  We study the effects of external magnetic field on the properties of an
ordered Heisenberg antiferromagnet with the Dzyaloshinskii-Moriya (DM)
interaction. Using the spin-wave theory quantum correction to the energy,
on-site magnetization, and uniform magnetization are calculated as a function
of the field H and the DM anisotropy constant D. It is shown that the spin-wave
excitations exhibit an unusual field-evolution of the gaps. This leads to
various non-analytic dependencies of the quantum corrections on H and D. It is
also demonstrated that, quite generally, the DM interaction suppresses quantum
fluctuations, thus driving the system to a more classical ground state. Most of
the discussion is devoted to the spin-S, two-dimensional square lattice
antiferromagnet, whose S=1/2 case is closely realized in K2V3O8 where at H=0
the DM anisotropy is hidden by the easy-axis anisotropy but is revealed in a
finite field. The theoretical results for the field-dependence of the
spin-excitation gaps in this material are presented and the implications for
other systems are discussed.
