Magnetic anisotropies of Ho(III) and Dy(III) single-molecule magnets experimentally determined polarized neutron diffraction.

We present the magnetic anisotropy of two isostructural pentagonal-bipyramidal complexes, [Ln(HO)(HMPA)]I·2HMPA (HMPA = hexamethylphosphoramide, Ln = Dy, Ho). Using ac magnetic susceptibility measurements, we find magnetic relaxation barriers of 600 K and 270 K for the Dy- and Ho-compounds, respectively. This difference is supported by polarized neutron diffraction (PND) measured at 5 K and 1 T which provides the first experimental evidence that the transverse elements in the magnetic anisotropy of the Ho-analogue are significant, whereas the Dy-analogue has a near-axial magnetic anisotropy with vanishing transverse contributions.