  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.
