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Abstract:
We report a combined experimental and theoretical study of the spin S = 1/2 nanomagnet Cu-5(OH)(2)(NIPA)(4)center dot 10H(2)O (Cu-5-NIPA). Using thermodynamic, electron spin resonance, and H-1 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 Cu-5-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 = 1/2 doublet ground state (GS), with an exactly vanishing zero-field splitting (by Kramers' theorem), and a very large excitation gap of Delta similar or equal to 68 K. Thus, Cu-5-NIPA is a good candidate for achieving long electronic spin relaxation (T-1) and coherence (T-2) 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 nonfrustrated nature of the magnetic couplings, the GS is far from the classical collinear ferrimagnetic configuration. Finally, Cu-5-NIPA is a rare example of a S = 1/2 nanomagnet showing an enhancement in the nuclear spin-lattice relaxation rate 1/T-1 at intermediate temperatures.
