We construct the general free energy governing long-wavelength magnetism in
two-dimensional oxide heterostructures, which applies irrespective of the
microscopic mechanism for magnetism. This leads, in the relevant regime of weak
but non-negligible spin-orbit coupling, to a rich phase diagram containing
in-plane ferromagnetic, spiral, cone, and skyrmion lattice phases, as well as a
nematic state stabilized by thermal fluctuations. The general conclusions are
vetted by a microscopic derivation for a simple model with Rashba spin-orbit
coupling.Comment: 4+2 pages, 5 figures, version as accepted by Phys. Rev. Let