  We study in this paper magnetic properties of a system of quantum Heisenberg
spins interacting with each other via a ferromagnetic exchange interaction J
and an in-plane Dzyaloshinskii-Moriya interaction D. The non-collinear ground
state due to the competition between J and D is determined. We employ a
self-consistent Green'function theory to calculate the spin-wave spectrum and
the layer magnetizations at finite T in two and three dimensions as well as in
a thin film with surface effects. Analytical details and the validity of the
method are shown and discussed.
