Neutron study of mesoscopic magnetic clusters:Mn12O12

Neutron diffraction and inelastic neutron scattering in zero field have been performed on mesoscopic magnetic ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$ clusters, well known for their macroscopic quantum effects observed at low temperature. In addition to the static spin correlations of the cluster in its ground state, we have observed some energy levels related both to the anisotropy and to the exchange energies of the cluster, with their respective dynamical form factor. The temperature and $Q$ dependences of the anisotropy energy levels can be qualitatively explained using a quantum model. Besides these expected modes, the most striking result is the observation of extended energy modes at energy values below those related to anisotropy. Their temperature and $Q$ dependences differ from those expected for the energy levels of usual magnetic clusters. They indicate an additional spin coupling which must play a role in tunneling properties.