Review of NMR studies of nanoscale molecular magnets composed of geometrically frustrated antiferromagnetic triangles
by Yuji Furukawa
International Journal of Nanotechnology (IJNT), Vol. 13, No. 10/11/12, 2016

Abstract: 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 K₆[V₁₅As₆O₄₂(H₂O)]·8H₂O (in short V15), (2) the spin ball [Mo₇₂Fe₃₀O₂₅₂(Mo₂O₇(H₂O))₂(Mo₂O₈H₂(H₂O)) (CH₃COO)₁₂(H₂O)₉₁]·150H₂O (in short Fe30 spin ball), and (3) the twisted triangular spin tube [(CuCl₂tachH)₃Cl]Cl₂ (in short Cu3 spin tube). In V15t, from ⁵¹V 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 ge; 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/T₁) 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/T₁ at very low temperatures is observed in the frustrated S_T = 1/2 ground state. Possible origins for the peculiar behaviour of 1/T₁ 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/T₁ measurements. From the temperature dependence of 1/T₁, 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 ¹H NMR spectra below 0.6 K. Finally, ¹H NMR data in Cu3 will be described. An observation of magnetic broadening of ¹H NMR spectra at low temperatures below 1 K directly revealed a gapless ground state. The 1/T₁ measurements revealed a usual slow spin dynamics in the Cu3 spin tube.

Online publication date: Wed, 16-Nov-2016

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