Low-temperature X-ray diffraction

The main aim of this thesis is to measure the distortion of the crystallographic unit cell due to the onset of magnetic order. After measuring the standard material, tungsten, which was used as a calibration material, a number of measurements was performed on rare earth metallic compounds crystallizing in the cubic MgCu2-structure. Two compounds, YCo2, and YAl2, both of which show no magnetic order down to lowest temperatures were investigated for the purpose of comparison. The lattice parameters of these two compounds show a monotonic decrease with decreasing temperature. In contrast to such a behaviour the lattice parameters of NdCO2 and HoCo2, are characterized by a distortion which appears below the magnetic ordering temperature. Below the magnetic ordering temperature the cubic unit cell shows a tetrahedral distortion in both cases which is followed by an orthorhombic distortion below the spin-reorientation temperature, Tsub(R) (Tsub(R) = 42 K in case of NdCo2 and Tsub(R) = 15 K in case of HoCo2). The type and magnitude of these distortions was calculated from the size and magnitude of corresponding X-ray lines. Further measurements were also performed on UMn2, which yields a pronounced monotonic distortion of the MgCu2-unit cell although no magnetic order appears down to He-temperature. (Author)