We report x-ray synchrotron experiments on epitaxial films of uranium, deposited on niobium and tungsten seed layers. Despite similar lattice parameters for these refractory metals, the uranium epitaxial arrangements are different and the strains pro
pagated along the a-axis of the uranium layers are of opposite sign. At low temperatures these changes in epitaxy result in dramatic modifications to the behavior of the charge-density wave in uranium. The differences are explained with the current theory for the electron-phonon coupling in the uranium lattice. Our results emphasize the intriguing possibilities of producing epitaxial films of elements that have complex structures like the light actinides uranium to plutonium.
SQUID magnetometry and polarised neutron reflectivity measurements have been employed to characterise the magnetic properties of U/Fe, U/Co and U/Gd multilayers. The field dependence of the magnetisation was measured at 10K in magnetic fields from -7
0kOe to 70kOe. A temperature dependent study of the magnetisation evolution was undertaken for a selection of U/Gd samples. PNR was carried out in a field of 4.4kOe for U/Fe and U/Co samples (at room temperature) and for U/Gd samples (at 10K). Magnetic dead layers of about 15 Angstrom were observed for U/Fe and U/Co samples, consistent with a picture of interdiffused interfaces. A large reduction in the magnetic moment, constant over a wide range of Gd layer thicknesses, was found for the U/Gd system (about 4 Bohr magnetons compared with 7.63 for the bulk metal). This could be understood on the basis of a pinning of Gd moments arising from a column-like growth mechanism of the Gd layers. A study of the effective anisotropy suggests that perpendicular magnetic anisotropy could occur in multilayers consisting of thick U and thin Gd layers. A reduction in the Curie temperature was observed as a function of Gd layer thickness, consistent with a finite-size scaling behaviour.
This paper addresses the structural characterisation of a series of U/Fe, U/Co and U/Gd multilayers. X-ray reflectivity has been employed to investigate the layer thickness and roughness parameters along the growth direction and high-angle diffractio
n measurements have been used to determine the crystal structure and orientation of the layers. For the case of uranium/transition metal systems, the interfaces are diffuse and the transition metals are present in a polycrystalline form of their common bulk phases with a preferred orientation along the closest packed planes; Fe, bcc (110) and Co, hcp (001), respectively. The uranium is present in a poorly crystalline orthorhombic, alpha-U state. In contrast, the U/Gd multilayers have sharp interfaces with negligible intermixing of atomic species, and have a roughness, which is strongly dependent on the gadolinium layer thickness. Diffraction spectra indicate a high degree of crystallinity in both U and Gd layers with intensities consistent with the growth of a novel hcp U phase, stabilised by the hcp gadolinium layers.
