Spin dynamics of the giant polyoxometalate molecule {Mn₄₀W ₂₂₄} studied by NMR

⁷Li nuclear magnetic resonance (NMR) studies have been performed to investigate magnetic properties and spin dynamics of Mn³⁺ (S = 2) spins in the giant polyoxometalate molecule {Mn40W224}. The ⁷Li-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 Mn³⁺ spins. The temperature dependence of T1 for both ¹H and ⁷Li 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 Mn³⁺ spins is derived from the nuclear relaxation data.