We investigate the mechanism of spin-lattice relaxation of Er ions encapsulated in polyoxometalate clusters, which below 4 K can only reverse its spin via quantum tunneling processes. The temperature-independent rate ${\tau }^{-1}$ is, at zero field, ten orders of magnitude larger than the rates predicted for direct phonon-induced processes. In addition, we observe that ${\tau }^{-1}$ is suppressed by external magnetic bias and hyperfine interactions but enhanced by increasing the concentration of Er ions. The observed relaxation agrees with predictions for pure quantum tunneling, showing that this phenomenon drives the thermalization of electronic spins. A possible link between these two phenomena is discussed, involving the collective emission of phonons from particular spin configurations attained via quantum tunneling.

• Received 10 June 2010

DOI:https://doi.org/10.1103/PhysRevB.82.060403