Matthew T. Bryan

Forgery: ‘Fingerprinting’ documents and packaging

We have found that almost all paper documents, plastic cards and product packaging contain a unique physical identity code formed from microscopic imperfections in the surface. This covert ‘fingerprint’ is intrinsic and virtually impossible to modify controllably. It can be rapidly read using a low-cost portable laser scanner. Most forms of document and branded-product fraud could be rendered obsolete by use of this code.

Magnetic domain wall propagation in nanowires under transverse magnetic fields

We have investigated the propagation of transverse domain walls in magnetic nanowires under axial and transverse magnetic fields using three-dimensional micromagnetic modeling. Transverse magnetic fields change the domain wall width and, below the Walker field, either increase or decrease the domain wall velocity depending when the field and wall magnetization are parallel or antiparallel, respectively. Furthermore, differences in the Walker field also appear for opposite transverse fields, and a surprising result is that under relatively high axial and transverse fields, Walker breakdown can be completely suppressed and the domain wall velocity returns to several hundreds of ms−1.

Mechanoresponsive Networks Controlling Vascular Inflammation

Atherosclerosis is a chronic inflammatory disease of arteries that develops preferentially at branches and bends that are exposed to disturbed blood flow. Vascular function is modified by flow, in part, via the generation of mechanical forces that alter multiple physiological processes in endothelial cells. Shear stress has profound effects on vascular inflammation; high uniform shear stress prevents leukocyte recruitment to the vascular wall by reducing endothelial expression of adhesion molecules and other inflammatory proteins, whereas low oscillatory shear stress has the opposite effects. Here, we review the molecular mechanisms that underpin the effects of shear stress on endothelial inflammatory responses. They include shear stress regulation of inflammatory mitogen-activated protein kinase and nuclear factor-κB signaling. High shear suppresses these pathways through the induction of several negative regulators of inflammation, whereas low shear promotes inflammatory signaling. Furthermore, we summarize recent studies indicating that inflammatory signaling is highly sensitive to pulse wave frequencies, magnitude, and direction of flow. Finally, the importance of systems biology approaches (including omics studies and functional screening) to identify novel mechanosensitive pathways is discussed.

Symmetric and asymmetric domain wall diodes in magnetic nanowires

Micromagnetic simulations reveal how transverse domain walls couple with triangular diodes in magnetic nanowires. For symmetric diodes, the coupling explains the observed differences in the magnetic field required to depin domain walls traveling in opposite directions. In asymmetric diodes, the wall-triangle interaction can lead to order-of-magnitude differences in the depinning fields of oppositely magnetized walls traveling in the same direction. The asymmetric structures therefore combine the diode function of the symmetric structures with domain wall chirality filtering. We also show how two back-to-back diodes may be used to trap a domain wall and form a memory element.

Stress-based control of magnetic nanowire domain walls in artificial multiferroic systems

Artificial multiferroic systems, which combine piezoelectric and piezomagnetic materials, offer novel methods of controlling material properties. Here, we use combined structural and magnetic finite element models to show how localized strains in a piezoelectric film coupled to a piezomagnetic nanowire can attract and pin magnetic domain walls. Synchronous switching of addressable contacts enables the controlled movement of pinning sites, and hence domain walls, in the nanowire without applied magnetic field or spin-polarized current, irrespective of domain wall structure. Conversely, domain wall-induced strain in the piezomagnetic material induces a local potential difference in the piezoelectric, providing a mechanism for sensing domain walls. This approach overcomes the problems in magnetic nanowire memories of domain wall structure-dependent behavior and high power consumption. Nonvolatile random access or shift register memories based on these effects can achieve storage densities >1 Gbit/In2, sub-10...

Pinning induced by inter-domain wall interactions in planar magnetic nanowires

Pinning Induced by Inter-Domain Wall Interactions in Planar Magnetic Nanowires T.J. Hayward 1 , M.T. Bryan 1 , P.W. Fry 2 , P.M. Fundi 1 , M.R.J. Gibbs 1 , D.A. Allwood 1 , M.-Y. Im 3 and P. Fischer 3 Department of Engineering Materials, University of Sheffield, Sheffield, UK Nanoscience and Technology Centre, University of Sheffield, Sheffield UK Center for X-ray Optics, Lawrence Berkeley Natl Lab, Berkeley, CA, USA PACS: 07.85.Tt, 75.60.Ch, 75.75.+a, 85.70.Kh We have investigated pinning potentials created by inter-domain wall magnetostatic interactions in planar magnetic nanowires. We show that these potentials can take the form of an energy barrier or an energy well depending on the walls’ relative monopole moments, and that the applied magnetic fields required to overcome these potentials are significant. Both transverse and vortex wall pairs are investigated and it is found that transverse walls interact more strongly due to dipolar coupling between their magnetization structures. Simple analytical models which allow the effects of inter- domain wall interactions to be estimated are also presented. There is great interest in developing memory [1] and logic [2] devices based upon the controlled motion and interaction of domain walls (DWs) in ferromagnetic planar nanowires. Such domain walls have particle-like properties which allow them to be propagated around complex circuits using rotating magnetic fields [3,4] or short electric current pulses [5], and hence they may be used to represent binary data in a similar way to electric charge in conventional microelectronics. DWs in planar magnetic nanowires have head-to-head (H2H) or tail-to-tail (T2T) character (Fig 1(a)), and consequently they carry a net monopole moment (i.e. a localised excess of north (H2H) or south (T2T) magnetic poles). Therefore, to a first approximation DWs in adjacent nanowires will interact via a Coulomb-like potential: if the DWs have like monopole moments there will be a repulsive interaction, whereas if they have opposite monopole moments their interaction will be attractive. Understanding these effects and how they affect DW propagation is likely to be important to the development of DW based devices, where large nanowire densities will be desirable. So far there have been relatively few investigations into these effects, with studies characterizing attractive coupling between walls with opposite monopole moments for a limited range of nanowire geometries and DW structures [6,7]. We have also previously demonstrated that DW interaction energies are dependent to some degree on the DWs magnetization structure and chirality [8].

The effect of trapping superparamagnetic beads on domain wall motion

Domain walls may act as localized field sources to trap and move superparamagnetic beads for manipulating biological cells and DNA. The interaction between beads of various diameters and a wall is investigated using a combination of micromagnetic and analytical models. Domain walls can transport beads under applied magnetic fields but the mutual attraction between the bead and wall causes drag forces affecting the bead to couple into the wall motion. Therefore, the interaction with the bead causes a fundamental change in the domain wall dynamics, reducing the wall mobility by five orders of magnitude.

Switchable Cell Trapping Using Superparamagnetic Beads

Ni81Fe19 microwires are investigated as the basis of a switchable template for positioning magnetically labeled neural Schwann cells. Magnetic transmission X-ray microscopy and micromagnetic modeling show that magnetic domain walls can be created or removed in zigzagged structures by an applied magnetic field. Schwann cells containing superparamagnetic beads are trapped by the field emanating from the domain walls. The design allows Schwann cells to be organized on a surface to form a connected network and then released from the surface if required. As aligned Schwann cells can guide nerve regeneration, this technique is of value for developing glial-neuronal coculture models in the future treatment of peripheral nerve injuries.

Transverse and vortex domain wall structure in magnetic nanowires with uniaxial in-plane anisotropy

Micromagnetic and analytical models are used to investigate how in-plane uniaxial anisotropy affects transverse and vortex domain walls in nanowires where shape anisotropy dominates. The effect of the uniaxial anisotropy can be interpreted as a modification of the effective wire dimensions. When the anisotropy axis is aligned with the wire axis (θ(a) = 0), the wall width is narrower than when no anisotropy is present. Conversely, the wall width increases when the anisotropy axis is perpendicular to the wire axis (θ(a) = π/2). The anisotropy also affects the nanowire dimensions at which transverse walls become unstable. This phase boundary shifts to larger widths or thicknesses when θ(a) = 0, but smaller widths or thicknesses when θ(a) = π/2.

A sound idea: Manipulating domain walls in magnetic nanowires using surface acoustic waves

We propose a method of pinning and propagating domain walls in artificial multiferroic nanowires using electrically induced surface acoustic waves. Using finite-element micromagnetic simulations and 1D semi-analytical modelling, we demonstrate how a pair of interdigitated acoustic transducers can remotely induce an array of attractive domain wall pinning sites by forming a standing stress/strain wave along a nanowires length. Shifts in the frequencies of the surface acoustic waves allow multiple domain walls to be synchronously transported at speeds up to 50 ms−1. Our study lays the foundation for energy-efficient domain wall devices that exploit the low propagation losses of surface acoustic waves to precisely manipulate large numbers of data bits.