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 provide a quantitative account for some puzzling phenomena such as the onset of a uniform magnetization below Hc and the persistence of the excitation gap above Hc. The gap was accurately determined from the activation energy of the nuclear relaxation rate.