Roger Ward

Abstract:

NiFe/FeMn exchange-biased thin films were grown in an applied magnetic field of approximately 200 Oe using molecular beam epitaxy. In order to obtain the optimum conditions for the epitaxial growth of the thin films, various chemical treatments of the Si(1 1 1) substrates and insertion of a Cu buffer layer were used. The 7 × 7 surface reconstruction of Si(1 1 1) was found out to be the crucial factor for the epitaxial growth. This surface promotes the reaction of Si with the Cu buffer layer to form epitaxial η″-Cu3Si which then allows good epitaxy of NiFe and FeMn. In situ reflection high-energy electron diffraction and high resolution electron microscopy observation confirmed the epitaxial growth of the films to be of high quality with well defined flat interface between the layers. © 2002 Elsevier Science Ltd. All rights reserved.

Abstract:

We present the results of muon-spin-relaxation experiments for two materials which show geometric frustration. ZnCr22O4 has a spinel structure with S = 3/2 spins on a lattice of corner-sharing tetrahedra. Our experiments show that a local magnetic field which is quasi-static on the muon timescale develops below Tc = 12.5 K, a transition which has been associated with a three-dimensional analogue of the spin-Peierls transition. In contrast, Gd3Ga5O12 has a garnet structure with S = 7/2 spins arranged on interpenetrating triangular sublattices. In this material the muon data exhibit a temperature-dependent spin-relaxation rate indicative of slow spin fluctuations. We discuss these differing behaviours and relate them to the underlying physics in the two materials.

Abstract:

The detailed mechanisms of the structural phase transformations that occur in epitaxial Y-hydride switchable mirrors are revealed with high resolution transmission electron microscopy (both cross sectional and plan view). The triangular ridge network that develops in Y prior to the α-β transformation is a result of 101̄2 deformation twinning. The basal plane that is originally parallel to the film/substrate interface is rotated by twinning over 5.6° and transformed into a prismatic plane and similarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to β, nearly vertical Σ3111 twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the β domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible β-γ switching of the mirror. © 2002 American Institute of Physics.

Abstract:

We have performed room temperature Conversion Electron Mössbauer Spectroscopy (CEMS) on thin films and multilayers of DyFe2, YFe2 and HoFe2 under applied magnetic fields of 0 T and 0.25 T in plane. The samples were constructed by M.R. Wells and R.C.C. Ward using Molecular Beam Epitaxy on sapphire substrates giving single crystals with the [110] plane parallel to the surface. The samples produced and studied are: thin films of DyFe2, YFe2 and HoFe2; multilayers of DyFe2/YFe2 with respective layer thicknesses of 50 Å/50 Å, 40 Å/20 Å, 20 Å/40 Å and 20 Å/20 Å. In all spectra the fits obtained are different to those found in bulk samples: all the iron sites in the cubic Laves Phase unit cell are magnetically inequivalent and the spins on the iron atoms always have a component perpendicular to the layers, With a field of 0.25 T applied in the plane of the sample the iron spins in the YFe2 thin film are rotated into the plane by a large amount (8.5°) compared to only a minor change in the DyFe2 thin film (1°). In multilayer systems the iron atom spins in the YFe2 layers appear to be locked to those in the DyFe2 layers.

Abstract:

A [Gd30/La10]60 superlattice has been studied using SQUID magnetometry and polarised neutron reflectivity. Zero-field cooling results in the coexistence of an antiferromagnetic alignment of the ferromagnetic Gd blocks, and 3D superconductivity at low temperature. Field cooling from room temperature results in ferromagnetic coupling between the Gd blocks, and under these conditions, the superconducting transition in the La is suppressed. © 2002 Elsevier Science B.V. All rights reserved.