Cluster-size dependent internal dynamics and magnetic anisotropy of Ho ions in HoM2N@C80 and Ho2MN@C80 families (M = Sc, Lu, Y)

Abstract

The paramagnetic NMR study of HoM₂N@C₈₀-I_h and Ho₂MN@C₈₀-I_h nitride cluster fullerenes (M = Sc, Lu, Y) reveals strong dependence of Ho-induced paramagnetic shifts (δ^para) in ¹³C NMR spectra on the size of the diamagnetic metal in the cluster. In particular, the δ^para value in HoY₂N@C₈₀ is almost doubled in comparison to that in HoSc₂N@C₈₀. X-ray magnetic circular dichroism studies show that all Ho–nitride cluster fullerenes have the same magnetic ground state of Ho³⁺. Point-charge ligand-field splitting calculations show that the increase of the M³⁺ radius in going from Sc to Y results in a considerable increase of the energy splitting between different J_z states. This leads to a 19% higher magnetic anisotropy of Ho³⁺ in HoY₂N@C₈₀ than in HoSc₂N@C₈₀ at 300 K. Variations of the molecular geometry and cluster dynamics with the size of the cluster are found to have even greater influence on δ^para values. This work shows that the magnetic properties of the species confined inside the fullerene cages can be tuned using the geometrical factors such as the cluster and the cage size.