Inelastic-neutron-scattering study of excited spin multiplets and low-energy phonons in the Fe8 nanomagnet: Implications for relaxation

Inelastic neutron scattering is used to study excited spin multiplets and low-energy phonons in the Fe-8 nanomagnet. The energies of the three lowest-lying excited multiplets are determined. In addition, we find weakly dispersive optical phonons with energies lower than the anisotropy splitting of the ground S=10 spin multiplet. We show that these optical phonons may produce sharp minima in the field dependence of the relaxation time. The positions of the calculated minima agree with those that had been inferred from frequency-dependent specific heat and ac-susceptibility measurements.