  We report results of 11B NMR and susceptibility measurements on the quasi 2D
frustrated dimer spin system SrCu2(BO3)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.
