Neutron Spectroscopy for the Magnetic Anisotropy of Molecular Clusters

Neutron spectroscopy has been used to observe transitions between the energy levels for the 21 different orientations of the $S=10\mathrm{}$ spin ground state of an octanuclear iron molecular cluster $\left({\mathrm{Fe}}_8\right)$ exhibiting quantum tunneling of the magnetization. The results obtained on nondeuterated samples provide the first direct measurement of the zero-field splitting in a large cluster nanomagnet. A reliable and accurate determination of the composition of the cluster's spin wave functions and the spin-Hamiltonian parameters are deduced from the experimental observations, giving crucial information for the study of the macroscopic quantum tunneling process.