EFFECTS OF SPIN VALUE AND TOPOLOGY ON THE SPIN DYNAMICS OF ANTIFERROMAGNETIC MOLECULAR RINGS UNRAVELLED BY ¹H NMR.

The research outcomes reported in the present work are focused on the understanding of the magnetic properties and the spin dynamics of molecular Antiferromagnetic (AFM) rings. The experimental investigation was performed mainly by proton Nuclear Magnetic Resonance (NMR) as a function of temperature and applied magnetic field. The systems investigated include semi-integer spin Cr-based AFM rings, both closed (i.e. Cr₈, Cr₉) and open ones (i.e. Cr₈Zn, and Cr₇Cd), and integer-spin V-based AFM rings, namely V₇Ni and V₇Zn. The results are analyzed separately for three different temperature regions: (1) high temperature (kʙT >> J), where the magnetic moments of the molecular ring are weakly correlated and the finite spin system is found to behave like a simple paramagnet with some evidence for one dimensional spin diffusion effects; (2) intermediate temperature (kʙT ≈ J), where the evolution of the magnetic properties and of the spin dynamics of the system when the temperature is decreased, reflects the progressively increasing correlation of the spins towards a collective ground state. In this temperature range we found the first experimental evidence of more than one correlation frequency for the relaxation of the magnetization of the ring; (3) low temperature (kʙT << J), where the system occupies mainly the collective ground state with total spin Sᴛ. In this low temperature region, interesting effects are found at the level crossings between the ground state and the first few excited magnetic states induced by the external magnetic field. The results are discussed in terms of the effects of the spin topology. No measurable difference is found between the behavior of the spin dynamics in semi-integer spin (s = 3/2) AFM rings compared with integer spin (s = 1) ones, in the high and intermediate temperature ranges.