Roger Ward

Abstract:

A simple method for characterizing spring-type magnets is proposed by using a standard magneto-optical Kerr effect (MOKE) rig. As we previously reported, a spiral structure with parallel field (longitudinal) and normal field (transverse) components can easily develop in Laves-phase spring-type superlattice magnets once an external field is applied. Based on a phenomenological formula both longitudinal and transverse components in the exchange-springs can be probed simultaneously by MOKE. Out finding shows that not only the Kerr angle but the reflectivity change induced from transverse components can be used as good indicators for the formation of spiral structure in the spring-magnet. © 2013 Elsevier B.V.

Abstract:

We report on the positive magnetoresistance (MR) that accompanies the onset of fan-type magnetic structures in an epitaxially grown Dy/Y superlattice (SL). We find that MR ratios for current perpendicular to plane (CPP) and in-plane (CIP) geometries yield comparable values and a similar nonmonotonic dependence on the applied magnetic field H and temperature, which result in a maximum MR∼0.45% at T=110 K. We demonstrate that the rise in resistance is due to the increase in the number of superzone band gaps that accompanies the magnetization process of the anisotropic spin-spiral magnet, which reflects on the increasing complexity of the field-induced modulated magnetic structures. Furthermore, we find that the suppression of the giant MR in Dy/Y SL is due mainly to the combined action of chemical modulation and epitaxial strain effects. Finally, MR presents a linear (negative slope) and unsaturated scaling with H in the magnetized state, which is a clear fingerprint of magnon MR in highly anisotropic magnetic materials. © 2013 American Physical Society.

Abstract:

Thin films (250-4500 Å) of epitaxial UO2 were produced by reactive sputtering on two different substrate materials: LaAlO3 and CaF2. Using the large enhancement present with resonant x-ray scattering using photons at the uranium M4 absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature TN is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order bulk transition. For LaAlO3-based films, an additional strong diffuse magnetic disorder is observed, which is reminiscent of the second-length scale, associated with structural disorder and/or strain. By using a formulation accounting for the strong absorption and coherent nature of the photons, the energy widths at the U M4 resonances can be related to the thickness of the AF region. The LaAlO3-based films do not order magnetically over more than ∼600 Å, whereas the CaF2-based film orders throughout. Further, for thicker films (>1000 Å) the fitting procedure shows that the AF order is located at the top of the LaAlO3-based film. This points to the formation in thicker films of a nonmagnetic layer of UO2 adjacent to the substrate, which may have tetragonal symmetry. © 2013 American Physical Society.

Abstract:

We have determined the magnetic structures of single-crystal thin-films of IrMn 3 for the crystallographic phases of chemically-ordered L1 2, and for chemically-disordered face-centred-cubic, which is the phase typically chosen for information-storage devices. For the chemically-ordered L1 2 thin-film, we find the same triangular magnetic structure as reported for the bulk material. We determine the magnetic structure of the chemically-disordered face-centred-cubic alloy for the first time, which differs from theoretical predictions, with magnetic moments tilted away from the crystal diagonals towards the face-planes. We study the influence of these two antiferromagnetic structures on the exchange-bias properties of an epitaxial body-centred-cubic Fe layer showing that magnetization reversal mechanism and bias-field in the ferromagnetic layer is altered significantly. We report a change of reversal mechanism from in-plane nucleation of 90domain-walls when coupled to the newly reported cubic structure towards a rotational process, including an out-of-plane magnetization component when coupled to the L1 2 triangular structure.

Abstract:

Ar+ ions have been implanted into Laves phase epitaxial thin films of YFe2 and DyFe2. Magneto-optical Kerr effect and vibrating sample magnetometry experiments show that the easy and hard axes of magnetization in both materials rotate through an in-plane angle of 90°, whilst the strength of the magnetic anisotropy remains unaltered. This is supported by OOMMF computational modelling. Atomic force microscopy confirms that the film roughness is not affected by implanted ions. X-ray diffraction data show that the lattice parameter expands upon ion implantation, corresponding to a release of strain throughout the entire film following implantation with a critical fluence of 1017 Ar+ ions cm-2. The anisotropy of the films is linked to the strain and from these data it is concluded that the source of anisotropy alters from one where magnetoelastic and magnetocrystalline effects compete to one which is governed solely by magnetocrystalline effects. The ability to locally tune the source of magnetic anisotropy without affecting the film surface and without inducing or eliminating anisotropy could be important in the fabrication of high density magnetic data storage media, spintronic devices and magneto-optical materials. © 2013 IOP Publishing Ltd.