  The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a
function of temperature and magnetic field (up to 15 T) in the molecular
magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic
field values corresponding to a crossing between the ground state and the
excited states of the molecule. We propose that this is due to a
cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir
of the Zeeman levels of the molecule. This effect provides a powerful tool to
investigate quantum dynamical phenomena at level crossing.
