Intra and Inter-Multiplet Neutron Transitions in an Fe4 Magnetic Cluster

We report the results of neutron spectroscopy for a tetranuclear iron cluster, Fe-4(OCH3)(6)(dpm)(6))6 (Hdpm = dipivaloylmethane), having an S = 5 spin ground state. The experiments were carried out on a deuterated powder sample at ILL, Grenoble. The transitions within the ground multiplet, below 0.3 meV, were measured with a high-resolution time-of-flight spectrometer. Taking into account the presence of three different isomers, the parameters of the zero-field-splitting Hamiltonian have been determined. The results are discussed with reference to high-field electron paramagnetic resonance data. A non-dispersive peak at 7.8 meV, detected at T = 1.4 K using a three-axis spectrometer with polarization analysis, is interpreted as the transition between the S = 5 ground state and the two degenerate S = 4 excited multiplets. The Q dependence of the cross section is discussed in comparison with the theoretical model.