Control of the switching behavior of ferromagnetic nanowires using magnetostatic interactions

Magnetostatic interactions between two end-to-end Permalloy (Ni80Fe20) nanowires have been studied as a function of their separation, end shape, and width. The change in switching field increases as the wires become closer, with deviations from the switching field of an isolated wire of up to 40% observed. The sign of the change depends on the relative magnetization orientation of the two wires, with higher fields for parallel magnetization and lower fields for antiparallel magnetization. A wire end shape has a strong influence, with larger field variations being seen for flat-ended wires than wires with tapered ends. The micromagnetic modeling and experiments performed here were in good qualitative agreement. The experimental control of switching behavior of one nanowire with another was also demonstrated using magnetostatic interactions

Enhanced longitudinal magnetooptic Kerr effect contrast in nanomagnetic structures

We report on enhanced longitudinal magnetooptic Kerr effect signal contrast in thin-film nanomagnetic disks with in-plane magnetization when combined with dielectric layers that provide impedance matching to the structure and the underlying substrate. Kerr signals can increase by a factor of three, while substrate reflectance is almost completely suppressed. This leads to an increase in Kerr ellipticity relative to the background intensity and a subsequent improvement in the measured signal-to-noise ratio. Measurements using a beam focused on opaque 400-nm Ni disks yield contrast improvements of a factor of 8. Arrays of nanodisks demonstrate more complex behavior due to diffraction effects

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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3428775

Laser direct writing (LDW) of magnetic structures

Laser direct writing (LDW) has been used to pattern 90nm thick permalloy (Ni 81 Fe 19 ) into 1-D and 2-D microstructures with strong shape anisotropy. Sub-nanosecond laser pulses were focused with a 0.75 NA lens to a 1.85μm diameter spot, to achieve a fluence of approximately 350 mJ.cm -2 and ablate the permalloy film. Computer-controlled sample scanning then allowed structures to be defined. Scan speeds were controlled to give 30% overlap between successive laser pulses and reduce the extent of width modulation in the final structures. Continuous magnetic wires that adjoined the rest of the film were fabricated with widths from 650 nm - 6.75μm and magneto-optical measurements showed coercivity reducing across this width range from 47 Oe to 11 Oe. Attempts to fabricate wires narrower than 650nm resulted in discontinuities in the wires and a marked decrease in coercivity. This approach is extremely rapid and was carried out in air, at room temperature and with no chemical processing. The 6-kHz laser pulse repetition rate allowed wire arrays across an area of 4 mm x 0.18 mm to be patterned in 85 s

Transverse field-induced nucleation pad switching modes during domain wall injection

We have used magnetic transmission soft X-ray microscopy (M-TXM) to image in-field magnetization configurations of patterned Ni80F20 domain wall "nucleation pads" with attached planar nanowires. Comparison with micromagnetic simulations suggests that the evolution of magnetic domains in rectangular injection pads depends on the relative orientation of closure domains in the remanent state. The magnetization reversal pathway is altered by the inclusion of transverse magnetic fields. These different modes explain previous results of domain wall injection into nanowires

Direct imaging of domain-wall interactions in Ni80Fe20 planar nanowires

We have investigated magnetostatic interactions between domain walls in Ni80Fe20 planar nanowires using magnetic soft x-ray microscopy and micromagnetic simulations. In addition to significant monopole-like attraction and repulsion effects we observe that there is coupling of the magnetization configurations of the walls. This is explained in terms of an interaction energy that depends not only on the distance between the walls, but also upon their internal magnetization structure

Magnetic domain walls : types, processes and applications

Domain walls (DWs) in magnetic nanowires are promising candidates for a variety of applications including Boolean/unconventional logic, memories, in-memory computing as well as magnetic sensors and biomagnetic implementations. They show rich physical behaviour and are controllable using a number of methods including magnetic fields, charge and spin currents and spin-orbit torques. In this review, we detail types of DWs in ferromagnetic nanowires and describe processes of manipulating their state. We look at the state of the art of DW applications and give our take on the their current status, technological feasibility and challenges

Controlled motion of domain walls in submicron amorphous wires

© 2016 Author(s). Results on the control of the domain wall displacement in cylindrical Fe77.5Si7.5B15 amorphous glass-coated submicron wires prepared by rapid quenching from the melt are reported. The control methods have relied on conical notches with various depths, up to a few tens of nm, made in the glass coating and in the metallic nucleus using a focused ion beam (FIB) system, and on the use of small nucleation coils at one of the sample ends in order to apply magnetic field pulses aimed to enhance the nucleation of reverse domains. The notch-based method is used for the first time in the case of cylindrical ultrathin wires. The results show that the most efficient technique of controlling the domain wall motion in this type of samples is the simultaneous use of notches and nucleation coils. Their effect depends on wire diameter, notch depth, its position on the wire length, and characteristics of the applied pulse

Control of ferromagnetic properties of Ni80Fe20 thin films by voltage-induced oxidation

We demonstrate large voltage-induced changes of magnetic properties in thin films of Ni80Fe20(permalloy) when gated using an ionic liquid medium [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)]. The coercivity and magnetic moment of 5 nm thick permalloy films could be reduced by 75% and 35%, respectively, by using applied voltages. These changes were partially restored by reversing the potential polarity. Electrochemical, time-course magnetometry and surface analysis measurements suggest that the voltage-induced changes are due to changes in the oxidation state at the surface of the film, causing a thinning of the permalloy layer. The control of soft magnetic properties with low voltages may be of use in tuneable devices

Effect of annealing on the electrical and magnetic properties of electrodeposited Ni and permalloy nanowires

The influence of annealing on the microstructure and the electrical and magnetic properties of cylindrical nickel-based nanowires has been investigated. Nanowires of nickel of ~275 nm diameter and of permalloy (Py) of ~70 nm diameter were fabricated by electrochemical deposition into nanoporous templates of polycarbonate and anodic alumina, respectively. Characterization was carried out on as-grown and up to 650 °C heat-treated nanowires. Transmission electron microscopy imaging and diffraction of the nanowires showed a temperature-correlated grain growth of an initially nanocrystalline structure (untreated) with <8 nm (Ni) and <20 nm (Py) grains towards coarser poly-crystallinity after heat treatment with grains up to ~160 nm (Ni) and ~70 nm (Py), the latter being limited by the nanowire width. The electrical conductivity of individual as-grown and 650 °C annealed Ni nanowires was measured in-situ by scanning electron microscopy. At low current densities, the conductivity of annealed nanowires was estimated to have doubled over as-grown nanowires. We attribute this increase to the observed grain growth. The annealed nanowire was subsequently subjected to increasing current densities. Above 120 kA.mm−2 the nanowire resistance started to rise. At 450 kA.mm−2, the nanowire melted and current flow ceased. Magnetometry of as-grown and annealed nanowire arrays showed them to display quasi-thin film magnetic properties. Coercivity and saturation field were inversely correlated in annealed wires and a 25% tunability in these properties was achieved at just 200 °C