Unusual Scaling of Kondo Spin Relaxation

The relation between the Kondo spin relaxation rate ${\tau_{sK}}^{-1}$ and the Kondo momentum relaxation rate ${\tau_{eK}}^{-1}$ is explored by using nonlocal spin valves with submicron copper channels that contain dilute iron impurities. A linear relation between ${\tau_{sK}}^{-1}$ and ${\tau_{eK}}^{-1}$ is established under varying temperatures. However, under varying impurity concentrations, ${\tau_{sK}}^{-1}$ remains nearly constant despite variation of ${\tau_{eK}}^{-1}$ by a factor of 10. This surprising relation can be understood by considering spin relaxation through overlapping Kondo screening clouds and supports the physical existence of the elusive Kondo clouds.