Multilayered Nanowires Research Articles

Characterizing the structure of Co/Cu and NiFe/Cu multilayered magnetic nanowires

Multilayered magnetic Co/Cu and NiFe/Cu nanowires were grown via template-assisted electrodeposition. Structures were described by means of focused ion beam/scanning electron microscopy and energy dispersive X-ray spectroscopy.

CoB/Ni-Based Multilayer Nanowire with High-Speed Domain Wall Motion under Low Current Control

The spin-transfer torque motion of magnetic domain walls (DWs) in a CoB/Ni-based nanowire driven by a low current density of (1.12±0.8)×1011 A m-2 has been observed indirectly by magnetotransport measurements. A high DW velocity of 85±4 m/s at zero field was measured at the threshold current density. Upon increasing the current density to 2.6×1011 A m-2, the DW velocity increases to 197±16 m/s before decreasing quickly in the high-current-density regime attributed to nonadiabatic spin-transfer torque at a low damping factor and weak pinning. The addition of B atoms to the Co layers decreased the magnitude of saturation magnetization, Gilbert damping factor, and density of pinning sites, making the CoB/Ni multilayer nanowire favorable for practical applications.

High Efficiency Dye-Sensitized Solar Cells Based on Three-Dimensional Multilayered ZnO Nanowire Arrays with “Caterpillar-like” Structure

A 3D ZnO nanowire-based dye-sensitized solar cell (DSSC) with unique "caterpillar-like" structure was designed. Because of the significant improvement of the total ZnO nanowire surface area, the amount of light absorption was substantially increased. This increase in the light harvesting efficiency enables us to achieve an overall power conversion efficiency as high as 5.20%, which is the highest reported value to date for ZnO nanowire-based DSSCs. A branched-multilayered design of ZnO nanowire arrays grown from ZnO nanofiber seed layers proves to be very successful in fabricating 3D ZnO nanowire arrays. Practically, electrospun ZnO nanowires were used as the seeds in multilayer growth of ZnO nanowire arrays with a unique "caterpillar-like" structure. This unique structure significantly enhances the surface area of the ZnO nanowire arrays, leading to higher short-circuit currents. Additionally, this design resulted in closer spacing between the nanowires and more direct conduction pathways for electron transfer. Thus, the open-circuit voltage was so significantly improved as a direct result of the reduction in electron recombination.

Layer thickness dependent tensile deformation mechanisms in sub-10 nm multilayer nanowires

Using molecular dynamics simulations, the tensile deformation behavior for two types of sub-10 nm multilayer nanowires (NWs) have been investigated. For the structure with interfaces perpendicular to the wire axis, the deformation mechanism is changed from interface crossing by dislocations to interface rotation as the layer thickness is decreasing, causing a significant reduction in yield strength. However, the deformation mechanisms are all accommodated through interface crossing by dislocations regardless of layer thickness for the structure with interfaces parallel to the wire axis. Moreover, the yield strengths in the second structure are found to be controlled by two competing mechanisms: the interface strengthening by increased repulsive force and interface softening by increased dislocation source sites. The sudden stress drop after yielding point in NWs could be explained by the dislocation source-limited hardening mechanism: the more atomic fraction of newly formed stacking faults (SF) after stress drop, the larger normalized stress drop and the larger uniform tensile elongation. For the second structure, the larger total tensile elongation for larger layer thickness could be related to the twinning induced plasticity at the necking position. These findings should have implications for designing functionalized structures and devices in nanoelectromechanical systems.

Fabrication and magnetic study of Co/Pt multilayer nanowires and Co–Pt alloy nanowires electrodeposited into porous Si substrates

In this work, we grow composite structures consisting of magnetic and non-magnetic metal or alloy nanowires electrodeposited into the ion etched tracks previously created inside Si substrates. The holes are then filled by Co–Pt alloys and Co/Pt multilayers using electrodeposition technique making a large number of parallel nanowires. This process takes place in a single electrolyte containing Co+2 and Pt+4 ions by applying a proper deposition potential using a computer control potentiostat. The magnetic properties of the sample were studied using vibrating sample magnetometer. Magnetoresistive behaviour of the nanowire samples was then studied by subjecting the samples to an external magnetic field. The results show that the Co/Pt multilayered nanowires exhibit a large magnetoresistance, while the Co–Pt alloys only show anisotropic magnetoresistance. This result could be of a great interest for the sensor fabrication community as they will provide a view on a very important direction of the development of the wide spread sensor industry, and more importantly for understanding the physical phenomena underlying the magnetic/non-magnetic nanostructures.

Reproducible domain wall pinning by linear non-topographic features in a ferromagnetic nanowire

We demonstrate that for multilayered magnetic nanowires, where the thickness and composition of the individual layers have been carefully chosen, domain walls can be pinned at non-topographic sites created purely by ion irradiation in a focused ion beam system. The pinning results from irradiation induced alloying leading to magnetic property modification only in the affected regions. Using Lorentz transmission electron microscopy, we have studied the pinning behavior of domain walls at the irradiation sites. Depending on the irradiation dose, a single line feature not only pinned the domain walls but also acted to control their structure and the strength of their pinning.

CPP-GMR of Co/Cu Multilayered Nanowires Electrodeposited into Anodized Aluminum Oxide Nanochannels with Large Aspect Ratio

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Mechanical Characterization of Co/Cu Multilayered Nanowires

The mechanical deformation properties of (110) Co/Cu multilayered nanowires were studied by Molecular Dynamics under uniaxial tensile and compressive stresses. The potential of the immiscible CoCu system was modeled by a second-moment tight-binding approximation. Stress-strain curves at different conditions were obtained and the elastic modulus and yield stress were analyzed. Both magnitudes are approximately independent of the strain rate, except at high values. They decrease linearly with increasing temperature. Below a volume-to-surface-area ratio, their values drastically increase and diverge from the bulk values. If the thickness of the Cu sublayers increases, the Young's modulus and yield stress decrease, although in a different way. The elastic modulus decreases linearly and the yield stress falls steeply whenever Cu is present in the nanowire, since the lattice distortion takes place firstly and fundamentally in Cu sublayers. The change in the axial stress at the interface is little significant on average and rather localized. Unlike, the transverse stress has a non-uniform distribution along the Cu sublayer, especially at the yield point. The Young's modulus and yield stress are larger in tension than in compression. Under tensile stress, nanowires slip via partial dislocation nucleation and propagation. Unlike, compressive deformation of nanowires takes place via both partial and full dislocations.

High-Efficiency Solid-State Dye-Sensitized Solar Cells Based on TiO2-Coated ZnO Nanowire Arrays

Replacing the liquid electrolytes in dye-sensitized solar cells (DSCs) with solid-state hole-transporting materials (HTMs) may solve the packaging challenge and improve the long-term stability of DSCs. The efficiencies of such solid-state DSCs (ss-DSCs), however, have been far below the efficiencies of their counterparts that use liquid electrolytes, primarily due to the challenges in filling HTMs into thick enough sensitized films based on sintered TiO(2) nanoparticles. Here we report fabrication of high-efficiency ss-DSCs using multilayer TiO(2)-coated ZnO nanowire arrays as the photoanodes. The straight channel between the vertically aligned nanostructures combined with a newly developed multistep HTM filling process allows us to effectively fill sensitized films as thick as 50 μm with the HTMs. The resulting ss-DSCs yield an average power conversion efficiency of 5.65%.

Current-induced domain wall motion in a multilayered nanowire for achieving high density bit

We herein propose a multilayered nanowire in order to achieve a high density bit or short bit length. The multilayered nanowire consists of a continuous layer and a granular layer which are coupled by interlayer exchange and magnetostatic interactions. The continuous layer has a role of data transfer based on current-induced domain wall motion (CIDWM). The granular layer has a role of stabilizing data, and the current flows only through the continuous layer. We demonstrate CIDWM in the multilayered nanowire by micromagnetic simulation. The domain wall width in the multilayered nanowire is narrower than that of the single-layer nanowire because of weak exchange coupling in the granular layer. As a result, the smaller bit length can be obtained in the multilayered nanowire. Moreover, the critical current density in the multilayered nanowire almost equals to the critical current density of the single-layer nanowire with the same domain wall width as that of the multilayered nanowire.

Magnetic anisotropy and magnetization reversal in Co/Cu multilayers nanowires

The Co/Cu multilayer nanowires fabricated in an array using anodized aluminum oxide (AAO) template by electrodeposition method, have been investigated. It has been observed that the magnetization reversal mode and magnetic anisotropy depend upon the Co and Cu layer thicknesses. Magnetization reversal occurs by curling mode at around Co = 400 nm and Cu = 10 nm, while for Co = 30 nm and Cu = 60 nm, magnetization reversal occurs by nucleation mode. A change of magnetic anisotropy from out of plane to in plane is observed when thickness of Cu layer tCu = 60 nm and that of Co tCo = 30 nm. Magnetic anisotropy is lost when thickness of the Co layer tCo = 400 nm and that of Cu tCu= 10 nm. Magnetic properties have been explained by the competition among shape anisotropy, magnetostatic interactions and magnetocrystalline anisotropy. Magnetic properties can be tuned accordingly depending upon the thickness of the Co and Cu nanodisks.

Angular dependence of current perpendicular to plane giant magnetoresistance in multilayer nanowire

We use incoherent scattering matrices to calculate the angle dependent magnetoresistance and find analytic expressions for a tri-layer current-perpendicular-to-plane nanowire. We find that the non-linearity of magnetoresistance versus β = cos2(θ/2) can be quantitatively explained by reflected electrons using only experimental spin polarization as input. We extend this model to a multilayer nanowire and find the simulation approximately matches experiment.

Magnetization reversal mechanisms in 35-nm diameter Fe1-xGax/Cu multilayered nanowires

In this work, magnetization reversal mechanisms in various 35 nm diameter Fe80Ga20/Cu multilayered nanowire arrays were studied by a vibrating sample magnetometer (VSM) equipped with vector coils, making it possible to monitor both x- and y-components of the sample moment during reversal. When reversal fields were applied in the low angular range (0 – 60°), all nanowire structures, irrespective of Fe80Ga20 or Cu aspect ratios, experienced reversal by nucleation and propagation of a vortex domain wall. However, when fields were applied in the high angular range of 60–90°, reversal occurred by coherent rotation. Using vector-VSM, it was further shown that in structures with pancake-like Fe80Ga20 segments, the extent to which moments in adjacent segments rotate cooperatively decreased as the Cu thickness increased.

Electrical properties of individual NiFe/Pt multilayer nanowires measured in situ in a scanning electron microscope

The electrical properties of individual NiFe/Pt multilayer nanowires have been measured in situ by nanomanipulators in a scanning electron microscope. The electrical measurement of ∼50 nm diameter individual NiFe/Pt multilayer nanowires with polycrystalline microstructure shows that the nanowires have a resistivity of ∼2.2 × 10−7 Ω m (corresponding to a conductivity of ∼4.5× 106 Ω−1 m−1) and average resistance of individual NiFe-Pt interfaces of ∼0.2 Ω. The maximum failure current density of an individual NiFe/Pt nanowire was measured to be ∼9.63 × 1011 A m−2.

Diffusion‐Controlled Solid‐State Formation of CoSb Phase from Co/Sb‐Multilayered Nanowires

The physical properties of materials on a nanometer scale are known to be different from those of bulk form due to dimensional confinement effects and high specific surface‐to‐volume ratio. In this study, the size effects on the diffusion and reaction at the interface of dissimilar nanowires (NWs) were investigated. Co/Sb‐multilayered NWs of 15 ± 1 and 19 ± 2 nm in radius have been grown within anodic aluminum oxide templates using a pulsed potentiodynamic electrodeposition method. XRD and TEM results demonstrated that the multilayered NWs were transformed to a CoSb phase through a solid‐state reaction at the temperature in the range of 653 to 693 K. The kinetics of the solid‐state reaction was analyzed and found to be diffusion controlled at the interface between Co and Sb phases. The reaction was controlled by the dominant diffusion of Sb atoms. The activation energies for the solid‐state reaction were estimated to be 0.9 and 0.7 eV/atom for the multilayered NWs with radii of 15 and 19 nm, respectively.

On the Formation of 50 nm Diameter Free-Standing Silicon Wires Produced by Ion Irradiation

Ion irradiation in conjunction with electrochemical etching is a promising silicon (Si) machining technique for three-dimensional nanofabrication. We present a study of factors influencing the formation of silicon nanowires fabricated by this technique, such as ion energy, fluence, proximity of adjacent wires, location within an irradiated area and wafer resistivity. A better understanding of these factors in different resistivity wafers has enabled us to produce wire diameters and gaps between adjacent wires of about 50 nm using 50 keV protons. Multilayer silicon nanowire arrays are also achieved, so extending the use of this process for three dimensional nanoscale silicon machining.

Magnetic behavior in arrays of Ni79Fe21 and Ni79Fe21/Cu nanowires

Ordered hexagonal arrays of NiFe homogeneous nanowires and NiFe/Cu multilayer nanowires were fabricated by electrodeposition into alumina membranes with pore diameter about 70nm and 35nm and 105nm interpore distance, with a NiFe nanodisc thickness of 60 and 70nm, and a Cu thickness of 50 and 10nm. The hysteresis loops were measured with the applied magnetic field parallel and perpendicular to the wire axis using a VSM. The homogeneous NiFe nanowire array exhibits a nearly isotropic magnetic behavior. In the multilayer nanowire arrays of 70nm diameter, the preferential direction of magnetization is perpendicular to the wire axis and the measured coercive field decreases as the tCu/tNiFe increases. The magnetization processes in multilayer nanowire arrays of 70nm diameter have been investigated by means of micromagnetic simulations of isolated, 2 and up to 9, wires. The magnetization distributions for the two 21(NiFe60/Cu50) and 21(NiFe70/Cu10) multilayer nanowire arrays of 70nm diameter, with the applied magnetic field in the wire axis direction, indicate that magnetization occurs by vortex nucleation at the ends in all the wire nanodiscs. For the samples of the same composition mentioned above, but of 35nm diameter, the reversal magnetization starts by nucleation of a domain wall. A random orientation of the magnetization in the different nanodiscs for the sample of 21(NiFe60/Cu50) can be observed.

GMR Sensors: Technologies and Medical Applications

Giant Magnetoresistance (GMR) is a quantum mechanical effect found in thin film materials that are composed of alternating nano-size ferromagnetic and non magnetic layers. When a GMR material is in the presence of a magnetic field a change in electrical resistance is observed. The GMR effect has been used to produce magnetic sensors that can be used in a variety of applications. Although there are some sensors that have greater sensitivity and lower magnetic detection limits, GMR sensors have more commercial potential due to their small size, low power use, low fabrication cost, and their reproducible operation in very narrow magnetic ranges. The medical field is rapidly adopting the use of GMR magnetic sensors for many medical applications; this is leading to an increase in the number of patents that include GMR sensors. Two of the most promising medical application areas for GMR sensors are medical diagnostics and implant communication. This article reviews the GMR mechanism, measurement, fabrication techniques, and discusses a variety of medical applications and patents that incorporate GMR sensors. Keywords: GMR Sensors, Bioassays, Biosensors, Giant Magnetoresistance, Implant Communication, Magnetic Immunoassays, Magnetic Labeling, Magnetic Relaxation, Magnetic Sensor, Magnetic Sorting, Magnetic Switching, Magnetic Thin Films, Nanowires, Near Field Communication, Multilayer Nanowires

Photoemission electron microscopy of three-dimensional magnetization configurations in core-shell nanostructures

We present a photoemission electron microscopy method that combines magnetic imaging of the surface and of the inner magnetization in three-dimensional core-shell nanostructures. The structure investigated consists of a cylindrical nickel core that is completely surrounded by a shell of iron oxide and silicon oxide layers. The method enables one to image the magnetization configuration of the nickel core even though the shell is thicker than the mean-free path of the photoelectrons. Characteristic ${L}_{3}$ and ${L}_{2}$ edges can be observed not only in the yield of the photoelectrons emitted from the surface of the nanostructure but also in its shadow. X-ray magnetic circular dichroism in the electron yield of the x rays absorbed and transmitted by the multilayered nanowire allows for the individual imaging of the magnetization configurations of the iron oxide tube and the nickel core. The method suggests novel approaches for the characterization of the magnetic and material properties of complex three-dimensional nanostructures.

Mechanism of Magnetization Reversal in Arrays of Multilayered Nanowires

The effect of the shape anisotropy and dipolar interactions between NiFe magnetic layers on the magnetization reversal of NiFe/Cu multilayered nanowire arrays electrodeposited into the nanopores of anodic aluminum oxide (AAO) templates with diameters of 35 nm has been studied. The magnetization reversal mechanism has been investigated from the magnetic hysteresis loops of NiFe/Cu nanowires, for various angles θ between the applied field and the nanowires axis. The magnetization reversal strongly depends on the variation of the aspect ratio (thickness/diameter) between NiFe and Cu layers. Based on the Aharoni's model and the Stoner-Wohlfarth model, we calculated the coercive field (Hc) of multilayered nanowires, considering also the intra-nanowires interaction. The calculated results are in good agreement with the experimental ones even for thin NiFe layers, when the calculated results without considering the magnetostatic coupling between the adjacent ferromagnetic layers are unable to follow the experimental results. Additionally, the angular variation of the normalized remanence (Mr(θ)/Ms) for NiFe/Cu nanowires with various NiFe layers thickness was studied. The experimental Hc and Mr(θ)/Ms values were derived from the hysteresis loops.