Hard X-ray Resonant Scattering for Studying Magnetism

The development of resonant X-ray magnetic scattering is reviewed. Resonant X-ray magnetic scattering has become a unique probe among several useful techniques for studying magnetism in matter since the pioneering works of the 1980s. In contrast to neutron diffraction, which gives quantitative information on the spatial distribution of magnetic moments, resonant X-ray scattering is sensitive to the orbital and spin states of magnetic ions. The wavelengths of the K-edge resonance in 3d/4d transition elements and the L\(_{2,3}\)-edge resonance in lanthanide elements are located in the hard X-ray range, and are hence sufficiently short for diffraction measurement in most magnetic crystals, while the resonant enhancement is not very large. Recently, the electric dipole excitation between 2p and 5d orbitals has become a remarkably useful technique to explore novel electronic states in 5d transition-metal oxide compounds, where the spin and orbital are found to be strongly entangled with each other. Unique X-ray responses caused by an interference between the electric dipole and quadrupole transitions are also discussed.