Spin Anisotropy in ZnCu_3(OH)_6Cl_2

The spin anisotropic exchange interaction is suggested to contribute significantly to the abnormal upturn of the magnetic susceptibility in the ZnCu${}_3$(OH)${}_6$Cl${}_2$. The saturation of the magnetic susceptibility below 300 mK observed in the muon spin resonance ($\mu$SR) experiment is the quantum effect of the spin flipping process.