Magnetic order and sign of the Dzyaloshinskii-Moriya interaction in 2D antiferromagnet Ba2CoGe2O7 under applied magnetic field

The Dzyaloshinskii-Moriya interaction (DMI), that is the antisymmetric part of the exchange coupling tensor, favors the perpendicular arrangement of magnetic moment, thus, induces 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 a 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.