Spin-freezing in the two-dimensional spin-gap systems SrCu2-xMgx(BO3)2 (x=0, 0.04, 0.12)

The magnetic properties of the two-dimensional dimer spin-gap system SrCu2(BO3)2 were investigated by the μ+SR technique. The relatively slow fluctuations of spin-dimers slow down with decreasing temperature and an unusual spin-freezing process is unraveled at Tf<3.75 K, well within the spin-gap temperature range (TSG∼20 K). This quasi-static phase displays a Gaussian field distribution with a remarkable stability with applied longitudinal fields. In support of the criticality of the SrCu2(BO3)2 spin-gap ground state towards an antiferromagnetic transition, Knight-shift measurements suggest that implanted muons may liberate spin density at T<TSG that undergoes spin-freezing at very low temperatures. On the other hand, non-magnetic impurity-doping of the copper sublattice does not suppress the spin-gap ground state and does not lead to magnetic ordering effects of static nature