  We report a combined experimental and theoretical study of the spin S=1/2
nanomagnet 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 five-fold 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 Kramer's
theorem), and a very large excitation gap of \Delta~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=1/2 nanomagnet
showing an enhancement in the nuclear spin-lattice relaxation rate 1/T1 at
intermediate temperatures.

Materials Science;Strongly Correlated Electrons