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

We present high resolution ($\Delta E=120\mathrm{meV}$) resonant inelastic x-ray scattering data measured at the Ni $L_3$ edge ($2p_{3/2}\to 3d$) on the paradigmatic antiferromagnetic oxide NiO. Spectra reveal clear signatures of magnetic excitations at $\sim 95$ and $\sim 190\mathrm{meV}$ whose energy seems independent from transferred momentum. These spectral features are well reproduced by a single ${\mathrm{Ni}}^{2+}$ 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 ($\Delta S_{\alpha }$) of one and two units. The $\Delta S_{\alpha }=2$ case has different nature from bimagnons observed in optical Raman spectra, for which $\Delta S_{\alpha }=0$.