Using polarized neutron reflectometry (PNR) and high angle neutron scattering from Fe/Cr(001) superlattices, we demonstrate how the non-collinear exchange coupling between the Fe layers is caused by a frustration between antiferromagnetic Cr domains. This induces a spiral modulation of the Cr not observed in bulk. PNR and magnetization measurements show that the noncollinear coupling vanishes above the Néel temperature of this commensurate Cr order. The results are consistent with a recent model for non-collinear exchange coupling over antiferromagnetic interlayers.

The solubility isotherms of hydrogen in epitaxial Nb͑110͒ films have been determined via in situ x-ray lattice-parameter measurements. The Nb films were grown by molecular-beam epitaxial techniques on Al 2 O 3 (1120) substrates with thicknesses ranging from 32 to 527 nm. All isotherms can be well described by the standard expression for the solubility of a monatomic lattice gas. Aside from the metal-hydrogen interaction, which appears to be bulklike, all other properties exhibit a strong thickness dependence, including the hydrogen-hydrogen interaction, the critical temperature for the ␣-␣Ј transition, and the maximum latticeparameter change that can be reached in saturation. For the critical temperature we find a scaling with the film thickness, which most likely is due to the elastic boundary condition provided by the rigid substrate. The clamping of the epitaxial film to the substrate hinders critical fluctuations of macroscopic hydrogen density modes from developing, thereby lowering the critical temperature for the ␣-␣Ј transition.

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We have studied the structural properties of Molecular Beam Epitaxy grown thin Co films on α-sapphire (112̄0) substrates by high resolution x-ray scattering measurements in the thickness range of 100 Å to 5000 Å. The electron density profile perpendicular to the film plane was determined by x-ray specular reflectivity measurements. The profile contains information about the total metal film thickness, an oxide layer on top of the film and about the surface and interface roughness. The total metal film thickness was compared with the number of coherently scattering atomic planes by taking radial scans through Bragg peaks parallel to the growth direction. Samples up to 400 Å total thickness (out-of-plane mosaicity 0.02○) show very strong Laue-oscillations about the Co (0002)/(111) Bragg peak. Above a total thickness of 400 Å the oscillations vanish due to a loss in structural coherence. The combination of specular reflectivity and Bragg scans presents a powerful means to analyze the structural coherence of thin films.

Platinum films in the thickness range 20-500 Å have been sputtered onto sapphire substrates at different substrate temperatures . The structural properties of these films have been studied by high-resolution x-ray scattering techniques. Information about the total thickness of the film and about the surface and interface roughnesses has been determined by means of x-ray specular reflectivity measurements. The films sputtered at have shown Laue oscillations about the Pt(111) Bragg peak. The average thickness of coherently stacked lattice planes has been compared with the total film thickness , obtained from the reflectivity measurements. The epitaxial relationship between the platinum film and the sapphire substrate has been determined using in-plane x-ray diffraction. It was demonstrated that Pt epitaxial films include two in-plane orientations related by a rotation. Both anisotropic compression in the direction and anisotropic expansion in the [111] direction were observed for these films. A correlated variation of some structural parameters with the substrate temperature was found.