Observation of the distribution of molecular spin states by resonant quantum tunneling of the magnetization

Below 360 mK, Fe(8) 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(xi(H)) Of magnetic energy bias xi(H) acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby "digging a hole" in P(xi(H)) (around the resonant condition xi(H) = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.