Spin-freezing in the two-dimensional spin-gap systems SrCu 2− xMg x(BO 3) 2 ( x=0, 0.04, 0.12)

Abstract

The magnetic properties of the two-dimensional dimer spin-gap system SrCu 2(BO 3) 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 T f<3.75 K, well within the spin-gap temperature range ( T SG≈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 SrCu 2(BO 3) 2 spin-gap ground state towards an antiferromagnetic transition, Knight-shift measurements suggest that implanted muons may liberate spin density at T< T SG 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.