Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction5862. Spin-polarised current587 2.1. Tunnelling current spin polarisation589 2.2. Ballistic current spin polarisation592 2.3. Diffusive current spin polarisation5933. Magnetic Domain Walls598 3.1. Basics of domain walls598 3.1.1. Domain wall thickness and...

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Spin-polarised currents and magnetic domain walls

Herein, a new and facile patterning method is demonstrated for the scalable fabrication of gold elliptical rings (ERs) in a controlled manner over large areas. In this method, well-ordered hexagonally arrayed polystyrene (PS) rings, fabricated by colloidal lithography, were used as masters to generate poly(dimethylsiloxane) (PDMS) stamps with circular apertures. The stamps were then stretched and utilized as molds for creating elliptical PS rings by a capillary filling process. Through subsequent reactive ion etching and chemical wet-etching, the elliptical PS rings could be readily transferred into an underlying gold film, leading to the formation of gold ERs. Since the aspect ratio (AR) of the elliptical PS rings could be controlled by varying the applied strain during the capillary filling process, gold ERs with different ARs could be fabricated in a scalable manner. The optical properties of the gold ERs were characterized by UV–vis/NIR and IR extinction measurements. The ERs exhibited only odd modes ...

Fabrication of elliptical nanorings with highly tunable and multiple plasmonic resonances.

The main results of studying the linear and nonlinear dynamics of a domain wall in ferromagnets are presented beginning with the first publications. The experimental data obtained on polycrystals, single crystals, and films differing in terms of their anisotropy, as well as on magnetic nanostructures, are compared with theoretical results obtained by various methods.

Domain wall dynamics in ferromagnets

High-resolution magnetic patterning of a thin CoFe layer has been performed by irradiation using a focused-ion-beam system. Features <50nm wide were formed reproducibly. The irradiated pattern comprised sets of alternating 3.0- and 1.0-μm-long magnetic wires, 100 nm wide. During magnetization reversal, the longer wires reversed at a lower field resulting in the formation of an ordered array of domains with density 10μm−1 supporting antiparallel magnetization. The ability to create domains at predefined locations is important both for fundamental studies and technological applications.

Nanopatterning of a thin ferromagnetic CoFe film by focused-ion-beam irradiation

The magnetic and magnetoresistive behaviour of circular and elliptical thin film multilayer rings is discussed. Rings are particularly interesting because they can adopt several stable and metastable magnetic states characterized by different numbers of domain walls. Electrically contacted rings made of Co/Cu/NiFe or IrMn/Co/Cu/NiFe, with diameters of ~0.5–5 µm and widths of ~100 nm were fabricated by electron-beam lithography, sputtering and liftoff processing. We show that multilayer rings switch their magnetization by a qualitatively different mechanism compared with that of a single-layer ring; how the magnetization circulation direction around the ring can be controlled; how the rings can be electrically contacted to show large fractional field-induced changes in resistance and how these structures may be used in magnetic random access memories or magnetic logic devices.

https://iopscience.iop.org/article/10.1088/0022-3727/41/11/113002/pdf

Magnetism in multilayer thin film rings

Magnetic interactions between permalloy elliptical rings in different arrangements of two-dimensional arrays were investigated. The edge-to-edge distances between units along the long axis were changed from 2μm down to 100nm. Formation of domain walls during magnetization reversals results in magnetostatic coupling between adjacent units. When two neighboring rows were shifted one-half pitch along the short axis to form a shift-type array, large stable field range for vortex states was obtained.

Magnetic interaction in nanometer line width elliptical ring arrays

We have studied the current induced domain wall motion by using patterned U geometry. The observation of two magnetic states of the U-pattern was evidenced. One is the vortex domain wall at the center of semicircular arc in U-pattern and the other is the continuous magnetic state without wall. We investigated the influence of the positive and negative dc current injection on domain wall motion before switching field with density on the order of ∼107A∕cm2. The critical current varied dramatically as the bias field is close to the switching field and only gradually as the field is far away from the switching field.

Current driven domain wall motion in magnetic U-pattern

Patterned Permalloy submicron-size structures have been fabricated by e-beam lithography in the shape of a ring with a tip. A tip was intentionally added into the ring as a geometrical defect to interrupt the continuity of the magnetization, which aligns along the ring, in order to pin the domain wall. Magnetic switching and reversal process have been measured by the magnetoresistance measurement. The switching field about 260 Oe was obtained. At the remanent state, there was a 0.21% difference in the magnetoresistance between the angles of 90° and 0° that was equivalent to the domain wall magnetoresistance. By applying an external field, the domain wall moved along the ring under a lower field (100 Oe), which is smaller than the switching field. A drop (0.24%) in the resistance between the angle of 70°–120° has been observed that means the domain wall was moving into the voltage measuring region during the rotation.

https://ir.nctu.edu.tw:443/bitstream/11536/26738/1/000221657900070.pdf

Magnetic switching and reversal process in a tip ring structure

Results on both field driven and current induced domain wall motion in Co/Cu/Ni80Fe20 trilayered half-ring in-series wires with different linewidths were presented. The switching behavior depends linearly on the linewidth of Co/Cu/Ni80Fe20 trilayered half-ring structures. A larger bias field (Hb) applied to the samples results in a smaller critical current needed to reverse magnetization. The critical current density was observed to be 3×107 A/cm2 for Ni80Fe20 and 2×108 A/cm2 for Co at zero field. The results of the numbers of giant magnetoresistance jump (digital) versus external magnetic field and current densities (analog) of this design have potential utility as an analog-digital converter.

Current-driven domain wall in giant magnetoresistance half-ring series wires with varied linewidth

Domain wall pinning force in the junctions (corners), with different shapes of square, semicircle, or triangle, of half-ring in-series wires is considered to study the current injection induced wall movements. This geometry has less thermal activation at the region of domain wall nucleation in contrast to notch structures. The wires with square corners have the largest domain pinning force to resist polarized current-induced magnetization reversal, judging from the largest slope in the current-field dependence (ΔI∕ΔH=0.274).

https://ir.nctu.edu.tw:443/bitstream/11536/12363/1/000237404200381.pdf

K. W. Cheng

Papers

Magnetic interaction in nanometer line width elliptical ring arrays

Current driven domain wall motion in magnetic U-pattern

Magnetic switching and reversal process in a tip ring structure

Current-driven domain wall in giant magnetoresistance half-ring series wires with varied linewidth

Vortex domain wall depinning by polarized current in submicron half-ring wires