Scaling of Kondo spin relaxation: Experiments on Cu-based nonlocal spin valves with Fe impurities

The relation between the Kondo spin-relaxation rate ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ and the Kondo momentum-relaxation rate ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ is explored by using nonlocal spin valves with submicron copper channels that contain dilute iron impurities. A linear relation between ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ and ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ is established under varying temperatures for any given device. Among 20 devices, however, ${{\ensuremath{\tau}}_{sK}}^{\ensuremath{-}1}$ remains nearly constant, despite variation of ${{\ensuremath{\tau}}_{eK}}^{\ensuremath{-}1}$ 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.