Strategies for precise parametrisation of the Hamiltonians of magnetic molecules in the strong-coupling regime

When the energy gap, ∆, between the lowest-lying spin manifolds within a spin-exchange coupled molecule approaches ∆/k_B ≈ 300 K, one cannot always rely on the traditional temperature-dependence (T < 400 K) of the molar magnetic susceptibility to get a good estimate of intra-molecular exchange couplings. In this work we develop an experimental strategy, based on advanced spectroscopies, capable of accurately parametrising complex magnetic Hamiltonians by exploiting the separation of the energy scales associated with magnetic anisotropy and exchange in strongly coupled magnetic molecules. Specifically, we combine inelastic neutron scattering, high-frequency electron paramagnetic resonance, far infrared magneto-spectroscopy and magnetometry and obtain detailed information about the magnetic properties of a series of diiron complexes derived from [[Fe(cth)]2(dxbq)]3+ (H2dhbq: 2,5-dihydroxy-1,4benzoquinone, H2dcbq: 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone, cth: 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). These molecules have well-isolated S = 9/2 ground states due to strong direct antiferromagnetic exchange between the Fe3+ centres (S = 5/2) and the radical bridging benzoquinone ligand (S = 1/2). The specific sensitivities and transition selection rules of the applied methods allows us to determine the parameters of the microscopic Hamiltonian including exchange coupling, fourth order Stevens' operators and g-factors. The level of detail we obtain exemplifies the capabilities of our methodology, which is directly portable to other studies of molecule-based magnetic compounds.