Decreasing Wire Width Research Articles

Current-induced linear motion of antiferromagnetically coupled skyrmion-like bubble domains stabilized without Dzyaloshinskii–Moriya interaction

Magnetic domains are used to represent binary digits in magnetic memories. In domain-motion magnetic memories, the domain must move longitudinally along a ferromagnetic nanowire. Antiferromagnetically coupled magnetic skyrmions, which are domains meeting this requirement, can be stabilized via the Dzyaloshinskii–Moriya interaction (DMI) in an antiferromagnetically coupled bi-layered perpendicularly magnetized thin wire. We find that antiferromagnetically coupled skyrmion-like bubble domains (BDs) may be produced in such a wire without DMI. The stability and the dynamics of such domains are studied by micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. The BDs in the two layers have clockwise and anti-clockwise Bloch type domain walls, so the BD cores have anti-parallel magnetic moments and are antiferromagnetically coupled. Domain size decreases with decreasing wire width and/or increasing antiferromagnetic coupling strength of the wire. The minimum value of the BD radius is about 10 nm. When a spin-polarized current flows through the wire, the BD moves with the current.

Magnetoelectric Mapping as Observed in Quantum Coherence Phenomena under Strong Spin-Orbit Interaction

A magnetoelectric mapping is demonstrated between dephasing effects of the magnetic vector potential and effects of an effective vector potential describing spin-orbit interaction. The experiments use spin-dependent mesoscopic quantum transport experiments on the narrow-gap semiconductor InSb and the semimetal Bi, both materials with strong spin-orbit interaction. The spin-orbit-induced antilocalization signature in transport allows determination of spin coherence lengths in narrow InSb and Bi wires. Spin coherence lengths are observed to increase with decreasing wire widths. The geometrical effect of width can be understood from the magnetoelectric duality between the Aharonov-Bohm phase and the Aharonov-Casher phase.

Wire width dependence of hot carrier degradation in silicon nanowire gate-all-around MOSFETs

The increase of hot carrier degradation with decreasing wire width in nanowire gate-all-around (GAA) MOSFETs has been investigated through experiment and device simulation. From the systematical analysis of measurement and simulation, it is found that the increase of device degradation in narrow devices is dominantly governed by the increased current density, the large lateral and vertical fields, and the increased interface state generation rather than by the reduced floating body effects. The more significant hot carrier degradation with decreasing wire width is likely to be proportional to the surface-to-volume ratio of nanowires.

Enhancing the Curie Temperature of Ferromagnetic Semiconductor (Ga,Mn)As to 200 K via Nanostructure Engineering

We demonstrate by magneto-transport measurements that a Curie temperature as high as 200 K can be obtained in nanostructures of (Ga,Mn)As. Heavily Mn-doped (Ga,Mn)As films were patterned into nanowires and then subject to low-temperature annealing. Resistance and Hall effect measurements demonstrated a consistent increase of T(C) with decreasing wire width down to about 300 nm. This observation is attributed primarily to the increase of the free surface in the narrower wires, which allows the Mn interstitials to diffuse out at the sidewalls, thus enhancing the efficiency of annealing. These results may provide useful information on optimal structures for (Ga,Mn)As-based nanospintronic devices operational at relatively high temperatures.

Domain wall resistance in epitaxial Fe wires

We studied the magnetoresistance behavior of epitaxial Fe wires grown on GaAs(110) with varying widths at room temperature. Single nanowires show a wire width (w) dependence of the coercive field, which increases with 1/w for decreasing wire widths. This enables the pinning of a single domain wall in the connection area of two wires with different widths. Magnetoresistance measurements of such wire structures clearly reveal resistance contributions arising from a domain wall. The presence of the domain wall is proven by photoemission electron-microscopy with synchrotron radiation. Moreover, micromagnetic simulations are performed to determine the spin orientations, especially within the domain wall. This permits us to calculate the anisotropic magnetoresistance caused by the domain wall. Taking this into account, we determine the intrinsic domain wall resistance, for which we found a positive value of 0.2%, in agreement with theoretical predictions.

Anisotropic spin splitting in InGaAs wire structures

We report anisotropic spin splitting in gate-fitted InGaAs wires along different crystal orientations. Anisotropic magnetoconductance minima reflecting spin splitting is observed by decreasing wire width at the same carrier density. Anisotropy of spin splitting is reduced by applying negative gate voltages, while the strength of spin splitting is enhanced in the present InGaAs wire structures. This gate voltage dependence of spin splitting shows qualitative agreements with the theoretical calculations taking both Rashba SOI and Dresselhaus SOI into account.

Geometry-dependent scaling of critical current densities for current-induced domain wall motion and transformations

In a combined theoretical and experimental study, we investigate the critical current densities for vortex domain walls in magnetic nanowires. We systematically determine the critical current densities for continuous motion of vortex walls as a function of the wire width for different wire thicknesses and we find that the critical current density increases monotonously with decreasing wire width. Theoretically we present a mechanism that predicts a threshold current density based on wall transformations and this leads to a scaling of the critical current density ${j}_{c}\ensuremath{\propto}1/\text{width}$. The origin of this scaling is found to be the different dependence of the spin torque energy and the vortex nucleation energy on the wire width and good agreement with the experimental observations is found.

Superparamagnetic Behavior of Pt/CoFe/Pt Nanowires With Decreasing Wire Width

We present experimental demonstration that the thermal unstability grows with reducing the width of Pt/CoFe/Pt nanowires on the way to the superparamagnetic transition. The magnetic hysteresis loops measured at room temperature using a magneto-optical Kerr effect system reveal that the coercive field decreases drastically when the nanowire width is reduced smaller than 160 nm. To understand the origin of the coercive field reduction, the coercive field is measured with varying the sweep rate of the external magnetic field and then, analyzed with a thermally-activated magnetization reversal model. It is revealed that the reduction of the coercive field is mainly ascribed to the decrement of the thermal stability parameter, which in turn induces the abrupt reduction of the retention time down to a few seconds.

Semiclassical path integral approach for spin relaxations in narrow wires

The semiclassical path integral (SPI) method has been applied for studying spin relaxation in a narrow two-dimensional strip with the Rashba spin-orbit interaction. Our numerical calculations show good agreement with the experimental data, although some features of experimental results are not clear yet. We also calculated the relaxation of a uniform spin-density distribution in the ballistic regime of very narrow wires. With the decreasing wire width, the spin polarization exhibits a transition from the exponential decay to the oscillatory Bessel-type relaxation. The SPI method has also been employed to calculate the relaxation of the particularly long-lived helix mode. Good agreement has been found with calculations based on the diffusion theory.

Surface and size effects on the electrical properties of Cu nanowires

Copper nanowires were patterned with e-beam lithography and fabricated with a copper film deposited by e-beam evaporation. Various electrical properties of these nanowires (including resistivity, temperature coefficient of resistance, and failure current density) were characterized. It was experimentally found that surface and size have apparent effects on the electrical properties. Smaller values for the temperature coefficient of resistance and higher failure current density were found for Cu nanowires with decreasing wire width. The experimental finding of width dependent failure current density also agrees with finding for theoretical heat transfer of the nanowire and substrate system as calculated with the finite element method.

Magnetization reversal in epitaxial Fe nanowires on GaAs(110)

The anisotropy constants of epitaxial Fe films prepared on GaAs(110) are determined using ferromagnetic resonance. The films are structured into wires by employing electron-beam lithography with subsequent Argon-ion etching. Magnetoresistance measurements for wires with widths ranging from 250--2000 nm show the increasing influence of the shape anisotropy with decreasing wire widths. The magnetocrystalline anisotropy constants determined from these measurements agree well with the values obtained for the continuous films. These results are additionally confirmed by magnetic force microscopy measurements and micromagnetic simulations using the object oriented micromagnetic framework (OOMMF). We show that we are able to prepare wires with a remanent magnetization oriented perpendicular to their long axis due to the interplay of cubic and uniaxial in-plane magnetic anisotropies. Moreover, we can tune the effective easy axis by adjusting the width of the wires.

Study on Nonlinear Electrical Characteristics of GaAs-Based Three-Branch Nanowire Junctions Controlled by Schottky Wrap Gates

The nonlinear electrical characteristics of GaAs-based three-branch nanowire junction (TBJ) devices having Schottky wrap gates (WPGs) are investigated experimentally and theoretically, focusing on the nonlinear mechanism at room temperature in devices with large dimensions and the improvement of voltage transfer efficiency. Input–output voltage transfer curve, Vout–Vin, is characterized by changing nanowire width, W, temperature, T, and WPG gate voltage, VG, systematically. At room temperature, a bell-shaped Vout–Vin curve is observed even in the device having a nanowire width of 1,500 nm, which is ten times larger than the electron mean free path. With decreasing wire width or temperature, the output curves are sharpened and curvature in the low-input-voltage region increases. The curvature rapidly increases and voltage transfer efficiency, ΔVout/ΔVin, approaches unity when VG is decreased into the subthreshold region. A simple and compact model for the nonlinear characteristics in the nonballistic regime is introduced. The rapid change of the curvature and complex curve in the subthreshold region under VG control is due to the switching of the branch condition from resistive to capacitive by depletion underneath the WPG.

Direct imaging of current-induced domain wall motion in CoFeB structures

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1×1012A∕m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in.

Analysis of the size effect in electroplated fine copper wires and a realistic assessment to model copper resistivity

The size effect in electroplated copper wires has been widely studied recently. However, there is no consensus on the role of various scattering mechanisms. Therefore, an in-depth analysis to reveal the origin of the size effect is needed. In this article, we study the resistivity of fine copper wires whose feature sizes shrink in two dimensions. It is shown that the residual resistivity (at 5 K) increases with decreasing wire width or height and the temperature-dependent resistivity slightly deviates from that of bulk copper. This is mainly attributed to surface scattering rather than grain boundary scattering. In fact, the influence of grain boundary scattering in these well annealed copper wires is relatively small. In addition, for copper wires with a constant height, a linear dependence of the copper resistivity on 1/width (w) or 1/cross-sectional area (A), namely ρ=ρic+c*∕w (or ρ=ρic+c**∕A), is derived from the classic surface and grain boundary scattering models and validated experimentally. In this simple description, the contributions of different scattering mechanisms, such as surface reflectivity, p, and grain boundary reflection coefficient, R, defect and impurity density, combine together in parameters of ρic and c* (or c**). Especially, c* is a good indicator of scattering strength, from which one can quantitatively analyze the impact of nonsurface scattering contribution with a reference slope of c*=32.14.

Electron Mobility in Silicon Nanowires

The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schr\"{o}dinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 nm to 8 nm, the width range capturing a crossover between two-dimensional (2D) and one-dimensional (1D) electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly.

Size-dependent multilayer relaxation of nanowires and additional effect of surface stresses

A theoretical investigation was carried out on multilayer relaxation, surface energy and surface stress in [001] oriented rectangular f.c.c metal nanowires using the modified embedded atom method. Surprisingly, the multilayer relaxation behavior depends on the wire sizes in absolutely distinct trends for inward-relaxation metals Ag, Cu, and for outward-relaxation metals Ir and Ni. Specifically, the outmost interlayer relaxation | Δ d 12 | increases with decreasing cross-section areas for Ag, Cu nanowires, while it decreases for Ir, Ni nanowires. This is due to the additional effect of surface stress, which, with the surface energy, increases with decreasing wire width monotonically.

Size-dependent theoretical tensile strength and other mechanical properties of [001] oriented Au, Ag, and Cu nanowires

A uniaxial tensile loading process was simulated on rectangular [001] oriented single-crystal Au, Ag, and Cu nanowires using the modified embedded atom method. The calculated theoretical tensile strength as well as elastic modulus and “yield strength” increases with decreasing wire width almost logarithmically, which is qualitatively consistent with relevant experimental results. According to the present observed linear relationship among these three parameters, we think, the size dependent mechanical behaviors in nanowires may be due to the enhanced attraction between atoms, which is caused by the accumulation of electron charges along wire axial direction.

Rashba effect in InGaAs∕InP parallel quantum wires

We report on the Rashba effect in InGaAs∕InP quantum wires with an effective width ranging from 1.18μm down to 210nm. By measuring 160 wires in parallel universal conductance, fluctuations could be suppressed so that the characteristic beating effect in the magnetorestistance was observable down to very low magnetic fields. A characteristic shift of the nodes in the beating pattern was found for decreasing wire width. By assuming a realistic soft-wall potential, the experimentally observed node positions could be reproduced. For the range of measured wires, our study confirms that the Rashba coupling parameter does not change with wire width.

Influence of thickness and cap layer on the switching behavior of single Co nanowires

We have measured the low-temperature magnetoresistance (MR) of single cobalt nanowires of various lengths, thicknesses, and widths prepared by electron-beam lithography. The MR measurements exhibit peaks at the coercive fields Hc where Hc increases with decreasing wire width. From the shape of the peaks different switching mechanisms can be distinguished. While the resistance jumps for platinum-covered Co nanowires are sharp, pure Co nanowires exhibit broadened peaks due to domain-wall pinning at the Co/CoO interface. Covering the Co nanowires with insulating carbon yields a sharp switching behavior while simultaneously offering the possibility to investigate the resistance behavior of pure cobalt. By reducing the thickness of the wires the sharpness of the switching behavior continuously degenerates.

Spin-Splitting Transport in Narrow-Gap Heterojunction Narrow Wires

We systematically studied spin-orbit (SO) interaction in narrow wires with a variety of widths, using two different approaches. Etched wires were used in the first approach, while side-gate wires were used in the second. To determine the SO coupling constant, α, we measured magneto-resistances at 1.6 K. α remained almost constant for the etched wires with a width wider than 0.4 μm. On the other hand in the side-gate wires, α increased with decreasing wire width from 2 μm down to about 1 μm with the application of asymmetric side-gate voltage. The increase of α observed in the side-gate wire could be attributed to the enhanced total asymmetric electric field, Easym, total, which is the vector sum of the vertical electric field EV originally existing at the hetero-interface and the horizontal electric field EH resulting from the side-gate voltage application. A rough estimation of α based on this assumption was made and compared with the experimental results.