Refined model of the {Fe9} magnetic molecule from low-temperature inelastic neutron scattering studies

We present a refined model of the ${{\mathrm{Fe}}_{9}}$ tridiminished icosahedron magnetic molecule system. This molecule was originally modeled as being composed of two (${{\mathrm{Fe}}_{3}}$ and ${{\mathrm{Fe}}_{6}}$) clusters, with the ${\mathrm{Fe}}^{3+}$ ions within each cluster being coupled via exchange interactions, but with no coupling between the clusters. The present inelastic neutron scattering (INS) measurements were used to probe the low-lying energy spectrum of ${{\mathrm{Fe}}_{9}}$, and these results demonstrate that the previously published model of two uncoupled clusters is incomplete. To achieve agreement between the experiment and theory, we have augmented the model with relatively small exchange coupling between the clusters. A combination of Lanczos matrix diagonalization and quantum Monte Carlo simulations have been used to achieve good agreement between the experimental data and the improved model of the full ${{\mathrm{Fe}}_{9}}$ system despite the complexity of this model (with Hilbert space dimension $g{10}^{7}$).