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

Abstract

We present a comprehensive review of nuclear magnetic resonance (NMR) studies performed on three nanoscale molecular magnets with different configurations of geometrically frustrated antiferromagnetic (AFM) triangles, new spin frustration systems with different novel structures: (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 the V15 nanomagnet, from 51 V NMR spectra observed at only below ∼ 0.1 K, we directly determined the local spin configuration in both the nonfrustrated total spin ST = 3/2 state at higher magnetic fields (H ≥ 2.7 T) and the two nearly degenerate ST = 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 ST = 3/2 state, 1/T1 shows thermally activated behavior as a function of temperature. On the other hand, a temperature independent behavior of 1/T1 at very low temperatures is observed in the frustrated ST = 1/2 ground state below 2.7 Tesla. Possible origins for the peculiar behavior of 1/T1 will be discussed in terms of magnetic fluctuations due to spin frustrations. In Fe30 spin ball, static and dynamical properties of Fe 3+ (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 Fe 3+ 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 1 H NMR spectra below 0.6 K. Finally 1 H NMR data in the Cu spin tube will be described. An observation of magnetic broadening of 1 H NMR spectra at low temperatures below 1 K directly revealed a gapless ground state of the system. The 1/T1 measurements revealed an usual slow spin dynamics in the quasi one dimensional Cu3 spin tube.

Magnetocaloric properties of V6 molecular magnet

“…One of their experimental realizations is based on a wide class of molecular magnets [1][2][3], within which single molecule magnets provide fascinating examples of zero-dimensional magnetic clusters [4,5]. Among the huge variety of molecular magnetic materials, systems containing a triangle as a fundamental unit constitute a highly interesting category [6,7]. This is, mainly, due to the fact that antiferromagnetically coupled triangle is an archetypical example of magnetically frustrated system [8].…”