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 {Cr 3 Dy 3 }, where a central fluorido bridge (µ 3 -F⁻) suppresses zerofield quantum tunneling of magnetization (QTM). Utilizing inelastic neutron scattering (INS), heat capacity (HC) and ac susceptibility techniques, we have probed the low-lying magnetic excitations and their response to applied magnetic fields in polycrystalline samples. Full ab initio calculations, incorporating the experimental data, have provided an unambiguous determination of the ferromagnetic Dy-Dy coupling (J Dy-Dy = 0.016 meV) mediated by the µ 3 -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 single-molecule magnet property.