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Dive into the research topics where D. A. Allwood is active.

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Featured researches published by D. A. Allwood.


EPL | 2004

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

Nicolas Vernier; D. A. Allwood; D. Atkinson; M.D. Cooke; Russell P. Cowburn

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.


Applied Physics Letters | 2004

Domain wall diodes in ferromagnetic planar nanowires

D. A. Allwood; Gang Xiong; Russell P. Cowburn

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.


Journal of Applied Physics | 2002

Domain wall injection and propagation in planar Permalloy nanowires

Russell P. Cowburn; D. A. Allwood; Gang Xiong; M.D. Cooke

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.


Applied Physics Letters | 2001

Magnetic nanoelements for magnetoelectronics made by focused-ion-beam milling

Gang Xiong; D. A. Allwood; M.D. Cooke; Russell P. Cowburn

Focused-ion-beam (FIB) milling has been used to structure magnetic nanoelements from 5 nm thick films of permalloy. We have used focused 30-keV Ga+ ions to define small arrays (6u200aμm×6u200aμm) of wires, circles, and elongated hexagons in the size range 100–500 nm. High-sensitivity magneto-optical measurements combined with atomic force microscopy show that very high quality magnetic nanostructures can be fabricated by FIB milling even in thin films of soft magnetic materials. This finding could be significant for the future commercialization of certain aspects of magnetic nanotechnology and magnetoelectronics.


Journal of Applied Physics | 2004

Artificial domain wall nanotraps in Ni81Fe19 wires

Colm C. Faulkner; M.D. Cooke; D. A. Allwood; D. Petit; D. Atkinson; Russell P. Cowburn

We report on the controlled pinning and depinning of head-to-head domain walls with individual artificial nanotraps in rounded L-shaped Ni81Fe19 wires. Domain walls were nucleated and injected into one arm of an L-shaped planar wire structure with a wire width of 200 nm and a thickness of 5 nm. The domain walls were propagated through a rounded corner into an orthogonal output wire by a 27 Hz anticlockwise rotating field. A highly sensitive magneto-optical Kerr magnetometer system was used to detect magnetization reversals around single wedge shaped nanotraps in the output wire of different samples. Domain wall propagation occurred at a mean measured x-field value of 6.8 Oe in the output wire arm when not interacting with a trap. Domain wall nanotraps with dimensions as small as depth Dt=35u2009nm and width Wt=55u2009nm were found to effectively pin domain walls. In general, the depinning field of a domain wall from a trap increased with trap size. Hysteresis loops and plots of domain walls depinning fields as a ...


Applied Physics Letters | 2002

Shifted hysteresis loops from magnetic nanowires

D. A. Allwood; N. Vernier; Gang Xiong; M.D. Cooke; D. Atkinson; Colm C. Faulkner; Russell P. Cowburn

We demonstrate that positively and negatively field-shifted magnetic hysteresis loops can be obtained from a single continuous L-shaped magnetic nanostructure. This is achieved by controlling the coercivity of one arm of the L-shape structure with the magnetization direction of the orthogonal arm. Furthermore, a memory effect is demonstrated by reversing the magnetization direction of one arm while leaving the other unchanged. Good discrimination between the different switching field magnitudes and the ease of fabrication make these continuous magnetic structures more suitable than chains of discrete magnetic dots for performing logical operations.


IEEE Transactions on Magnetics | 2003

Controlled switching of ferromagnetic wire junctions by domain wall injection

Colm C. Faulkner; D. A. Allwood; M.D. Cooke; Gang Xiong; D. Atkinson; Russell P. Cowburn

The switching of submicrometer ferromagnetic wire junctions is investigated by controlled injection of domain walls (DWs). A three-terminal continuous Ni/sub 80/Fe/sub 20/ structure is described, consisting of two input wires and one output wire. Separate structures were fabricated by focused ion beam (FIB) milling, to inject either zero, one, or two DWs to the junction inputs. Introduction of DWs to the junction was performed using one or two DW injection pads with low switching fields. Hysteresis loops measured by magnetooptical Kerr effect (MOKE) magnetometry on the device output wires showed the coercivity of the output is strongly dependent on the number of DWs incident at the junction. By injecting either one or two DWs into the junction, it is possible to switch the output wire at two distinct field values, each markedly lower than the nucleation field of the junction. Results presented are relevant to the future development of spintronics DW logic systems, and for fundamental studies into DW resistance and interaction between DWs and wire morphology.


IEEE Transactions on Magnetics | 2003

Magnetic domain wall dynamics in a permalloy nanowire

D. Atkinson; D. A. Allwood; Colm C. Faulkner; Gang Xiong; M.D. Cooke; Russell P. Cowburn

Experimental results of domain wall propagation velocity as a function of applied field in a 200-nm-wide permalloy nanostructure are presented. The structure was fabricated by focused-ion beam milling and designed to separate the domain wall nucleation from propagation. The measured velocities are very high, up to 1500 m/s at 49 Oe and the field dependence of the velocity is interpreted in terms of thermally activated wall motion through a series of pinning sites and a wall geometry that changes as a function of field.


Journal of Applied Physics | 2004

Characterization of submicrometer ferromagnetic NOT gates

D. A. Allwood; Gang Xiong; M.D. Cooke; Colm C. Faulkner; D. Atkinson; Russell P. Cowburn

We present operation phase diagrams of all-metallic submicrometer ferromagnetic NOT–gate devices. The phase diagrams summarize four different types of behavior, in which devices can operate correctly with either one or three domain walls propagating through them, nucleate domain walls, or pin a domain wall, leading to its annihilation with a succeeding domain wall. We use these phase diagrams to investigate the influence of junction dimensions on domain wall nucleation and pinning, and determine optimized junction parameters for NOT–gate operation. Furthermore, we demonstrate how changing the NOT–gate orientation to an applied field affects the operating phase diagram and may assist the integration of NOT-gates with other types of junction in the near future to realize a full magnetic logic scheme. By fabricating the NOT–gate junctions within a magnetic feedback loop, the direction of domain wall propagation is shown to be reversible and the input and output wires therefore interchangeable.


Journal of Applied Physics | 2008

Tuning of biased domain wall depinning fields at Permalloy nanoconstrictions

C. C. Faulkner; D. A. Allwood; Russell P. Cowburn

The propagation of a transverse domain wall (DW) through single asymmetric nanoconstrictions is investigated in continuous ferromagnetic planar wire structures, by localized magneto-optical Kerr effect magnetometry. Using focused ion beam (FIB) milled planar Permalloy nanowires, we propagate a DW in opposite directions through single asymmetric nanoconstriction. The DW depinning field from an asymmetric nanoconstriction strongly depends on the direction of motion of the DW relative to the trap. A depinning field bias effect is tuned by varying the constriction width from 50to200nm in 300nm wide nanowires. FIB milled nanoconstrictions, with a width as narrow as 50nm, demonstrate a bias effect. Narrowing the width of the constriction enhances the depinning field bias effect.

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D. Petit

University of Cambridge

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