Spin dynamics in the two-dimensional spin system SrCu2(BO3)2

Muon spin relaxation measurements have been performed in the two dimensional spin system SrCu2(BO3)2. As the temperature decreases, the relaxation rate increases below T=10K, and shows a saturation below T=3K. The line shape at low temperatures is nearly Gaussian, yet it cannot be easily decoupled by an application of longitudinal field. These results indicate the existence of magnetically active spins that show slowing down of fluctuations with decreasing temperatures, yet remain dynamic even at 0.1K. We have also examined a sample doped with small amount of non-magnetic Zn2+ impurity, Sr(Cu0.99Zn0.01)2(BO3)2. The result was quite similar to the pure sample. Thus, anomalous behavior of the pure sample observed in low temperatures is not due to magnetic moments induced by impurities or a lattice defect.