RESONANT MAGNETIC-X-RAY SCATTERING - A NEW PROBE FOR ACTINIDES

The magnetic cross-section for photons is small and difficult to obser ve; however, when the X-ray energy is tuned to certain absorption edge s an enormous enhancement of the magnetic scattering cross-section is observed. This process is now understood in terms of atomic resonances . By chance, the largest enhancements, and most convenient for diffrac tion experiments, occur at the M edges (3.5-4.5 keV in the actinides, We shall give some examples of the work performed at the NSLS, Brookha ven National Laboratory. Our recent experiments have been on small sin gle crystals of NpAs. The higher resolution available with X-ray synch rotron beams, compared with that at neutron sources, has allowed us to determine new features in bath the magnetic structure and the critica l fluctuations just above T-N. Because of the high absorption of the X -rays, the technique is extremely sensitive to the near-surface region , and the results often exhibit significant differences to those deter mined from neutron diffraction, which examines the bulk of the materia l. Thus, the two techniques are complementary. Very small samples are all that is required for the synchrotron experiments; we estimate that the beam illuminates no more than 1 mu g of NpAs in this study, so th at studies of the magnetic properties of transcurium materials become possible for the first time.