Spin dynamics at the level crossing in the molecular antiferromagnetic ring [Cr8F8Piv16] from proton NMR

The H1 NMR average effective spin-lattice relaxation rate 1 T1 has been measured as a function of magnetic field (4.5-15.6T) at T=1 and 0.3K, and as a function of temperature (0.07-1K) around the first crossing field Hc1=7.43T, in the molecular antiferromagnetic ring [Cr8F8Piv16] (Cr8). T1-1(H) shows a single peak centered at Hc1 and a double peak for HHc2. The field dependence of 1 T1 away from the level crossing can be explained by a nuclear relaxation mechanism due to quasielastic scattering in the S=1 excited state thus yielding the energy gap between S=0 and S=1, MS=-1 states. An additional contribution to T1-1 can be detected near Hc1, due to inelastic scattering. Information about the H and T dependence of the width of the molecular magnetic levels is obtained from the fit of the T1-1 data.