  We use Resonant Inelastic X-ray Scattering (RIXS) to compare the doping
dependence of magnetic excitations of an Infinite-Layer nickelate to those of a
prototypical superconducting cuprate. The polarization analysis of RIXS spectra
establishes the dominant spin-flip nature of the mid-infrared peak in both
cases. Hole doping leads to opposite behavior of the magnetic energy in the two
materials. By fitting the data with an original Hubbard-based model for dynamic
susceptibility, we find that t is comparable in the two materials while U is
about twice larger in the nickelate. This finding accounts for the smaller
magnetic bandwidth of nickelates and for its decrease upon doping.
