Finding lanthanide magnetic anisotropy axes in 3d-4f butterfly single-molecule magnets using inelastic neutron scattering

A fundamental aspect of lanthanide-containing single-molecule magnets (SMMs) is determining the orientation of the magnetic anisotropy axes. However, the experimental determination is challenging. Here, we present an inelastic neutron scattering (INS) study on a family of 3d-4f MIII2LnIII2 butterfly compounds, with M = AlIII or FeIII and LnIII = YIII, ErIII, or DyIII. The rich INS spectra presented here provide significantly more experimental information in comparison to those of pure 4f clusters. As a main result, for Fe2Er2 and Fe2Dy2, the relative angle between the LnIII and the FeIII main anisotropy axes is determined. Models for the description of the magnetic properties are developed and found to reproduce the INS data, as well as the magnetic susceptibility and magnetization curves, with good accuracy. The results provide benchmarks for testing high-level ab initio theoretical approaches.