Field-Induced Effects of Anisotropic Magnetic Interactions in   SrCu2(BO3)2
  We observed a field-induced staggered magnetization in the 2D frustrated dimer-singlet spin system SrCu2(BO3)2 by 11B NMR, from which the magnitudes of the intradimer Dzyaloshinsky-Moriya interaction and the staggered g-tensor were determined. These anisotropic interactions cause singlet-triplet mixing and eliminate a quantum phase transition at the expected critical field Hc for gap closing. They also provide a quantitative account for some puzzling phenomena such as the onset of a uniform magnetization below the and the persistence of the excitation gap above Hc. The gap was accurately determined from the activation energy of the nuclear relaxation rate. 
K. Kodama, S. Miyahara, M. Takigawa, M. Horvatic, C. Berthier, F.   Mila, H. Kageyama, and Y. Ueda