Cobalt Nanowires Research Articles

Crystalline Phase Mapping Associated to the Magnetic Flux in Cobalt Nanowires

Crystalline Phase Mapping Associated to the Magnetic Flux in Cobalt Nanowires

Magnetization reversal modes in fourfold Co nano-wire systems

Magnetic nano-wire systems are, as well as other patterned magnetic structures, of special interest for novel applications, such as magnetic storage media. In these systems, the coupling between neighbouring magnetic units is most important for the magnetization reversal process of the complete system, leading to a variety of magnetization reversal mechanisms. This article examines the influence of the magnetic material on hysteresis loop shape, coercive field, and magnetization reversal modes. While iron nano-wire systems exhibit flat or one-step hysteresis loops, systems consisting of cobalt nano-wires show hysteresis loops with several longitudinal steps and transverse peaks, correlated to a rich spectrum of magnetization reversal mechanisms. We show that changing the material parameters while the system geometry stays identical can lead to completely different hysteresis loops and reversal modes. Thus, especially for finding magnetic nano-systems which can be used as quaternary or even higher-order storage devices, it is rational to test several materials for the planned systems. Apparently, new materials may lead to novel and unexpected behaviour - and can thus result in novel functionalities.

Mapping the magnetic and crystal structure in cobalt nanowires.

Using off-axis electron holography under Lorentz microscopy conditions to experimentally determine the magnetization distribution in individual cobalt (Co) nanowires, and scanning precession-electron diffraction to obtain their crystalline orientation phase map, allowed us to directly visualize with high accuracy the effect of crystallographic texture on the magnetization of nanowires. The influence of grain boundaries and disorientations on the magnetic structure is correlated on the basis of micromagnetic analysis in order to establish the detailed relationship between magnetic and crystalline structure. This approach demonstrates the applicability of the method employed and provides further understanding on the effect of crystalline structure on magnetic properties at the nanometric scale.

Nanoporous cobalt oxide nanowires for non-enzymatic electrochemical glucose detection

Nanoporous Co3O4 nanowires (NCoNWs) were synthesized via a two-step process with hydrothermal growth of cobalt nanowires (NWs) precursors and subsequent calcination of corresponding precursors in air. The structures and morphologies of the resulting cobalt NWs precursors and NCoNWs were characterized by X-ray diffraction (XRD), scanning electron microscopic (SEM) and transmission electron microscopic (TEM). NCoNWs show high electrocatalytic activity toward glucose oxidation in alkaline medium and be used for non-enzymatic electrochemical detection of glucose. The sensor displays a high sensitivity of 300.8μAmM−1cm−2, a low detection limit of 5μM (S/N=3), and a wide linear dynamic range (5.0×10−6–5.7×10−4M). Furthermore, the sensor exhibits a high selectivity for glucose against the common interferences including ascorbic acid, uric acid and dopamine which co-exist with glucose in human serum samples.

Functional helical silica nanofibers with coaxial mixed mesostructures for the fabrication of PtCo nanowires that display unique geometry-dependent magnetism

Helical mesoporous silicas are notable from a self-assembly and applications points of view. We report a novel confinement-free synthesis of chloropropyl-functionalized helical mesoporous silica nanofibers (CP-HMSNFs) possessing straight channels at the fiber center surrounded by concentric short-pitch helical channels. The chloropropyl groups are found mainly distributed at the central cylindrical portion of the fibers, allowing the selective inclusion of guest species to fabricate novel nanocomposite fibers. Moreover, the chloropropyl-functionalized nanofibers were applied as a hard template to fabricate helical platinum–cobalt (PtCo) alloy nanowires with small and narrowly distributed radii of gyration. The helical metal nanowires exhibited distinct ferromagnetic properties as compared with their straight counterpart. Platinum–cobalt nanowires can be constructed into twisted, magnetically active helical shapes using a novel silica template. Mesoporous silica is a sieve-like material, ideal for directing the growth of metal nanowires. Previously, researchers have developed helical mesoporous silicas to grow spiral metal nanowires. Now, Chia-Min Yang from National Tsing Hua University in Taiwan and co-workers have made these templates more versatile. By introducing silanes bearing chloropropyl groups into the synthesis mixture, the materials self-assembled into nanofibres having a straight central core surrounded by concentric short-pitch helical channels. Because the hydrophobic chloropropyl groups localized near the nanofibre core, the team used them to selectively deposit various materials into the helical templates — iron oxide, pure platinum metal and ferromagnetic platinum–cobalt nanocomposites with possible high-density storage applications. A novel confinement-free synthesis of CP-HMNSFs possessing straight channels at fiber center surrounded by concentric short-pitch helical channels is discovered. The chloropropyl groups are found mainly located at central cylindrical part of the nanofibers. Helical PtCo nanowires with small and narrowly distributed radii of gyration are also fabricated, showing distinct ferromagnetic properties as compared with the straight counterparts.

Electrodeposited Co93.2P6.8 nanowire arrays with core-shell microstructure and perpendicular magnetic anisotropy

We demonstrate the formation of an unusual core-shell microstructure in Co93.2P6.8 nanowires electrodeposited by alternating current (ac) in an alumina template. By means of transmission electron microscopy, it is shown that the coaxial-like nanowires contain amorphous and crystalline phases. Analysis of the magnetization data for Co-P alloy nanowires indicates that a ferromagnetic core is surrounded by a weakly ferromagnetic or non-magnetic phase, depending on the phosphor content. The nanowire arrays exhibit an easy axis of magnetization parallel to the wire axis. For this peculiar composition and structure, the coercivity values are 2380 ± 50 and 1260 ± 35 Oe, parallel and perpendicular to the plane directions of magnetization, respectively. This effect is attributed to the core-shell structure making the properties and applications of these nanowires similar to pure cobalt nanowires with an improved perpendicular anisotropy.

Poly(vinylidene fluoride)-Based Flexible and Lightweight Materials for Attenuating Microwave Radiations

Two unique materials were developed, like graphene oxide (GO) sheets covalently grafted on to barium titanate (BT) nanoparticles and cobalt nanowires (Co-NWs), to attenuate the electromagnetic (EM) radiations in poly(vinylidene fluoride) (PVDF)-based composites. The rationale behind using either a ferroelectric or a ferromagnetic material in combination with intrinsically conducting nanoparticles (multiwall carbon nanotubes, CNTs), is to induce both electrical and magnetic dipoles in the system. Two key properties, namely, enhanced dielectric constant and magnetic permeability, were determined. PVDF/BT-GO composites exhibited higher dielectric constant compared to PVDF/BT and PVDF/GO composites. Co-NWs, which were synthesized by electrodeposition, exhibited saturation magnetization (Ms) of 40 emu/g and coercivity (Hc) of 300 G. Three phase hybrid composites were prepared by mixing CNTs with either BT-GO or Co-NWs in PVDF by solution blending. These nanoparticles showed high electrical conductivity and significant attenuation of EM radiations both in the X-band and in the Ku-band frequency. In addition, BT-GO/CNT and Co-NWs/CNT particles also enhanced the thermal conductivity of PVDF by ca. 8.7- and 9.3-fold in striking contrast to neat PVDF. This study open new avenues to design flexible and lightweight electromagnetic interference shielding materials by careful selection of functional nanoparticles.

Crystallographically driven magnetic behaviour of arrays of monocrystalline Co nanowires

Cobalt nanowires, 40 nm in diameter and several micrometers long, have been grown by controlled electrodeposition into ordered anodic alumina templates. The hcp crystal symmetry is tuned by a suitable choice of the electrolyte pH (between 3.5 and 6.0) during growth. Systematic high resolution transmission electron microscopy imaging and analysis of the electron diffraction patterns reveals a dependence of crystal orientation from electrolyte pH. The tailored modification of the crystalline signature results in the reorientation of the magnetocrystalline anisotropy and increasing experimental coercivity and squareness with decreasing polar angle of the ‘c’ growth axis. Micromagnetic modeling of the demagnetization process and its angular dependence is in agreement with the experiment and allows us to establish the change in the character of the magnetization reversal: from quasi-curling to vortex domain wall propagation modes when the crystal ‘c’ axis tilts more than 75° in respect to the nanowire axis.

2D and 3D ordered arrays of Co magnetic nanowires

Cobalt nanowire arrays spatially distributed in 2D and 3D arrangements have been performed by pulsed electrodeposition into the pores of planar and cylindrical nanoporous anodic alumina membranes, respectively. Morphological characterization points out the good filling factor reached by electroplated Co nanowires in both kinds of alumina membranes exhibiting hexagonally self-ordered porous structures. Co nanowires grown in both kinds of alumina templates exhibit the same crystalline phases. DC magnetometry and First Order Reversal Curve (FORC) analysis were carried out in order to determine the overall magnetic behavior for both nanowire array geometries. It is found that when the Co nanowires of two kinds of arrays are perpendicularly magnetized, both hysteresis loops are identical, suggesting that neither the intrinsic magnetic behavior of the nanowires nor the collective one depend on the arrays geometry. FORC analysis performed along the radial direction of the Co nanowire arrays embedded in the cylindrical alumina template reveals that the contribution of each nanowire to the magnetization reversal process involves its specific orientation with respect to the applied field direction. Furthermore, the comparison between the magnetic properties for both kinds of Co nanowire arrays allows discussing about the effect of the cylindrical geometry of the template on the magnetostatic interaction among nanowires.

Effects of pH on the crystallographic structure and magnetic properties of electrodeposited cobalt nanowires

Anodic aluminum oxide templates with pore diameter of 40 nm and inter pore separation of 100 nm are prepared by two step anodization in 0.3 M oxalic acid solution. These templates are used to fabricate dc-deposited Co nanowires at different pH values of acidic bath. Continuous and densely packed nanowires having length ~8 µm are observed. The hcp configuration appeared at moderate and high pH whereas both fcc and hcp phases are observed at low pH. However the crystallinity distorted at high pH due to formation of polycrystalline structure of cobalt nanowires. Alignment of easy-axis of nanowires can be tailored by varying pH of solution.

Magnetic Properties of Cobalt Nanowires and Nanotubes

Development of novel magnetic materials has played an important role in modern day science and technology. Nanomagnetism research not only sparks interests in fundamental physics, but also renders promising technological applications. Ferromagnetic Co nanowires, synthesized via low voltage electrodeposition method. High resolution transmission electron microscopy and x-ray diffraction results show that the nanowires are uniform in size, and consist predominantly hcp structure with the magnetocrystalline easy axis (c-axis) perpendicular to the wire axis. SQUID measurement illustrates the dominance of shape anisotropy, manifested by the weak temperature dependence of the enhanced coercive field along the wire axis. Furthermore, the magnetic domain structures of individual, segmented or multiple nanowires are studied via magnetic force microscopy. It shows a strong dipole at the ends of the wire, together with a spatial magnetization modulation along the wire with a period around 700 nm. Based on theoretical modeling, such intrinsic modulation originates from the competition between the magnetocrystalline polarization along the easy axis and the shape anisotropy along the wire axis. Low temperature magnetoresistance measurement shows irreversible magnetization switching in addition to the anisotropic behavior due to magnetization orientation dependent electron scattering.On the other hand, Cobalt nanotube arrays embedded in hexagonally ordered anodic aluminum oxide templates are fabricated by direct electrodeposition with low current density under several mA/cm2. The wall thickness is about 15 nm and outer diameters are in accordance with pore diameter around 80 nm. Nearly 100% filling rate is achieved which benefits from the through pores which enable the efficient penetration of electrolyte. Transmission electron microscopy and selected-area electron diffraction are performed to show that the nanotubes are predominantly hcp single crystals with c-axis (easy axis) perpendicular to the tube axis. SQUID and MFM results indicate that the magnetization follows a circumferential direction around the tube, which is manifested in the weak magnetic signal in the MFM imaging as well as a sheared hysteresis dependence for field applied along the tube axis. This phenomenon has been confirmed by theoretical modeling taking into account the magnetocrystalline, shape demagnetization and magnetic exchange energies.

Magnetic anisotropy of 3 nm diameter Co nanowires embedded in CeO2/SrTiO3(001): a ferromagnetic resonance study

The magnetic anisotropy of 3-nm wide cobalt nanowires embedded in epitaxial CeO2/SrTiO3(001) layers is investigated by ferromagnetic resonance measurements. The measured magnetic shape and the magnetocrystalline anisotropies confirm that the Co nanowires have their main axes perpendicular to the film surface, and they are composed of hcp Co grains with the c-axes oriented along one of the directions of the CeO2 matrix. The effects of such a peculiar structure on the magnetic anisotropy are addressed experimentally. The results show that the magnetic anisotropy of the wires is dominated by the magnetostatic term. The inhomogeneous structure of the wires leads to an effective magnetocrystalline anisotropy smaller than the bulk value of hcp Co.

Platinum-Coated Cobalt Nanowires as Oxygen Reduction Reaction Electrocatalysts

Cobalt nanowires (CoNWs) are coated with platinum (Pt) by partial galvanic displacement, forming core/shell wires 200–300 nm in diameter and 100–200 μm in length. Pt-coated CoNWs (PtCoNWs) are characterized for activity in the oxygen reduction reaction (ORR) with rotating disk electrode half-cells in 0.1 M perchloric acid electrolytes. The resulting catalysts demonstrate ORR-specific activities in the range 2053–2783 μA cmPt−2, comparable to the specific activity of polycrystalline Pt. The specific activities of PtCoNWs increase with decreasing Pt content and exhibit a corresponding increase in Pt lattice compression. PtCoNWs have exhibited a maximum mass activity of 793 mA mgPt−1, 2.6 times greater than carbon-supported Pt nanoparticles.

Structure and magnetic properties of Co nanowires electrodeposited into the pores of anodic alumina membranes

Cobalt nanowires were obtained in the process of electrodeposition into pores of an alumina membrane. Structural research (XRD, TEM) of Co revealed the face-centered cubic structure. However, the existence of the hexagonal structure cannot be excluded due to strong texture. The influences of an external magnetic field and Al2O3 membrane geometry on magnetic properties of obtained wires were examined. It was found that cobalt nanowires exhibit pronounced shape anisotropy in a direction parallel to the wire axis. The highest influence on the magnetic properties is ascribed to the nanowires geometry i.e., height, diameter, and distances between single wires. Application of an external magnetic field in a perpendicular direction to the sample surface during cobalt electrodeposition increases magnetic anisotropy with a privileged direction along the wire axis. Application of the magnetic field in a parallel direction to the sample surface changes the direction of magnetization.

Ultrahigh-density sub-10 nm nanowire array formation via surface-controlled phase separation.

We present simple, self-assembled, and robust fabrication of ultrahigh density cobalt nanowire arrays. The binary Co-Al and Co-Si systems phase-separate during physical vapor deposition, resulting in Co nanowire arrays with average diameter as small as 4.9 nm and nanowire density on the order of 10(16)/m(2). The nanowire diameters were controlled by moderating the surface diffusivity, which affected the lateral diffusion lengths. High resolution transmission electron microscopy reveals that the Co nanowires formed in the face-centered cubic structure. Elemental mapping showed that in both systems the nanowires consisted of Co with undetectable Al or Si and that the matrix consisted of Al with no distinguishable Co in the Co-Al system and a mixture of Si and Co in the Co-Si system. Magnetic measurements clearly indicate anisotropic behavior consistent with shape anisotropy. The dynamics of nanowire growth, simulated using an Ising model, is consistent with the experimental phase and geometry of the nanowires.

High energy product developed from cobalt nanowires.

Cobalt nanowires with high aspect ratio have been synthesized via a solvothermal chemical process. Based on the shape anisotropy and orientation of the nanowire assemblies, a record high room-temperature coercivity of 10.6 kOe has been measured in Co nanowires with a diameter of about 15 nm and a mean length of 200 nm. As a result, energy product of the wires reaches 44 MGOe. It is discovered that the morphology uniformity of the nanowires is the key to achieving the high coercivity and high energy density. Nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.

Solution epitaxial growth of cobalt nanowires on crystalline substrates for data storage densities beyond 1 Tbit/in2.

The implementation of nano-objects in numerous emerging applications often demands their integration in macroscopic devices. Here we present the bottom-up epitaxial solution growth of high-density arrays of vertical 5 nm diameter single-crystalline metallic cobalt nanowires on wafer-scale crystalline metal surfaces. The nanowires form regular hexagonal arrays on unpatterned metallic films. These hybrid heterostructures present an important perpendicular magnetic anisotropy and pave the way to a high density magnetic recording device, with capacities above 10 Terabits/in(2). This method bypasses the need of assembling and orientating free colloidal nanocrystals on surfaces. Its generalization to other materials opens new perspectives toward many applications.

Growth of the cobalt nanowires using AC electrochemical deposition on anodized aluminum oxide templates

The nanopore arrays were fabricated by a two-step anodization of aluminum in an aqueous solution of 0.3 M oxalic acid at the temperature of 6 °C by applying a constant direct current (DC) of 40 V. A template-assisted synthesis of Co nanowire arrays was carried out by electrodeposition into the pores of prepared AAO templates. Cobalt (Co) nanowires of high density and uniformity were synthesized via an electrochemical deposition of metallic Co nanoparticles using a nanoporous template of anodized aluminum oxide, by applying an alternative current (AC).

A comparison of the magnetic properties of Ni and Co nanowires deposited in different templates and on different substrates

Nickel (Ni) and cobalt (Co) nanowire arrays (NWs) grown by electrodeposition in porous nano-templates are studied by the ferromagnetic resonance (FMR) technique at room temperature (RT) by comparing the effects of template type (alumina and polycarbonate) and the deposition substrate (i.e., metallic back contact). The line-width and resonance field of the FMR spectra strongly depends on the orientation of the applied field direction. A model is developed to analyze the spectra in order to extract the magnetic parameters such as g-values, spin–spin relaxation times (T2) and uniaxial anisotropy parameters. The experimental FMR spectra and their resonance field values were fitted using the imaginary part of magnetic susceptibility and a dispersion relation of magnetization, including the Bloch–Bloembergen type damping term. The easy axes of magnetization for all Ni and Co NWs were found to be perpendicular to the wire-axis. Surface spin modes have been observed only when pure Au was used as substrate. A discussion will be provided to explain the observed differences in terms of the anisotropic behavior and magnetic parameters of the NWs for different substrates and growth templates.

Growth of Cobalt Nanowires under External Magnetic Field

Metallic cobalt (Co) nanowires with a mean diameter of about 240 nm and lengths up to 30 μm are grown in solution by electroless deposition under external magnetic. Without magnetic field, only quasi-spherical Co nanoparticles are formed. In the presence of the magnetic field, strong attractive dipolar interactions are induced among the Co nanoparticles. This results in the preferential assembly of Co nanoparticles into nanowires with wire axes parallel to the magnetic field direction. Stronger magnetic field intensity produces longer and thinner Co nanowires. The Co nanowires exhibit ferromagnetic properties at room temperature with an enhanced coercivity of 800 Oe due to shape anisotropy.