F. Y. Ogrin

Spectroscopy on the wing: Naturally inspired SERS substrates for biochemical analysis

We show that naturally occurring chitinous nanostructures found on the wings of the Graphium butterfly can be used as substrates for surface-enhanced Raman scattering when coated with a thin film of gold or silver. The substrates were found to exhibit excellent biocompatibility and sensitivity, making them ideal for protein assaying. An assay using avidin/biotin binding showed that the substrates could be used to quantify protein binding directly from changes in the surface-enhanced Raman scattering (SERS) spectra and were sensitive over a concentration range comparable with a typical enzyme-linked immunosorbent assays (ELISA) assay. A biomimetic version of the wing nanostructures produced using a highly reproducible, large-scale fabrication process, yielded comparable enhancement factors and biocompatibility. The excellent biocompatibility of the wings and biomimetic substrates is unparalleled by other lithographically produced substrates, and this could pave the way for widespread application of ultrasensitive SERS-based bioassays.

Direct Observation of the Flux-Line Vortex Glass Phase in a Type II Superconductor

The order of the vortex state in La1.9Sr0.1CuO4 is probed using muon-spin rotation and small-angle neutron scattering. A transition from a Bragg glass to a vortex glass is observed, where the latter is composed of disordered vortex lines. In the vicinity of the transition the microscopic behavior reflects a delicate interplay of thermally induced and pinning-induced disorder.

Investigation of perpendicular anisotropy of a thin film using the planar Hall effect

We have used the planar Hall effect (PHE) with the applied field perpendicular to the surface of the plane in order to investigate the perpendicular anisotropy of a thin Co film. To characterise the perpendicular anisotropy we measured the field dependence of the PHE at a fixed angle and the angular dependence of the PHE at a fixed field. The latter dependence was fitted using a single-domain model. The effective perpendicular anisotropy was extracted and compared with that extracted by fitting torque measurements on the same film.

High field ferromagnetic resonance measurements of the anisotropy field of longitudinal recording thin-film media

The average value of the magnetocrystalline anisotropy field, Hk, is an important parameter for the characterization of magnetic recording media but is difficult to measure accurately due in part to the effect of interactions between the grains. In order to evaluate Hk we have studied two model CoCrPtTa magnetic films using a number of complementary techniques: high field ferromagnetic resonance (FMR) (35.0–45.0 kOe), low field (<20 kOe) vector vibrating-sample magnetometry and torque magnetometry. The FMR measurements were performed at a number of discrete frequencies in the range 75–93 GHz using a new quasi-optical spectrometer developed at the University of St. Andrews. The values of Hk derived by FMR (10.8 kOe) are approximately 10% greater than those obtained from conventional magnetometry (9.6 kOe). This difference is believed to be due to the presence of intergranular exchange coupling which reduces the measured value of anisotropy when the applied field is not sufficiently large to completely alig...

Magnetic imaging by x-ray holography using extended references.

We demonstrate magnetic lensless imaging by Fourier transform holography using extended references. A narrow slit milled through an opaque gold mask is used as a holographic reference and magnetic contrast is obtained by x-ray magnetic circular dichroism. We present images of magnetic domains in a Co/Pt multilayer thin film with perpendicular magnetic anisotropy. This technique holds advantages over standard Fourier transform holography, where small holes are used to define the reference beam. An increased intensity through the extended reference reduces the counting time to record the farfield diffraction pattern. Additionally it was found that manufacturing narrow slits is less technologically demanding than the same procedure for holes. We achieve a spatial resolution of ∼30 nm, which was found to be limited by the sample period of the chosen experimental setup.

Small angle x-ray and neutron scattering study of disordered and three dimensional–ordered magnetic protein arrays

The magnetic nanoparticles of Fe3O4-γ–Fe2O3 grown inside the cavity of globular proteins (apoferritin)-magnetoferritin proved to be a useful model system for studying the fundamental effects of magnetostatic interactions in nanoparticle assemblies. In this work the main focus is on structural characterization of such new nanocomposites by small angle x-ray scattering (SAXS) and small angle neutron scattering to evaluate interparticle separation (center to center) in two types of assemblies: three dimensional periodic arrays and disordered (amorphous) assemblies. Straightforward analysis of the face-centered cubic pattern of periodic arrays revealed that the interparticle spacing is 9.9 nm, whereas the SAXS pattern of disordered assembly reveals three correlation lengths, one of which is 10.5 nm and corresponds to the interparticle (center-to-center) nearest neighbor distance. The magnetic behaviors of the two systems are distinctly different. Given that the interparticle separation differs by only ∼0.6 nm...

FMR studies of cobalt thin films deposited by MOCVD

Abstract Ferromagnetic resonance has been used to investigate the magnetic anisotropy in thin films of cobalt and Co/ZnO multilayers deposited by MOCVD. In general, the samples exhibited an overall in-plane magnetic anisotropy, the strength of which decreased with decreasing cobalt layer thickness. A buffer layer of ZnO was found to decrease the strength of the anisotropy with respect to a film of the same thickness deposited onto glass. In some relatively thin single films, two resonance modes were observed. This was also the case in multilayers deposited onto a ZnO buffer layer. The origins of these modes are discussed.

Hollow carbon spheres in microwaves: Bio inspired absorbing coating

The electromagnetic response of a heterostructure based on a monolayer of hollow glassy carbon spheres packed in 2D was experimentally surveyed with respect to its response to microwaves, namely, the Ka-band (26–37 GHz) frequency range. Such an ordered monolayer of spheres mimics the well-known “moth-eye”-like coating structures, which are widely used for designing anti-reflective surfaces, and was modelled with the long-wave approximation. Based on the experimental and modelling results, we demonstrate that carbon hollow spheres may be used for building an extremely lightweight, almost perfectly absorbing, coating for Ka-band applications.

Asymmetry of spin wave dispersions in a hexagonal magnonic crystal

We report a study of the dispersion of spin waves in a hexagonal array of interacting ferromagnetic nanodisks for two orthogonal orientations of the in-plane applied magnetic field, i.e., either parallel or perpendicular to the direction of first neighbour disks. The experimental data were modelled using the dynamical matrix method, and the results were interpreted in terms of the effective wave vector model. We have found that spin waves propagating in the two orthogonal directions exhibit marked asymmetry concerning the existence of maxima/minima in their dispersion curves and the sign of their group velocities.

Ferromagnetic Resonance Investigation of Macroscopic Arrays of Magnetic Nanoelements Fabricated Using Polysterene Nanosphere Lithographic Mask Technique

A dense plane periodical array of cylindrical magnetic nanodots has been fabricated using a lithographic mask formed by self-organization of polystyrene nanospheres. In this paper, we study collective static and dynamic magnetic behavior of this array. We find that this technique produces samples with reasonably small dispersion of magnetic parameters of individual dots. This is evidenced by magnetometry and well-resolved discrete frequencies of standing spin waves measured with cavity and coplanar-waveguide ferromagnetic resonance. The standing spin wave resonances could be reliably observed in a large range of frequencies (4-15 GHz). However the measured linewidth of resonances is about ten times larger than for unpatterned Permalloy. This may be due to some variation in shape and magnetic parameters from dot to dot resulting in inhomogeneous broadening of the resonance lines.