Spin dynamics at the level crossing in the molecular antiferromagnetic ring [Cr8F8Piv16] from proton NMR -: art. no. 020405R

The H-1 NMR average effective spin-lattice relaxation rate 1/T-1 has been measured as a function of magnetic field (4.5-15.6 T) at T=1 and 0.3 K, and as a function of temperature (0.07-1 K) around the first crossing field H-c1=7.43 T, in the molecular antiferromagnetic ring [Cr(8)F(8)Piv(16)] (Cr8). T-1(-1)(H) shows a single peak centered at H-c1 and a double peak for H similar or equal to H-c2. The field dependence of 1/T-1 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, M-S=-1 states. An additional contribution to T-1(-1) can be detected near H-c1, 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 T-1(-1) data.