Multilayer Nanowire Arrays Research Articles

Modulated critical currents of spin-transfer torque-induced resistance changes in NiCu/Cu multilayered nanowires.

We present a novel method combining anodic aluminum oxide template synthesis and nanolithography to selectively deposit vertically patterned magnetic nanowires on a Si substrate. With this approach we fabricated three-dimensional nanowire-based spin valve devices without the need of complex etching processes or additional spacer coating. Through this method, we successfully obtained NiCu/Cu multilayered nanowire arrays with a controlled sequence along the long axis of the nanowires. Both magnetic switching and excitation phenomena driven by spin-polarized currents were clearly demonstrated in our NiCu/Cu multilayered nanowires. Moreover, the critical currents for switching and excitation were observed to be modulated in an oscillatory manner by the magnetic field in the nanowire-based devices. We present a toy model to qualitatively explain these observations.

Angular first-order reversal curve analysis of FeNi/Cu multilayered nanowire arrays with different diameters

The main goal of the emerging field of spintronics is the shrinkage of materials and devices while improving their magnetic performance for a variety of applications, especially three-dimensional information storage. This stipulates the understanding of fundamental magnetic mechanisms such as magnetization reversal and switching events in multilayer systems with tunable size. Here, a porous anodic alumina membrane-assisted electrochemical deposition method is employed to fabricate FeNi/Cu multilayered nanowire arrays (MNWAs) with diameters in the range of D = 35–80 nm. Angular magnetic properties of these MNWAs are investigated via hysteresis loop and first-order reversal curve (FORC) measurements for angle fields of 0° ≤ θ ≤ 90°. While the former indicates the existence of different contributions of vortex domain wall propagation for smaller (D ≤50 nm) and larger (D >50 nm) diameters, the latter reveals the occurrence of single vortex states, depending on θ and D. Moreover, at θ = 0°, a transition from single domain to multidomain-like behavior appears to occur with increasing D from 35 to 80 nm based on FORC diagrams, significantly influencing magnetization reversibility of magnetic FeNi segments with relatively high aspect ratios (>5). The variation of magnetostatic interactions with respect to θ is discussed and compared at each diameter as an effective factor in determining angular magnetic properties. Also, the axial variation behavior of coercivity as a function of D is correlated with changes in the reversible percentage, shedding light on the expectations from the vortex domain wall propagation.

Tuning magnetostatic interaction and coercivity distributions of FeCo/Cu multilayer nanowire arrays by variation of magnetic and nonmagnetic layer aspect ratios

FeCo/Cu multilayer NW arrays are fabricated using a pulsed electrodeposition method in 50 nm pore diameter anodic aluminum oxide templates. The magnetic layer (LFeCo) and nonmagnetic layer (LCu) thicknesses range between 23–225 and 2–50 nm, respectively. These layer designations make it possible to find a thin Cu layer, in which the magnetic behavior of multilayer NWs is similar to that of alloy NWs. A threshold LCu is also obtained, representing dual behavior of the coercive field in FeCo/Cu multilayer NWs. These behaviors, in turn, are associated with an increase in magnetizing and demagnetizing interactions, as evidenced by hysteresis curve and first-order reversal curve experiments. By extracting magnetic parameters such as the average coercive field, irreversible coercive field, magnetostatic interaction distribution, and reversibility, the magnetic behavior of FeCo/Cu multilayer NWs is comprehensively explored as a function of LFeCo.

Magnetization reversal properties and magnetostatic interactions of disk to rod-shaped FeNi layers separated by ultra-thin Cu layers

From fast magnetic memories with low-power consumption to recording media with high densities, realizing the magnetization reversal and interaction of magnetic layers would allow for manipulating the ultimate properties. Here, we use a pulsed electrochemical deposition technique in porous alumina templates (50 nm in pore diameter) to fabricate arrays of nanowires, consisting of FeNi layers (26−227 nm in thickness) with disk to rod-shaped morphologies separated by ultra-thin (3 nm) Cu layers. By acquiring hysteresis curves and first-order reversal curves (FORCs) of the multilayer nanowire arrays, we comprehensively investigate magnetization reversal properties and magnetostatic interactions of the layers at different field angles (0° ≤ θ ≤ 90°). These involve the extraction of several parameters, including hysteresis curve coercivity (H c Hyst ), FORC coercivity (H c FORC ), interaction field distribution width (ΔH u ), and irreversible fraction of magnetization (IF m ) as a function of θ. We find relatively constant and continuously decreasing trends of H c Hyst when 0° ≤ θ ≤ 45°, and 45° < θ ≤ 90°, respectively. Meanwhile, angular dependence of H c FORC and IF m shows continuously increasing and decreasing trends, irrespective of the FeNi layer morphology. Our FORC results indicate the magnetization reversal properties of the FeNi/Cu nanowires are accompanied with vortex domain wall and single vortex modes, especially at high field angles. The rod-shaped layers also induce maximum ΔH u during the reversal process, owing to enhancements in both magnetizing and demagnetizing-type magnetostatic interactions.

3D interacting magnetic multilayered nanowire arrays: the emergence and evolution of new first-order reversal curve features

The crucial role of magnetostatic interactions in tuning properties of storage devices based on magnetic nanowires (NWs) has recently been highlighted by advanced characterization techniques including the first-order reversal curve (FORC) analysis, evaluating physical entities constituting conventional 2D NW systems. Herein, FORC diagrams of ferromagnetic (FM)/non-magnetic (NM) multilayered NW arrays are simulated using Monte Carlo calculations, involving magnetostatic interactions between segments in 3D space. The FM length is constant to 6 µm whereas the NM length (LNM) varies from 10 to 300 nm, significantly influencing interwire and intrasegment interactions of neighboring NWs and coupled segments along the NW length. Intriguingly, this is accompanied with the emergence of two new FORC diagram features in addition to the typical demagnetizing-type feature, indicating complex behavior of the 3D interacting NWs with the same anisotropy field for each FM segment. The FORC coercivity of the emerging features is tracked individually, presenting evolution as a function of LNM. Our results also evidence an increase in interwire and intrasegment interactions when increasing NW diameter, being in accordance with total magnetostatic energy calculations.

CPP-GMR Performance of Electrochemically Synthesized Co/Cu Multilayered Nanowire Arrays with Extremely Large Aspect Ratio.

Anodized aluminum oxide (AAO) films, which have numerous nanochannels ca. 75 nm in diameter, D and ca. 70 µm in length, L (ca. 933 in aspect ratio, L/D), were used as a template material for growing Co/Cu multilayered nanowire arrays. The multilayered nanowires with alternating Cu layer and Co layers were synthesized by using an electrochemical pulsed-potential deposition technique. The thickness of the Cu layer was adjusted from ca. 2 to 4 nm while that of the Co layer was regulated from ca. 13 to 51 nm by controlling the pulsed potential parameters. To get a Co/Cu multilayered nanowire in an electrochemical in-situ contact with a sputter-deposited Au thin layer, the pulsed potential deposition was continued up to ca. 5000 cycles until the nanowire reached out toward the surface of AAO template. Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect reached up to ca. 23.5% at room temperature in Co/Cu multilayered nanowires with ca. 3500 Co/Cu bilayers (Cu: 1.4 nm and Co: 18.8 nm). When decreasing the thickness of Co layer, the CPP-GMR value increased due to the Valet–Fert model in the long spin diffusion limit.

Surface plasmon aided high sensitive non-enzymatic glucose sensor using Au/NiAu multilayered nanowire arrays

A novel plasmon aided non-enzymatic glucose sensor was first constructed based on the unique half-rough Au/NiAu multilayered nanowire arrays. These multilayered and half-rough nanowires provide high chemical activity and large surface area for glucose oxidation in an alkaline solution. Under visible light irradiation, the surface plasmons originated from Au part enhance the electron transfer in the vertically aligned nanowires, leading to high sensitivity and wide detection range. The resulting sensor exhibits a wide glucose detection concentration range, low detection limit, and high sensitivity for plasmon aided non-enzymatic glucose sensor. Moreover, the detection sensitivity is enhanced by almost 2 folds compared to that in the dark, which significantly enhanced the performance of Au/NiAu multilayered nanowire arrays sensor. An excellent selectivity and acceptable stability were also achieved. These results indicate that surface plasmon aided nanostructures are promising new platforms for the construction of non-enzymatic glucose sensors.

Inter- and intra-nanowire magnetic interaction in Co/Cu multilayer nanowires deposited by electrochemical deposition

Intra- and inter-wire magnetic interactions in Co/Cu multilayer nanowire arrays have been studied with the Henkel plot method. It has been found that the intra-nanowire interaction greatly decreases with an increase in the Cu spacer thickness. Magnetoresistance of 0.16% has been obtained in Co(200nm)/Cu(5nm)/Co(10nm) junction. However, the magnetoresistance effect is negligible both in the juctions with strong intra-nanowire interaction ( thickness of Cu spacer is less than 3nm) and the juctions with strong current-depolarization (thickness of Cu spacer is more than 17nm).

Tuning the coercivity of Cu/Ni multilayer nanowire arrays by tailoring multiple parameters

Cu/Ni multilayer nanowire arrays with well controlled diameters, Cu and Ni layer thickness and varying periodicity were fabricated by multi potential deposition technique in etched ion-track template at room temperature. A facile method was adopted to confirm the different layer thickness and corresponding component by only removing the Cu layer. The pure Ni and Cu layer were confirmed in the Cu/Ni multilayer nanowires by using the new method and XRD analysis. The relationships between the coercivity and nanowire diameter, layer thickness and periodicity were obtained by carrying out vibrating sample magnetometry (VSM) measurement. The results indicated that the coercivity of multilayer nanowire arrays can be tuned significantly by nanowire diameter, periodicity, Cu and Ni layer thickness. In order to explain the experimental phenomena, Pant's model was modified and adopted to successfully explain the variation of demagnetizing field in Cu/Ni multilayer nanowire arrays after tailoring multiple parameters.

Synthesis of Ni/Au multilayer nanowire arrays for ultrasensitive non-enzymatic sensing of glucose

Ni/Au multilayer nanowire arrays were fabricated by an electrochemical method using anodic aluminum oxide as template. Each nanowire has a length of 2.5μm and diameter of 70nm with alternating Ni and Au layers along its axis. A novel glucose sensor was then developed using a Ni/Au multilayer nanowire array immobilized on an ITO electrode. The resulting sensor exhibited two specific linear ranges at 0.25μM–2mM and 2–5.5mM, with very high sensitivities (3372 and 1906μAmM−1cm−2, respectively). The detection limit was as low as 0.1μM with a signal-to-noise ratio of 3. An excellent selectivity and acceptable stability were also achieved. These results indicate that Ni/Au multilayer nanowire arrays are promising new platforms for the construction of non-enzymatic glucose sensors.

Magnetostatic interaction in electrodeposited Ni/Au multilayer nanowire arrays**Project supported by the National Natural Science Foundation of China (Grant No. 11204246) and the Natural Science Foundation of CQCSTC (Grant No. cstc2014jcyjA50027).

Ordered Ni/Au multilayer nanowire arrays are successfully fabricated inside the nanochannels of anodic aluminum oxide template by pulse electrodeposition method. The thickness of the alternating layers is controlled to examine the magnetostatic interaction in Ni/Au multilayer nanowires. The magnetic easy axis parallel to the nanowires indicates that here the magnetostatic coupling along the wire axis dominates over the interactions perpendicular to the nanowires. However, the magnetostatic interaction between adjacent nanowires with larger magnetic layers is enhanced, leading to the existence of an optimum coercivity value.

Tuning the magnetic properties of multilayered CoPt-Pt nanowires via thickness of magnetic segments

We report on an effect of CoPt layer thickness on the magnetic properties of 3D CoPt-Pt multilayered nanowire arrays. The nanowires exhibit ferromagnetic behavior for the CoPt segments even only a few nanometers thick. It was found that decreasing thickness of CoPt nanosegments, by shortening the time of ‘magnetic material pulse’ during electrodeposition, leads to a stronger dependence of magnetization on temperature and the decrease of critical temperature. Also other magnetic parameters such as ground state magnetization, spin-wave parameter and Bloch exponent were determined for the CoPt-Pt nanowire arrays. The ground-state values of magnetization suggest that the mixing of Co and Pt occurs within the whole segment’s volume, however, the segments remain ferromagnetic. It is demonstrated that temperature dependencies of magnetization qualitatively follow the predictions of the spin-wave theory. Furthermore, based on the angle-dependent measurements of magnetization versus applied magnetic field, the coherent spin rotation was determined as a mechanism of magnetization reversal in the CoPt-Pt multilayered nanowire arrays.

Perpendicular Giant Magnetoresistance and Magnetic Properties of Co/Cu Nanowire Arrays Affected by Period Number and Copper Layer Thickness

One-dimensional magnetic nanowires have attracted much attention in the last decades due to their unique physical properties and potential applications in magnetic recording and spintronics. In this work, ordered arrays of Co/Cu multilayered nanowires which can be exploited to develop magnetoresistive sensors were successfully prepared using porous anodic alumina (PAA) templates. The structure and morphology of the multilayered nanowire arrays were characterized by transmission electron microscopy and scanning electron microscopy. The nanowire arrays are highly ordered and the average diameter is about 50 nm, which is controlled by the pore diameter of the PAA templates. The influences of period number and Cu layer thickness on the magnetic and the giant magnetoresistance (GMR) properties were investigated. The coercivity and remanence ratio increase first and then gradually tend to be stable with the increase of period number and the Cu layer thickness, while the GMR ratio increases first and then decreases with the increase of the period number accompanied by an oscillatory behavior of GMR as the Cu layer thickness changes, which are ascribed to the spin dependence electron scattering in the multilayers. The optimum GMR of −13% appears at Co (50 nm)/Cu (5 nm) with 200 deposition cycles in our experimental conditions.

Magnetic–optical bifunctional CoPt3/Co multilayered nanowire arrays**Project supported by the National Natural Science Foundation of China (Grant Nos. 51472165, 51471185, and 11274370).

CoPt3/Co multilayered nanowire (NW) arrays are synthesized by pulsed electrodeposition into nanoporous anodic aluminum oxide (AAO) templates. The electrochemistry deposition parameters are determined by cyclic voltammetry to realize the well control of the ratio of Co to Pt and the length of every segment. The x-ray diffraction (XRD) patterns show that both Co and CoPt3 NWs exhibit face-centered cubic (fcc) structures. In the UV-visible absorption spectra, CoPt3/Co NW arrays show a red-shift with respect to pure CoPt3NWs. Compared with the pure Co nanowire arrays, the CoPt3/Co multilayered nanowire arrays show a weak shape anisotropy and well-modulated magnetic properties. CoPt3/Co multilayered nanowires are highly encouraging that new families of bimetallic nanosystems may be developed to meet the needs of nanomaterials in emerging multifunctional nanotechnologies.

Effect of sub-layer thickness on magnetic and giant magnetoresistance properties of Ni–Fe/Cu/Co/Cu multilayered nanowire arrays

Ni–Fe/Cu/Co/Cu multilayered nanowire arrays were electrodeposited into anodic aluminum oxide template by using dual-bath method at room temperature. Scanning electron microscopy and transmission electron microscopy were used to characterize the morphology and structure of the multilayered nanowire arrays. Vibrating sample magnetometer and physical property measurement system were used to measure their magnetic and giant magnetoresistance (GMR) properties. The effect of sub-layer thickness on the magnetic and GMR properties was investigated. The results indicate that magnetic properties of electrodeposited nanowires are not affected obviously by Cu layer thickness, while magnetic layers (Ni–Fe and Co layers) have significant influence. In addition, GMR ratio presents an oscillatory behavior as Cu layer thickness changes. The magnetic and GMR properties of the multilayered nanowire arrays are optimum at room temperature for the material structure of Ni–Fe (25nm)/Cu (15nm)/Co (25nm)/Cu (15nm) with 30 deposition cycles.

Magnetoresistance of Electrodeposited Co/Cu Multilayer Nanowires

Co/Cu multilayer nanowires were electrodeposited potentiostatically on highly ordered porous anodic alumina oxide templates. Scanning electron microscopy (SEM) showed that wires have diameters of about 250 300 nm and length of 40 50 μm. Co/Cu multilayer nanowire arrays embedded in anodic alumina oxide were mechanically polished with diamond in a suspension of oil until the wires appeared. Then the top and bottom surfaces of the material were coated with a layer of Au with a thickness of 100 nm for electrical contacts. Magnetic eld was applied by an electromagnet and resistance was measured by four point technique.

Synthesis and magnetic properties of Ni–Fe/Cu/Co/Cu multilayer nanowire arrays

Ni–Fe/Cu/Co/Cu multilayer nanowire arrays were successfully electrodeposited into pores of an AAO template using a three-bath method. TEM and SEM were used to characterize structure and morphology of multilayered nanowire arrays. The results indicate that layer thickness is very uniform and multilayered structure is clear and regular. Multilayered nanowire arrays are highly ordered. Average diameter of multilayered nanowires is about 120 nm, which is the same as pore diameter of AAO templates. Vibrating sample magnetometer and physical property measurement system reveal that magnetic coercivity in an applied magnetic field perpendicular to surface of AAO templates is larger than the coercive field parallel to the surface of AAO templates. CPP GMR of Ni–Fe/Cu/Co/Cu nanowire arrays is affected by change of copper layer thickness.

Bi2Se3/Bi multiple heterostructure nanowire arrays formed by pulsed electrodeposition

Thermoelectric Bi2Se3/Bi multilayered nanowire arrays with a minimum period of about 100nm were first successfully fabricated by a template-assisted pulsed electrodeposition process. And the effect of ion concentration on electrodeposition current was investigated by using the cyclic voltammogram. Meanwhile, the nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy, respectively. Large-scale and continuous nanowire arrays with multi-segment characters and uniform diameters can be observed by FE-SEM and TEM, and the randomly selected neighboring segments were characterized to be Bi2Se3 and Bi, respectively, by EDS.

Effect of Nanowire Diameter and Period Number on Magnetic Properties and CPP-GMR of Ni-Fe/Cu/Co/Cu Multilayer Nanowire Arrays

Ni-Fe/Cu/Co/Cu multilayer nanowire arrays were electrodeposited into the pores of AAO template by using dual-bath method. The morphology and structure of the nanowire arrays were characterized by scanning electron microscopy and transmission electron microscopy. Vibrating sample magnetometer and physical property measurement system revealed that the magnetic properties and current perpendicular to the plane-giant magnetoresistance (CPP-GMR) of Ni-Fe/Cu/Co/Cu nanowire arrays were affected by the changes of nanowire diameter and period number. Along with the increase of the diameter, the coercivity and remanence ratio increased first, and then decreased. The properties of nanowire arrays were optimum and the maximum room temperature CPP-GMR ratio of 45.24% was achieved when the nanowire diameter was 80 nm and the period number was 300.

Structural and Magnetic Characterization of FeCoCu/Cu Multilayer Nanowire Arrays

A series of [FeCoCu/Cu(x)]10 (7 ≤ × ≤ 40 nm with FeCoCu layer thickness of 300 nm) and [FeCoCu(y)/Cu]10 (120 ≤ y ≤ 900 nm with Cu layer thickness of 15 nm) arrays of multilayer nanowires, 35 nm in diameter, were fabricated by electrodeposition into self-assembled pores of anodic alumina membranes. High-resolution transmission electron microscopy and X-ray diffraction analysis confirm the segregation of layered structures, with well-defined Cu layers (fcc cubic structure) separating FeCoCu-alloy segments (bcc cubic structure). Hysteresis loop measurements indicate an overall magnetization easy axis parallel to the nanowires in all the samples. For constant FeCoCu segment length, the coercivity, the remanence, and especially, the susceptibility increase with the Cu layer thickness, whereas for the series with constant Cu layer thickness, the susceptibility significantly decreases with FeCoCu segment length. Complementary Henkel curves indicate that the net inter/intrananowires magnetostatic interactions always contribute to the demagnetization of the nanowires. The variation of the susceptibility with FeCoCu and Cu layers thickness together with the Henkel plots data indicate that a reduced demagnetizing effect is achieved for multilayer nanowires with the thicker Cu layer and the shorter FeCoCu segment, for which a moderated reduction in saturation magnetization of around 11% is estimated compared to a continuous FeCoCu alloy nanowire array.