Below 360 mK, ${\mathrm{Fe}}_8$ 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 $P\left({\xi }_H\right)$ of magnetic energy bias ${\xi }_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby “digging a hole” in $P\left({\xi }_H\right)$ (around the resonant condition ${\xi }_H=0\mathrm{}$). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.

• Received 1 December 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.3903