Double-k phase of the Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7
  Neutron diffraction is used to re-investigate the magnetic phase diagram of the noncentrosymmetric tetragonal antiferromagnet Ba2CuGe2O7. A novel incommensurate double-k magnetic phase is detected near the commensurate-incommensurate phase transition. This phase is stable only for magnetic field closely aligned with the 4-fold symmetry axis. The results emphasize the inadequacy of existing theoretical models for this unique material, and points to additional terms in the Hamiltonian or lattice effects. 
S. M\"uhlbauer, S. N. Gvasaliya, E. Pomjakushina, A. Zheludev
cond-mat.str-el