Roger Ward

Abstract:

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 Å) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.

Abstract:

HoHy exhibits dramatic changes in both structural and optical properties as y is varied from zero to three by hydrogen loading. This work reports on the effect of such loading upon epitaxial single-crystal Ho films grown by molecular beam epitaxy on (110) Nb∥(112̄0) Al2O3 substrates. Upon loading, the film undergoes a structural transition from hcp metal (α-phase, H in solid solution) to the fcc dihydride (β-phase). There is a further transformation between the dihydride phase and the hexagonal trihydride (γ-phase). Three films of HoHy, where, nominally, y = 0, 2, 3, were studied by XRD, AFM, UFM, SEM and TEM. Triangular networks of features of width approximately 300 nm and height approximately 20-30 nm that align with the [01̄1], [1̄01] and [11̄0] directions of the dihydride sample are seen on the surface of both the dihydride and the trihydride samples, but have a much greater density on the surface of the latter. Such features are not present on the as-grown metal layer. In situ controlled-environment transmission electron microscopy (CETEM) studied the effects of hydrogen loading within the dihydride phase, in cross sectional geometry. During loading, slip within the dihydride phase was observed. The hypothesis that the triangular features are due to the precipitation of the di- and trihydride compounds, with expanded lattices, on the slip planes within the α- or β-phase host crystal is discussed.

Abstract:

Epitaxial thin-films of DyxPr1-x alloys have been studied using neutron diffraction and magnetization measurements. The crystal structure changes from HCP to Sm type to DHCP as x decreases; each crystal phase has different magnetic behaviour. Surprisingly, long-range order is suppressed in the DHCP alloys, a possible explanation is outlined.