RIETVELD REFINEMENT OF MAGNETIC-STRUCTURES FROM PULSED-NEUTRON-SOURCEPOWDER-DIFFRACTION DATA

The General Structure Analysis System, GSAS, has recently been modifie d to include magnetic neutron-scattering cross-sections. Low-temperatu re diffraction data have been taken on the hexagonal noncollinear anti ferromagnet UPdSn on both the HIPD and the NPD powder diffractometers at LANSCE. The low-resolution data reveal that the magnetic structure has orthorhombic symmetry between 23 K and 37 K, and monoclinic symmet ry below 23 K. The high-resolution data reveal that there are structur al distortions with corresponding symmetry changes in each of these ph ases, while the paramagnetic phase above 37 K remains hexagonal. Using GSAS, we have refined data sets from both diffractometers simultaneou sly, including both magnetic and structural cross-sections. The magnet ic results from Rietveld refinement are in good agreement with model f itting to the integrated intensities of seven independent magnetic ref lections and these, in turn, agree with measurements made on the same sample using the constant-wavelength reactor technique.