Review of NMR studies of nanoscale molecular magnets composed of geometrically frustrated antiferromagnetic triangles

This article presents a comprehensive review of nuclear magnetic resonance (NMR) studies performed on three nanoscale molecular magnets with different novel configurations of geometrically frustrated antiferromagnetic (AFM) triangles: (1) the isolated single AFM triangle K-6[V15As6O42(H2O)]center dot 8H(2)O (in short V15), (2) the spin ball [Mo72Fe30O252(Mo2O7(H2O)) 2(Mo2O8H2(H2O)) (CH3COO)(12) (H2O)(91)]center dot 150H(2)O (in short Fe30 spin ball), and (3) the twisted triangular spin tube [(CuCl(2)tachH)(3)Cl]Cl-2 (in short Cu3 spin tube). In V15t, from V-51 NMR spectra, the local spin configurations were directly determined in both the nonfrustrated total spin S-T = 3/2 state at higher magnetic fields (H >= 2.7 T) and the two nearly degenerate S-T = 1/2 ground states at lower magnetic fields (H <= 2.7 T). The dynamical magnetic properties of V15 were investigated by proton spin-lattice relaxation rate (1/T-1) measurements. In the S-T = 3/2 state, 1/T1 shows thermally activated behaviour as a function of temperature. On the other hand, the temperature independent behaviour of 1/T-1 at very low temperatures is observed in the frustrated S-T = 1/2 ground state. Possible origins for the peculiar behaviour of 1/T-1 will be discussed in terms of magnetic fluctuations due to spin frustrations. In Fe30, static and dynamical properties of Fe3+ (s = 5/2) have been investigated by proton NMR spectra and 1/T-1 measurements. From the temperature dependence of 1/T-1, the fluctuation frequency of the Fe3+ spins is found to decrease with decreasing temperature, indicating spin freezing at low temperatures. The spin freezing is also evidenced by the observation of a sudden broadening of H-1 NMR spectra below 0.6 K. Finally, H-1 NMR data in Cu3 will be described. An observation of magnetic broadening of H-1 NMR spectra at low temperatures below 1 K directly revealed a gapless ground state. The 1/T-1 measurements revealed a usual slow spin dynamics in the Cu3 spin tube.