Square-lattice spiral magnet Ba2CuGe2O7 in an in-plane magnetic field

The magnetic structure of Ba2CuGe2O7 is investigated by neutron diffraction in magnetic fields applied along several directions in the (a,b) plane of the crystal. In relatively weak fields, H less than or similar to 0.5 T, the propagation vector of the spin spiral rotates to form a finite angle with the field direction. This angle depends on the orientation of H itself. The rotation of the propagation vector is accompanied by a reorientation of the plane of spin rotation in the spiral. The observed behavior is well described by a continuous-limit form of a free-energy functional that includes exchange and Dzyaloshinskii-Moriya interactions, as well as the Zeeman energy and an empirical anisotropy term.