Observation of Two Nondispersive Magnetic Excitations in NiO by Resonant Inelastic Soft-X-Ray Scattering

We present high resolution (E ¼ 120 meV) resonant inelastic X-ray scattering data measured at the Ni L3 edge (2p3=2 ! 3d) on the paradigmatic antiferromagnetic oxide NiO. Spectra reveal clear signatures of magnetic excitations at 95 and 190 meV whose energy seems independent from transferred momentum. These spectral features are well reproduced by a single Ni2þ ion model in an effective exchange field. Within this local model the two magnetic excitations are characterized by a variation of the atomic magnetic moment along the local ordering direction (S) of one and two units. The S¼ 2 case has different nature from bimagnons observed in optical Raman spectra, for which S¼ 0.