Spin dynamics of the giant polyoxometalate molecule {Mn40W224} studied by NMR.

(7)Li nuclear magnetic resonance (NMR) studies have been performed to investigate magnetic properties and spin dynamics of Mn(3+) (S = 2) spins in the giant polyoxometalate molecule {Mn40W224}. The (7)Li-NMR line width is proportional to the external magnetic field H as expected in a paramagnetic state above 3 K. Below this temperature the line width shows a sudden increase and is almost independent of H, which indicates freezing of the local Mn(3+) spins. The temperature dependence of T1 for both (1)H and (7)Li reveals slow spin dynamics at low temperatures, consistent with spin freezing. The slow spin dynamics is also evidenced by the observation of a peak of 1/T2 around 3 K, where the fluctuation frequency of spins is of the order of ∼200 kHz. An explicit form of the temperature dependence of the fluctuation frequency of Mn(3+) spins is derived from the nuclear relaxation data.