We report a combined experimental and theoretical study of the spin S=12 nano magnet Cu5(OH)2(NIPA)4⋅10H2O (Cu5-NIPA). Using thermodynamic, electron spin resonance, and 1H nuclear magnetic resonance measurements on one hand, and ab initio density-functional band-structure calculations, exact diagonalizations, and a strong-coupling theory on the other, we derive a microscopic magnetic model of Cu5-NIPA and characterize the spin dynamics of this system. The elementary fivefold Cu2+ unit features an hourglass structure of two corner-sharing scalene triangles related by inversion symmetry. Our microscopic Heisenberg model comprises one ferromagnetic and two antiferromagnetic exchange couplings in each triangle, stabilizing a single spin S=12 doublet ground state (GS), with an exactly vanishing zero-field splitting (by Kramers' theorem), and a very large excitation gap of Δ≃68 K. Thus, Cu5-NIPA is a good candidate for achieving long electronic spin relaxation (T1) and coherence (T2) times at low temperatures, in analogy to other nanomagnets with low-spin GS's. Of particular interest is the strongly inhomogeneous distribution of the GS magnetic moment over the five Cu2+ spins. This is a purely quantum-mechanical effect since, despite the non frustrated nature of the magnetic couplings, the GS is far from the classical collinear ferrimagnetic configuration. Finally, Cu5-NIPA is a rare example of a S=12 nano magnet showing an enhancement in the nuclear spin-lattice relaxation rate 1/T1 at intermediate temperatures.
R. Nath, Tsirlin, A. A., Khuntia, P., Janson, O., Forster, T., Prof. M. Padmanabhan, Li, J., Skourski, Y., Baenitz, M., Rosner, H., and Rousochatzakis, I., “Magnetization and spin dynamics of the spin S=1/2 hourglass nanomagnet Cu 5 (OH) 2 (NIPA) 4 10H 2 O”, Physical Review B, vol. 87, p. 214417/115, 2013.