Neutron Magnetic Excitation Study of the Giant Magnetic Molecule {\{}Mo$_{72}$Fe$_{30}${\}}

We report cold-neutron inelastic scattering measurements on deuterated samples of {\{}Mo$_{72}$Fe$_{30}${\}} Keplerate. The nanocluster {\{}Mo$_{72}$Fe$_{30}${\}} is a molecular quasi-sphere in which 30 Fe$^{3+}$ ions (S = 5/2) occupy the vertices of an icosidodecahedron. The spins interact via an isotropic AF exchange coupling between nearest-neighbors. The measurements reveal a magnetic excitation spectrum that can be interpreted within the context of an effective three-sublattice Hamiltonian model. At mK temperatures, an excitation at E $\sim $ 0.63 meV is observed that can be attributed to the gap between the two lowest rotational-bands predicted by the model. Applied magnetic fields in the 0 -- 8.5 Tesla range confirm the magnetic origin of the spectrum. The temperature and field dependence of the energy spectra are discussed.