Probing superexchange interaction in a very hard 3d-4f single-molecule magnet by inelastic neutron scattering

We report a combined experimental and theoretical investigation of the spin dynamics in the very hard 3d-4f single-molecule magnet (SMM) {Cr₃Dy₃}, where a central fluorido bridge (µ₃-F^-) suppresses zero-field quantum tunneling of magnetization (QTM). Utilizing inelastic neutron scattering, heat capacity and ac susceptibility techniques, we have probed the low-lying magnetic excitations and their response to applied magnetic fields in polycrystalline samples. Fullab initiocalculations, incorporating the experimental data, have provided an unambiguous determination of the ferromagnetic Dy-Dy coupling (J_Dy-Dy= 0.016 meV) mediated by theµ₃-F^-bridge and antiferromagnetic Cr-Dy interactions (J_Cr-Dy= - 0.215 meV). The significant energy gap between the ferrimagnetic ground state and the lowest exchange-induced excited state (1.03 meV) provides a direct mechanistic basis for the suppression of QTM, leading to its exceptional SMM property.