  We present details of earlier studies (Zheng et al, Phys. Rev. Lett. 78, 310
(1997) and Das Sarma et al, ibid 79, 917 (1997)) and additional new results on
double-layer quantum Hall systems at a total filling \nu = 2 \nu_1, where a
single layer at filling \nu_1 forms a ferromagnetic, fully spin-polarized,
gapped incompressible quantum Hall state. For the case \nu_1 = 1, a detailed
Hartree-Fock analysis is carried out on a realistic, microscopic Hamiltonian.
Apart from the state continuously connected to the ground state of two well
separated layers, we find two double-layer quantum Hall phases: one with a
finite interlayer antiferromagnetic spin ordering in the plane orthogonal to
the applied field (the `canted' state), and the other a spin singlet. The
quantum transitions between the various quantum Hall states are continuous, and
are signaled by the softening of collective intersubband spin density
excitations. For the case of general \nu_1, closely related results are
obtained by a semi-phenomenological continuum quantum field theory description
of the low-lying spin excitations using a non-linear sigma model. Because of
its broken symmetry, the canted phase supports a linearly dispersing Goldstone
mode and has a finite temperature Kosterlitz-Thouless transition. We present
results on the form of the phase diagram, the magnitude of the canted order
parameter, the collective excitation dispersions, the specific heat, the form
of the dynamic light scattering spectrum at finite temperature, and the
Kosterlitz-Thouless critical temperature. Our findings are consistent with
recent experimental results.
