  We examine the robustness of a recently predicted exchange-induced zero-field
magnetic phase in semiconductor double quantum wells in which each well is
spin-polarized and the polarization vectors are antiparallel. Magnetic
instabilities are a general feature of Coulombic double quantum well systems at
low densities. We argue that this antiferromagnetic phase is stabilized
relative to ferromagnetic ones by an effective superexchange interaction
between the wells. Detailed self-consistent Hartree-Fock calculations using a
point-contact model for the interaction show that the antiferromagnetic phase
survives intra-subband repulsion matrix elements neglected in earlier work in a
large portion of the model's parameter space. We also examine the role of
asymmetry due to biasing or to differences in the widths of the two quantum
wells. The asymmetry creates a mode coupling between the intra- and
inter-subband collective spin-density excitations (SDEs) which changes the
Raman spectroscopy signature of the phase transition from a complete softening
of the inter-subband SDE to a cusp as the density is tuned through the
transition. This cusp may be detectable in inelastic light scattering
experiments in samples of sufficient quality at low enough temperatures and
densities.
