In this review we discuss conventional methods of performing biological assays and molecular identification and highlight their advantages and limitations. An alternative approach based on magnetic nanotechnology is then presented. Firstly, magnetic carriers are introduced and their biocompatibility and functionalisation discussed, with spotlights on functionalisation via self assembled monolayers and on methods of reducing nonspecific binding. In addition an introduction is provided to the basic physical concepts behind the various types of sensors used to detect magnetic labels. Finally, progress in the field of magnetic biosensors and the outlook for the future are discussed.
Ferromagnetic metal rings of nanometre range widths and thicknesses exhibit fundamentally new spin states, switching behaviour and spin dynamics, which can be precisely controlled via geometry, material composition and applied f eld. Following the discovery of the 'onion state', which mediates the switching to and between vortex states, a range of fascinating phenomena has been found in these structures. In this overview of our work on ring elements, we f rst show how the geometric parameters of ring elements determine the exact equilibrium spin conf guration of the domain walls of rings in the onion state, and we show how such behaviour can be understood as the result of the competition between the exchange and magnetostatic energy terms. Electron transport provides an extremely sensitive probe of the presence, spatial location and motion of domain walls, which determine the magnetic state in individual rings, while magneto-optical measurements with high spatial resolution can be used to probe the switching behaviour of ring structures with very high sensitivity. We illustrate how the ring geometry has been used for the study of a wide variety of magnetic phenomena, including the displacement of domain walls by electric currents, magnetoresistance, the strength of the pinning potential introduced by nanometre size constrictions, the effect of thermal excitations on the equilibrium state and the stochastic nature of switching events.
A lab-on-a-chip integrated microfluidic cell has been developed for magnetic biosensing, which is comprised of anisotropic magnetoresistance (AMR) sensors optimized for the detection of single magnetic beads and electrodes to manipulate and sort the beads, integrated into a microfluidic channel. The device is designed to read out the real-time signal from 9μm diameter magnetic beads moving over AMR sensors patterned into 18×4.5μm rectangles and 10μm diameter rings and arranged in Wheatstone bridges. The beads are moved over the sensors along a 75×75μm wide channel patterned in SU8. Beads of different magnetic moments can be sorted through a magnetostatic sorting gate into different branches of the microfluidic channel using a magnetic field gradient applied by lithographically defined 120nm thick Cu striplines carrying 0.2A current.
We present a magnetic multiplexed assay technology which encodes the identities of target biomolecules according to the moment of magnetic beads to which they are attached. An active digital technique based on a microfabricated magnetoresistive ring-shaped sensor is demonstrated, which can distinguish the magnetic moments of micron-sized superparamagnetic beads. We propose that this development is key to combining nonvolatile magnetic labeling with biochemical libraries for high-throughput bioassays and rapid multiplexed detection.
Arrays of interacting 2D nanomagnets display unprecedented electromagnetic properties via collective effects, demonstrated in artificial spin ices and magnonic crystals. Progress toward 3D magnetic metamaterials is hampered by two challenges: fabricating 3D structures near intrinsic magnetic length scales (sub-100 nm) and visualizing their magnetic configurations. Here, we fabricate and measure nanoscale magnetic gyroids, periodic chiral networks comprising nanowire-like struts forming three-connected vertices. Via block copolymer templating, we produce Ni75Fe25 single-gyroid and double-gyroid (an inversion pair of single-gyroids) nanostructures with a 42 nm unit cell and 11 nm diameter struts, comparable to the exchange length in Ni–Fe. We visualize their magnetization distributions via off-axis electron holography with nanometer spatial resolution and interpret the patterns using finite-element micromagnetic simulations. Our results suggest an intricate, frustrated remanent state which is ferromagnetic but without a unique equilibrium configuration, opening new possibilities for collective phenomena in magnetism, including 3D magnonic crystals and unconventional computing.
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