Comparison of the magnetic properties and the spin dynamics in heterometallic antiferromagnetic molecular rings

We present a comparison of the results obtained in the antiferromagnetic (AFM) homometallic ring Cr8 with the results in three heterometallic rings with a Cr ion replaced by Cd, Ni, and Fe ions, i.e., Cr7 Cd, Cr7 Ni, and Cr7 Fe, respectively. The experimental results include magnetic susceptibility, H1 nuclear-magnetic-resonance (NMR) spectra, spin-spin and spin-lattice relaxation rates data, collected in the temperature range 1.65<T<300K at two applied magnetic fields. The data include both new results and previously published data. The static magnetic properties derived from susceptibility and H1 NMR linewidth can be analyzed in a simple way in terms of the properties of the individual ions constituting the ring and their nearest-neighbor antiferromagnetic exchange coupling. The nuclear-spin-lattice relaxation rate at low temperature can be described phenomenologically by a model, which assumes a single-correlation time for the relaxation of the magnetization, as used previously for homometallic AFM rings. The correlation frequencies obtained from the fit of the data, increase by as much as two orders of magnitude in the Cr7 Ni and Cr7 Fe rings with respect to the homometallic ring Cr8 and the diamagnetically substituted Cr7 Cd ring. This result can be explained qualitatively in terms of a change in spin-phonon coupling due to the enhancement of crystal-field effects in the heterometallic rings. For a more quantitative analysis one should take into account the multi-Lorentzian behavior of the spin-spin correlation function for which detailed theoretical calculations are required. At temperatures higher than the magnetic exchange energy J/ kB, the mechanism for nuclear-spin-lattice relaxation changes since the fluctuations of the moments of the magnetic ions become weakly correlated. We find a fluctuation frequency much higher in the heterometallic rings as a result of the perturbation introduced by the substituted magnetic ion.