Hydrogen in yttrium is of fundamental interest as a model system for driving metal-insulator transitions including switchable optical properties from reflecting to transparent in the visible region. We report on the structural properties of hydrogenated and deuterated thin, monocrystalline Y͑0001͒ films grown by molecular beam epitaxy on Nb/Al 2 O 3 substrates. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The structural coherence in all three spatial directions as well as the epitaxial relation to the substrate are maintained, though the sample undergoes structural phase transitions between the different hydride phases. With neutron reflectivity measurements we have determined the hydrogen and deuterium content since the critical angle for total reflection depends on their concentration in the sample. Measurements on hydrogenated and deuterated films show that H and D are completely interchangeable within the trihydride phase. Neutron scattering also allows us to determine the position of the deuterium atoms within the yttrium matrix. All structural information gained on thin films is in agreement with the space group P3 c1 which was previously determined from powder samples.

We present an x-ray scattering study of the continuous B2-A2 order-disorder transition in semi-infinite FeCo using synchrotron radiation, which gives access to the weak one-electron contrast associated with the B2 ordering phenomena in this alloy. The asymptotic (001) Bragg scattering profiles disclose an oscillating Fe-Co layering profile, which alters the surface critical phenomena in a characteristic way: The surface-related order parameter persists above the bulk critical temperature and exhibits a temperature dependence which provides unambiguous evidence for the existence of a surface field ͑h 1 ͒ which couples to the surface susceptibility. [S0031-9007(97)03171-2]

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The magnetic splitting of Delta(2) valence states in the heavy lanthanide metals Gd, Tb, Dy, and Ho was studied in epitaxial films by angle-resolved photoemission, revealing an essentially Stoner-like temperature dependence in all cases. It scales linearly with the 4f spin moment, even in the case of the helical antiferromagnet Ho. Such a behavior can be explained by a substantial localization of the corresponding wave function in the c direction. The helical magnetic structure was confirmed for the thin Ho films by in situ resonant magnetic x-ray diffraction.

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