Intra- and inter-multiplet magnetic excitations in a tetrairon(III) molecular cluster

We used inelastic neutron scattering to determine the microscopic spin Hamiltonian of a tetrairon(III) molecular cluster with trigonal symmetry, [Fe-4(thme)(2)(dpm)(6)], obtained from a parent compound with C-2 symmetry by site-specific ligand substitution. Intra-multiplet excitations within the anisotropy split S=5 ground spin state, and inter-multiplet transitions toward the first S=4 excited states have been observed. The model spin Hamiltonian used to interpret the experimental data evidences an enhancement of the anisotropy barrier due to the chemical modification of the parent molecular cluster. A rhombic anisotropy term, forbidden in D-3 symmetry, is necessary to reproduce experimental data. The determination of this term is crucial for the understanding of quantum tunneling of the magnetization in this model compound. The calculated temperature dependence of heat capacity for different applied magnetic field values is in good agreement with experimental measurements. Within this model, the estimated temperature dependence of the relaxation time agrees with available experimental data.