Resonant magnetic x-ray-scattering studies of NpAs. I. Magnetic and lattice structure

The magnetic x-ray scattering from a transuranium material, NpAs, has been measured. We report the intensity of the scattering as a function of photon energy in the range of the ${\mathit{M}}_{\mathrm{IV}}$ and ${\mathit{M}}_{\mathrm{V}}$ absorption edges (3.5 to 4 keV). Large enhancements are found at the M-edge binding energies, as is the case in uranium compounds. We have used the strong ${\mathit{M}}_{\mathrm{IV}}$ resonance (3.852 keV) to examine the magnetic struture of NpAs as a function of temperature. Differences between the phase diagram as studied previously by neutron diffraction and the present experiments may be ascribed to the near-surface sensitivity of x-ray scattering. In particular we observe some unusual ‘‘domain’’ effects that may be attributed to surface anisotropy. The higher resolution available in the x-ray experiments, as compared to the neutron experiments, has led to an understanding of the wave vector dependence as a function of temperature. We suggest that the change of the wave vector from its commensurate value of 1/4 (corresponding to a square wave 4+, 4- arrangement of magnetic moments) may be understood by introducing ‘‘defects’’ into the magnetic structure. We confirm the earlier work that showed the lattice initially distorted to tetragonal (from the NaCl fcc structure in the paramagnetic state) and reverted to cubic in the low-temperature triple-q antiferromagnetic phase.