Magnetization and spin dynamics of the spin S=12 hourglass nanomagnet Cu₅(OH)₂(NIPA)₄·10H₂O

We report a combined experimental and theoretical study of the spin S=12 nanomagnet Cu₅(OH)₂(NIPA)₄·10H ₂O (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 nonfrustrated 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 nanomagnet showing an enhancement in the nuclear spin-lattice relaxation rate 1/T₁ at intermediate temperatures.