NEUTRON STUDY OF MESOSCOPIC MAGNETIC CLUSTERS - MN12O12

Neutron diffraction and inelastic neutron scattering in zero field hav e been performed on mesoscopic magnetic Mn12O12 clusters, well known f or their macroscopic quantum effects observed at low temperature. In a ddition to the static spin correlations of the cluster in its ground s tate, we have observed some energy levels related both to the anisotro py and to the exchange energies of the cluster, with their respective dynamical form factor. The temperature and Q dependences of the anisot ropy energy levels can be qualitatively explained using a quantum mode l. Besides these expected modes, the most striking result is the obser vation of extended energy modes at energy values below those related t o anisotropy. Their temperature and Q dependences differ from those ex pected for the energy levels of usual magnetic clusters. They indicate an additional spin coupling which;must play a role in tunneling prope rties.