Roger Ward

Abstract:

Multilayer samples of Ce/Fe, fabricated by sputtering and characterized by X-ray diffraction and reflectivity were studied by polarized neutron reflectivity and Mössbauer spectroscopy. Corresponding studies on sputtered U/Fe multilayers have been initiated. In Fe/Ce a paramagnetic phase, identified as amorphous Fe was observed at Fe thicknesses below 20 Å. Perpendicular magnetic anisotropy was observed for Fe thicknesses below 17 Å and studies in applied fields enabled magnetic anisotropies to be evaluated for Fe thicknesses up to 40 Å. The Ce/Fe interface anisotropy was evaluated as Ks = (0.98 ± 0.15) × 10^-3 Jm^-2, among the largest values reported. Neutron reflectivity results did not observe magnetic moments on Ce layers. Preliminary measurements on U/Fe multilayers have not observed induced moments on the U layer.

Abstract:

We present some universal results for exchange springs in multilayer systems with alternating hard and soft layers. In particular, the reduced magnetisation 〈cosθ〉, for both symmetric exchange springs (0→φ→0) and 180° exchange springs (0→π), is given in terms of elliptic functions. The result for the symmetric exchange spring is found to be in good accord with experimental results on MBE grown DyFe2/YFe2 multilayers. © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

X-ray magnetic resonant scattering studies of Nd/Pr superlattices have allowed the magnetic ordering in both contstituents of the superlattice to be studied seperately. However, the resonances at the L edges were found to be anomalous, and the aim of this experiment was to understand the lineshape of these energy dependencies. The dominant features, a large peak at the absorption edge and a high-energy shoulder, were found to be due to a dipolar transition to a broad 5d band © 2002 Published by Elsevier Science B.V.

Abstract:

Structural and magnetic characterizations have been performed on NiFe-FeMn exchange-biased bilayers grown by molecular beam epitaxy. The growth of the films showed high-quality epitaxy with well-defined flat interface between the layers. Magnetic measurements confirmed that the films showed not only a comparable exchange anisotropy field (Hex) to that of sputtered films but also a lowered threshold thickness of 2 nm at which Hex vanishes for the FeMn.

Abstract:

Epitaxial thin films of dysprosium-praseodymium and erbium-praseodymium alloys have been studied using neutron and x-ray diffraction. The magnetic and structural properties have been determined for DyxPr1-x for 1.0 ≥ x ≥ 0.5 and for Er0.5Pr0.5. The crystal structure is hexagonal close-packed (hcp) for 1.0 ≥ x ≥ 0.6 and changes to the Sm type for x ∼ 0.5. The hcp samples form basal-plane helices with a transition to ferromagnetism accompanied by a discontinuity in the lattice parameter c. The alloys with the Sm structure form commensurate magnetic structures similar to that of Sm metal with anisotropy dictated by the heavy-rare-earth constituent. The changes in transition temperatures, magnetoelastic effects and modulation wave vector with composition have been studied. The relationships between change in ordering wave vector and magnetization and between transition temperature and the de Gennes factor of the constituents have been established for light-heavy-rare-earth systems.