Russell P. Cowburn

Domain wall propagation in magnetic nanowires by spin-polarized current injection

We demonstrate movement of a head-to-head domain wall through a magnetic nanowire of permalloy (Ni81Fe19) simply by passing an electrical current through the domain wall and without any external magnetic field applied. The effect depends on the sense and magnitude of the electrical current and allows direct propagation of domain walls through complex nanowire shapes, contrary to the case of magnetic-field–induced propagation. The efficiency of this mechanism has been evaluated and the effective force acting on the wall has been found equal to 0.44 × 10−9 N A−1.

Magneto-optical Kerr effect analysis of magnetic nanostructures

We present a treatment of polarized light analysis and noise sources relevant to magneto-optical Kerr effect (MOKE) measurements of magnetic nanostructures and give performance details of our state-of-the-art MOKE magnetometer in these terms. By considering the various signal contributions, we are able to determine experimental conditions for optimal MOKE signal dynamic range and signal-to-noise ratio. A description of our MOKE instrument includes a novel facility to locate magnetic nanostructures by producing a susceptibility map of the sample surface. The performance of the magnetometer is demonstrated by hysteresis loops of single 100–200 nm wide magnetic nanowires, including a loop obtained with no time averaging during a single magnetic field cycle.

Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures

The control of magnetic order in nanoscale devices underpins many proposals for integrating spintronics concepts into conventional electronics. A key challenge lies in finding an energy-efficient means of control, as power dissipation remains an important factor limiting future miniaturization of integrated circuits. One promising approach involves magnetoelectric coupling in magnetostrictive/piezoelectric systems, where induced strains can bear directly on the magnetic anisotropy. While such processes have been demonstrated in several multiferroic heterostructures, the incorporation of such complex materials into practical geometries has been lacking. Here we demonstrate the possibility of generating sizeable anisotropy changes, through induced strains driven by applied electric fields, in hybrid piezoelectric/spin-valve nanowires. By combining magneto-optical Kerr effect and magnetoresistance measurements, we show that domain wall propagation fields can be doubled under locally applied strains. These results highlight the prospect of constructing low-power domain wall gates for magnetic logic devices.

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 ratchet for three-dimensional spintronic memory and logic

One of the key challenges for future electronic memory and logic devices is finding viable ways of moving from today’s two-dimensional structures, which hold data in an x–y mesh of cells, to three-dimensional structures in which data are stored in an x–y–z lattice of cells. This could allow a many-fold increase in performance. A suggested solution is the shift register—a digital building block that passes data from cell to cell along a chain. In conventional digital microelectronics, two-dimensional shift registers are routinely constructed from a number of connected transistors. However, for three-dimensional devices the added process complexity and space needed for such transistors would largely cancel out the benefits of moving into the third dimension. ‘Physical’ shift registers, in which an intrinsic physical phenomenon is used to move data near-atomic distances, without requiring conventional transistors, are therefore much preferred. Here we demonstrate a way of implementing a spintronic unidirectional vertical shift register between perpendicularly magnetized ferromagnets of subnanometre thickness, similar to the layers used in non-volatile magnetic random-access memory. By carefully controlling the thickness of each magnetic layer and the exchange coupling between the layers, we form a ratchet that allows information in the form of a sharp magnetic kink soliton to be unidirectionally pumped (or ‘shifted’) from one magnetic layer to another. This simple and efficient shift-register concept suggests a route to the creation of three-dimensional microchips for memory and logic applications.

Domain wall pinning and potential landscapes created by constrictions and protrusions in ferromagnetic nanowires

The potential experienced by transverse domain walls (TDWs) in the vicinity of asymmetric constrictions or protrusions in thin Permalloy nanowires is probed using spatially resolved magneto-optical Kerr effect measurements. Both types of traps are found to act as pinning centers for DWs. The strength of pinning is found to depend on the trap type as well as on the chirality of the incoming DW; both types of traps are seen to act either as potential wells or potential barriers, also depending on the chirality of the DW. Micromagnetic simulations have been performed that are in good qualitative agreement with the experimental results.

Magnetic nanodots for device applications

Structuring magnetic materials on the nanometre scale leads to magnetic nanodots which may, in the future, find a wide variety of technological applications. Many of these applications require the nanodots to be in the single domain state, to have well-controlled aniosotropy and low hysteresis. In this paper, the physics which controls these aspects of magnetic behaviour on the nanometre scale is described, with particular reference to a phenomenon known as configurational anisotropy. These physical principles are illustrated by two potential applications of magnetic nanodots: high sensitivity magnetic field sensors and high density low power magnetic logic gates.

Domain wall diodes in ferromagnetic planar nanowires

We demonstrate a lithographically defined magnetic structure through which domain walls from planar magnetic nanowires propagate in one direction only, under an appropriate magnetic field. This domain wall diode is of the form of an isosceles triangle, with one nanowire emanating from its apex and one from its base. A domain wall arriving at the triangle apex, under an applied magnetic field, is able to overcome minor pinning through the diode and continue through the opposite nanowire. However, a domain wall arriving at the triangle base is unable to overcome the significant pinning energy presented by the sudden change in track width. Domain wall diodes are of potential use in controlling domain wall propagation for fundamental investigations and technological applications.

Domain wall injection and propagation in planar Permalloy nanowires

We have used high-sensitivity magneto-optics to study the magnetic switching of a single 20 μm long, 100 nm wide, 5 nm thick planar Permalloy wire made by focused ion beam milling. It is found that the switching field of the wire can be dramatically reduced by the addition of a large end pad to the wire, which serves as a domain wall injector. We show that once injected, the domain wall is able to move very freely along the wire. No measurable pinning of the domain wall was observed at a shallow kink in the middle of the length of the wire. We thus show that such wires can be considered as domain wall conduits, with many applications in magnetoelectronic devices.

Three dimensional magnetic nanowires grown by focused electron-beam induced deposition.

Control of the motion of domain walls in magnetic nanowires is at the heart of various recently proposed three-dimensional (3D) memory devices. However, fabricating 3D nanostructures is extremely complicated using standard lithography techniques. Here we show that highly pure 3D magnetic nanowires with aspect-ratios of ~100 can be grown using focused electron-beam-induced-deposition. By combining micromanipulation, Kerr magnetometry and magnetic force microscopy, we determine that the magnetisation reversal of the wires occurs via the nucleation and propagation of domain walls. In addition, we demonstrate that the magnetic switching of individual 3D nanostructures can be directly probed by magneto-optical Kerr effect.