The Dzyaloshinskii-Moriya interaction (DMI), i.e., the antisymmetric part of the exchange
coupling tensor, favors the perpendicular arrangement of magnetic moment, thus inducing
canting in otherwise collinear structures. The DMI is the prerequisite for the emergence
of weak ferromagnetism in antiferromagnets, but can stabilize twisted magnetic textures,
such as spin spirals, soliton lattices, and magnetic skyrmions. While the magnitude
of the DMI determines the canting angle of adjacent spins, its sign dictates the sense
of the spin rotation. Based on focused polarized neutron diffraction (PND) study,
combined with symmetry analysis, we determine the sign of the DMI in the unconventional
multiferroic Ba2CoGe2O7 and reveal its detailed magnetic structure in magnetic fields
applied in the tetragonal plane. As PND gives unique access to the scattering contribution
from the phase-sensitive nuclear-magnetic interference, it is a valuable tool for
a straightforward DMI sign determination in bulk materials and allows to disclose
even very weak magnetic moments. Remarkably, the sign of the DMI could be determined
from the PND measurement of a single reflection, which is demonstrated to be reliable
for a large range of applied magnetic field directions and values.
