Observation of the Distribution of Molecular Spin States by Resonant Quantum Tunneling of the Magnetization

Below 360 mK, Fe₈ magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted ``square-root time'' relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(ξH) of magnetic energy bias ξH acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby ``digging a hole'' in P(ξH) (around the resonant condition ξH0ex0ex=0ex0ex0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.