Single-crystal high-frequency electron paramagnetic resonance investigation of a tetranuclear iron(III) single-molecule magnet

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.