  The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic
order thanks to a Dzyaloshinskii-Moriya (DM) anisotropy that is allowed by
crystal symmetry. Here we theoretically examine some finer issues such as the
experimentally observed lattice pinning of the propagation vector of helical
magnetic domains. We find that DM anisotropy alone would lead to incorrect
pinning, but agreement with experiment is restored upon including an additional
exchange anisotropy that is also consistent with symmetry. The present results
shed light on the so-called bisection rule which has been abstracted from
experiment in presence of an in-plane magnetic field.
