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) {CrDy}, where a central fluorido bridge (-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. Fullcalculations, incorporating the experimental data, have provided an unambiguous determination of the ferromagnetic Dy-Dy coupling = 0.016 meV mediated by the-Fbridge and antiferromagnetic Cr-Dy interactions = - 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.