Distinct effect of Kondo physics on crystal field splitting in electron and spin spectroscopies

Magnetic anisotropy is a key feature of rare earth materials from permanent magnets to heavy fermions. We explore the complex interplay of Kondo physics and anisotropy in a minimal impurity model using numerical renormalization group. While anisotropy suppresses Kondo physics, Kondo physics enhances the anisotropy. Importantly, we find distinct renormalization of the magnetic anisotropy measured via dynamical spin response (inelastic neutron scattering) versus electronic excitations in the impurity spectral function (resonant inelastic x-rays and scanning tunneling spectroscopy). The two measurement types have different responses and dependences on the temperature and Kondo scales.