Magnetic anisotropy and exchange coupling for a fullerene C₇₄ containing a nearly linear Tb-O-Tb unit

Fullerenes containing lanthanide elements have been extensively synthesized and characterized for increasing blocking temperature and coercive field in single-molecule magnets (SMMs) and for studies of lanthanide-lanthanide bonding which may be hard to achieve without fullerenes. Recently, the fullerene C₈₀ with one carbon replaced by nitrogen, azafullerene C₇₉N, containing a terbium (Tb) dimer has been shown to have high blocking temperature and large coercive as well as an unsually large exchange coupling between the Tb ions. Charge transfer between the Tb ions and the fullerene cage is crucial for these properties. Here we investigate magnetic properties of a newly synthesized fullerene C₇₄ containing a nearly linear Tb-O-Tb unit, within ab-initio multiconfigurational methods including spin-orbit coupling, using the structure obtained from single crystal X-ray diffraction. We find that the individual Tb ions retain strong axiality in the magnetic anisotropy which may favor SMM behavior, and that a small deviation of the Tb-O-Tb unit from the linearity contributes to transverse ligand-field interactions. Our study also shows that the fullerene cage significantly enhances the axial and transverse magnetic anisotropy terms. The exchange coupling between the Tb ions turns out to be small and it favors ferromagnetic coupling.