Magnetic anisotropy and low-energy spin waves in the   Dzyaloshinskii-Moriya spiral magnet Ba_2 Cu Ge_2 O_7
  New neutron diffraction and inelastic scattering experiments are used to investigate in detail the field dependence of the magnetic structure and low-energy spin wave spectrum of the Dzyaloshinskii-Moriya helimagnet Ba_2 Cu Ge_2 O_7. The results suggest that the previously proposed model for the magnetism of this compound (an ideal sinusoidal spin spiral, stabilized by isotropic exchange and Dzyaloshinskii-Moriya interactions) needs to be refined. Both new and previously published data can be quantitatively explained by taking into account the Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) term, a special magnetic anisotropy term that was predicted to always accompany Dzyaloshinskii-Moriya interactions in insulators. 
A. Zheludev, S. Maslov, G. Shirane, I. Tsukada, T. Masuda, K.   Uchinokura, I. Zaliznyak, R. Erwin, L. P. Regnault