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.
This article reports a detailed x-ray resonant scattering study of the bilayer iridate compound, Sr3Ir2O7, at the Ir L2 and L3 edges. Resonant scattering at the Ir L3 edge has been used to determine that Sr3Ir2O7 is a long-range ordered antiferromagn
et below TN 230K with an ordering wavevector, q=(1/2,1/2,0). The energy resonance at the L3 edge was found to be a factor of ~30 times larger than that at the L2. This remarkable effect has been seen in the single layer compound Sr2IrO4 and has been linked to the observation of a Jeff=1/2 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the programme SARAh, we have determined that the magnetic order consists of two domains with propagation vectors k1=(1/2,1/2,0) and k2=(1/2,-1/2,0), respectively. A raster measurement of a focussed x-ray beam across the surface of the sample yielded images of domains of the order of 100 microns size, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations (FDMNES), using the Greens function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L2,3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard, U, term. A large L3 to L2 edge intensity ratio (~5) was found for calculations including spin-orbit coupling. Adding the Hubbard, U, term resulted in changes to the intensity ratio <5%.
Recent calculations, concerning the magnetism of uranium in the U/Fe multilayer system have described the spatial dependence of the 5f polarization that might be expected. We have used the x-ray resonant magnetic reflectivity technique to obtain the
profile of the induced uranium magnetic moment for selected U/Fe multilayer samples. This study extends the use of x-ray magnetic scattering for induced moment systems to the 5f actinide metals. The spatial dependence of the U magnetization shows that the predominant fraction of the polarization is present at the interfacial boundaries, decaying rapidly towards the center of the uranium layer, in good agreement with predictions.
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.
