculations for these measurements. We also examine the ferromagnetic
transition in both single and double quantum wells within the
local-spin-density approximation to DFT and obtain a critical density which
depends on the well width and which is far below that of the antiferromagnetic
transition.
