Comparison of spin dynamics and magnetic properties in antiferromagnetic closed and open molecular Cr-based rings

We present magnetization and 1 H nuclear magnetic resonance (NMR) measurements performed in both closed Cr 8 and open Cr 8 Zn antiferromagnetic molecular rings in the temperature range 1.65 < T < 300 K at different external magnetic fields. The magnetization measurements on Cr 8 Zn are consistent with a small decrease of the exchange constant J Cr-Cr and a much smaller gap between the singlet ground state and the first magnetic excited state when compared with the same properties of the closed ring Cr 8 , in agreement with previous inelastic neutron scattering results. The temperature dependence of the 1 H NMR nuclear spin lattice relaxation rate (NSLR), 1/T 1 (T), was found to be similar in both open and closed rings with a magnetic field dependent peak centered at a temperature of the order of the corresponding exchange constant J Cr-Cr . Such main peak in the NSLR could be fitted with a single correlation frequency ω c1 as in most molecular magnets. At low temperature T < 4 K, a new feature not observed in previous NMR measurements on antiferromagnetic rings and consisting in a smaller peak of 1/T 1 (T) which is well resolved only in Cr 8 Zn, was singled out. This low-T peak indicates the presence of a second correlation frequency ω c2 of the magnetization, found to be quite different between the two rings and thus possibly reflecting the different low temperature level structure associated with the different spin topology. The presence of ω c2 is confirmed by the NMR spin-spin relaxation rate enhancement, which generates a two-steps wipe-out effect of the NMR signal intensity.