X-ray resonance exchange scattering in UAs

Resonant enhancement of the magnetic x-ray scattering cross section in UAs has been investigated by tuning the incident x-ray energy through the uranium ${\mathrm{M}}_{\mathrm{IV}}$ and ${\mathrm{M}}_{\mathrm{V}}$ absorption edges. At the ${\mathrm{M}}_{\mathrm{IV}}$ edge the intensity for the (0,0,(5/2) magnetic reflection is enhanced by ${10}^7$ relative to the nonresonant component far above the edge, and is a remarkable 1% of the intensity of the (0,0,2) charge peak. At the ${\mathrm{M}}_{\mathrm{V}}$ edge the enhancement is ${10}^5$. The dramatic enhancement at M edges has been predicted by Hannon et al., and arises from strong electric multipole transitions between atomic core states and the exchange-split Fermi edge states.