1H nuclear magnetic resonance and spin dynamics in the tetranuclear iron(III) cluster {Fe4}

The spin dynamics of a cluster of four iron(III) ions characterized by a total spin ground state S-T = 5 and Ising anisotropy have been investigated by the H-1 nuclear magnetic resonance (NMR) linewidth and H-1 NMR spin-lattice relaxation rate 1/T-1 as a function of the temperature (0.5-295 K) and external magnetic field (0.3-7.2 T). At very low T (0.5 K) the spectrum becomes very broad indicating freezing of the Fe3+ moments in a superparamagnetic state. The temperature dependence of T-1(-1) and T-2(-1) shows strong field dependent enhancement below 50 K which must be related to the spin dynamics of the Fe3+ moment freezing process, although no quantitative theory of this is currently available. (C) 2002 American Institute of Physics.