An Ding

Depinning of vortex domain walls from an asymmetric notch in a permalloy nanowire

The mechanism of depinning process of vortex domain wall (DW) in a planar magnetic nanowire with an asymmetric notch is investigated by using focused magneto-optical Kerr effect (MOKE) magnetometer and micromagnetic modeling. Two distinct depinning fields are directly observed by the single-shot MOKE measurements. Micromagnetic modeling provides physical insight into the details of how the depinning process develops at the asymmetric notch and reveals that the different depinning fields originate from the different chiralities of the vortex DWs.

MFM Observation of Twin Pinning Sites on NiFe Nanowires

A train of symmetric twin pinning sites of varying distances on Permalloy nanowires of different widths fabricated with electron beam lithography (EBL) have been investigated using magnetic force microscope (MFM) measurement. Nanowires with a width from 600 to 900 nm with twin pinning sites at a separation from 1.1 μm to 4.6 μm have been found to favor the formation of transverse domain walls under perpendicular, longitudinal, and 45° externally applied magnetic fields. The pinning sites were found to have potential for both head-to-head and tail-to-tail transverse domain wall (DW) pinning at the remanent status under perpendicular, 45° and longitudinal applied magnetic field, while the details of the head-to-head DW pinning area and the tail-to-tail DW structure are subject to the nanowire widths, pinning sites separations, as well as the externally applied magnetic field directions.

Development of an in situ magnetoelastic magneto-optical Kerr effect magnetometer

Reported here is the development and implementation of an integrated in situ magnetoelastic measurement setup with a MOKE magnetometer, repositionable electromagnet, and sample transfer/straining device. The former were used within a molecular beam epitaxial vacuum growth chamber. Consequently the magnetostriction constants for both Cr capped and uncapped Fe/GaAs(100) films were acquired without film oxidization occurring. Samples were bent in a four point bending geometry to produce a quantifiable tensile mechanical strain on the films during magnetoelastic measurements. In addition, a laser measurement system was developed to confirm the induced strain in the samples.

Vortex Domain Wall Formation in Nanowires With Twin Pinning Sites

A set of symmetric twin pinning sites of varying distances on Permalloy nanowires of different widths have been investigated and applied in a racetrack memory scenario using micromagnetic simulations. The nanowire width as well as pinning sites distance were found to affect the vortex domain wall formation, while the writing performance is subject to the writing head dimensions and the external field strength. Nanowires with a width of 600 nm and 800 nm with twin pinning sites at a distance of 1 m have been found to favor the formation of the vortex domain wall compared with 400 nm and 1 m nanowires. The detailed micromagnetic simulations show that a pinning site distance of 1 m and a nanowire width of 800 nm are optimal for an information storage device. The results of the write scenario simulations carried out for this device indicate that, for a successful writing process, an applied field length (AFL) of value 1 m and an applied field magnitude (AFM) of -0.05 are most suitable for information writing processes.

Shape and Proximity Effect of Magnetic Nanoelements Used as Biomolecular Labels for Magnetic Notched Nanowire Biosensors

Three magnetic nanoelements of different shapes and sizes were investigated as potential candidates for biological magnetic labels to be used on a biosensor, which incorporates magnetoresistive nanowires. Circular, square, and triangular nanoelements were positioned at varying distances from a Permalloy, single notched, nanowire to determine how the shape and proximity of the nanoelements affected the domain wall structure of the nanowire. All results were simulated using micro-magnetic simulations by the object oriented micro-magnetic framework. It is shown that both the proximity and the shape of each of the nanoelements produce changes in the domain structure that are specific to the nanoelement and could potentially generate measurable magnetoresistive changes. The 200 nm diameter circularly shaped nanoelement had the greatest proximity effect on the nanowire. It also had a closed, circular domain structure when isolated, which is potentially useful since this is likely to reduce the tendency of biologically labeled material in solution to form agglomerations.

Shape Effect of Magnetic Nanoelements as Biomolecular Labels for Magnetic Biosensors

Four magnetic nanoelements of different shapes were investigated at varying distances from a permalloy nanowire to see how the shape of the nanoelements affected the resultant demagnetizing field from the nanoelements onto the nanowire. All results were simulated using micromagnetic simulations by Object Oriented Micromagnetic Framework (OOMMF). It is shown that the square nanoelements performed the best and that the shape anisotropy has an important role in determining the external demagnetizing field for nanoelements.

Vortex domain wall formation in Permalloy nanowires with constriction based pinning

The potential for magnetic domain walls (DWs) within Permalloy nanowires to be used as non-volatile data storage has prompted a surge in research to meet the fabrication and operational challenges of these DW devices. Utilising a spin polarised current, it has been proposed that transporting vortex DWs is preferential over transverse DWs to accommodate lower currents. It is also believed that pinning sites - either a constriction or a doped area - must be used to allow reliable and controlled operation. In this paper, we present a number of micro-magnetic simulations which investigate the qualitative pinning potential of some constriction-based pinning sites and the implications for reliably forming vortex DWs in tight confinement within Permalloy nanowires.

Depinning behaviour of domain wall in magnetic nanowire with asymmetric notch

The magnetic nanowires have attracted great interests recently for their potential applications in novel logic and memory devices. The magnetization reversal of the magnetic nanowires is realized by the nucleation and the propagation of domain walls (DWs) along the wires. Both current and magnetic field driven DW motion have been proposed and investigated widely. One of the techniques to manipulate the DW is to introduce artificial defects to the nanowire structure such as different shapes of notches. Asymmetric notches have been reported to act as an asymmetric energy barrier for the DW and the depinning field and critical depin current depend on the direction of DW propagation, which is related to the slope of notch. The “Magnetic ratchet effect” has been addressed since the DW depins and moves easier in one direction than the other. Magnetic diodes were also proposed based on the asymmetric notch structure in magnetic nanowires. However, rather than constriction, the previous reports were based on the protruding shape of asymmetric notch. In this paper, we present a study on the depinning behaviour of the DW inside magnetic nanowires with a single inward asymmetric notch.

Current controlled magnetic AND/OR logic circuit

Logic gates are important as they are indispensable devices to all the electronic products. Controlling magnetization directly with an electric current rather than a magnetic field is one of the recent developments within spintronics. The magnetic logic operations are performed by domain wall propagation through ferromagnetic nanowire junctions, resulting in magnetization reversal. This domain wall propagation in the magnetic logic has been induced by an external applied field, which prevents the use of the voltage and current to control the logic operations. A potential alternative is current-induced dragging of a domain wall, which does not rely on a generated magnetic field. An AND/OR gate is a three-terminal device, made up of two input nanowires with constriction meeting at a junction region. The device is exposed by the electron beam with the SEM machine on a Si substrate. The logic function for a two-input gate is realized through the domain wall motion. To realize the OR function, critical current of the input constriction must be larger than that of the output constriction, this means that the constriction width of the input wire must be larger than that of the output wire. To realize the AND function, the width of both input constrictions must be smaller than that of output constrictions.