Spin Dynamics in Mesoscopic Size Magnetic Systems: A H-1 NMR Study in Rings of Iron(III) Ions

Two magnetic mol. clusters contg. almost coplanar rings of iron (III) ions with spin S = 5/2 were studied by 1H NMR and relaxation measurements. The 1st system, which will be referred to as Fe6, is a mol. [NaFe6(OCH3)12(C17O4H15)6]+ClO4- or [NaFe6(OCH3)12(C15H11O2)6]+ClO4- or [LiFe6(OCH3)12(C15H11O2)6]+ClO4- while the 2nd type of ring, denoted Fe10, corresponds to the mol. [Fe10(OCH3)20(C2H2O2Cl)10]. The 1H NMR linewidth is broadened by the nuclear dipolar interaction and by the dipolar coupling of the protons with the iron (III) paramagnetic moment. The nuclear spin-lattice relaxation rate, T1-1, of the proton is a sensitive probe of the Fe spin dynamics. In both clusters, T1-1 decreases with decreasing temps. from room temp., goes through a peak just .ltorsim.30 K in Fe6 and 10 K in Fe10, and it drops exponentially to very small values at helium temp. The temp. dependence of the relaxation rate is discussed in terms of the fluctuations of the local spins within the allowed total spin configurations in the framework of the weak collision theory to describe the nuclear relaxation. The authors use the calcd. energy levels for the Fe6 ring based on a Heisenberg Hamiltonian and the value of J obtained from the fit of the magnetic susceptibility to describe semiquant. the behavior of T1-1 vs. T. The exponential drop of T1-1 at low temp. is consistent with a nonmagnetic singlet ground state sepd. by an energy gap from the 1st excited triplet state. The values obtained for the gap energies are ET/k = 12 K for Fe10 and ET/k = 38 K for Fe6 which are almost twice as big as the values deduced from susceptibility measurements. At all temps. the relaxation rate decreases with increasing magnetic field, i.e., NMR resonance frequency. This effect could be related to the long time persistence of the spin correlation functions typical of diffusive modes in low dimensional magnetic systems. It is argued that the data presented are a direct exptl. study of spin dynamics in mesoscopic spin rings and should afford a test for exact anal. and/or numerical solns.