A Novel Ordered Phase in SrCu2(BO3)2 under High Pressure

We report results of 11 B NMR and susceptibility measurements on the quasi two-dimensional frustrated dimer spin system SrCu 2 (BO 3 ) 2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below T = 30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low- T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to T = 0.