The effects of composition and thermal treatment on the magnetic properties of Fe100-x Co x nanowire arrays based on AAO templates

A series of Fe_100-xCo_x nanowire arrays with about 6.0 μm in length and 60 nm in diameter have been fabricated successfully by AC deposition of Fe and Co atoms into anodic aluminum oxide (AAO) templates. Samples with different composition could be obtained by adjusting the concentration ratio of Fe²⁺ and Co²⁺ in the solution of the electrolyte. The composition of the samples were determined by atomic absorption spectroscopy. Structural analysis was performed using scanning electron microscopy and X-ray diffraction. The magnetic properties of the samples were examined by vibrating sample magnetometry. The effects of composition and thermal treatment on the magnetic properties of the nanowire arrays have been examined. XRD shows that the nanowires have a body-centred-cubic (bcc) structure with a preferred orientation of the <110> axis parallel to the nanowires. When x is between 80 and 90, nanowires undergo a phase transformation α → γ which is very different from Fe–Co bulk alloy. Furthermore, a localized magnetization reversal mechanism of the nanowire arrays was conformed. It is also found that the magnetic properties of the arrays are critically dependent on the compositions and thermal treatment. With suitable choices of these factors, a kind of soft ferromagnetic film can be produced while maintaining a high coercivity and squareness.     

Fabrication and magnetic properties of Fe<Subscript>3</Subscript>Co<Subscript>7</Subscript> alloy nanowire arrays

Fe₃Co₇ alloy nanowire arrays have been fabricated by direct current electrodeposition of Fe²⁺ and Co²⁺ into anodic aluminum oxide (AAO) templates. The phase structure and magnetic properties of the nanowires were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). Magnetic measurements show that the coercivity and remanence of the as-deposited Fe₃Co₇ Alloy nanowires increase dramatically after heat-treatment at 773 K for 2 h, and the nanowire arrays exhibit uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axes owing to the large shape anisotropy. The great difference between practical coercivity and ideal coercivity was also discussed in detail.     

Electrochemical fabrication and magnetic properties of Fe<Subscript>7</Subscript>Co<Subscript>3</Subscript> alloy nanowire array

A new type of magnetic material, Fe₇Co₃ nanowires, was successfully synthesized for the first time via a simple electrodeposition method. Highly uniform, self-ordered porous anodic aluminum oxide (AAO) membranes were prepared by the way of electrochemical. Fe₇Co₃ alloy nanowire arrays were fabricated in the porous alumina template in an aqueous solution of FeCl₂ and CoCl₂ by direct current electrodepositing. The microstructures of nanowires and AAO template were characterized by XRD, SEM, and TEM. The results show that a single Fe₇Co₃ nanowire is 40 nm in width and 2.5 μm in length with a preferred crystal face (110) during growing. The Fe₇Co₃ nanowire arrays have uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axis due to the large shape anisotropy. It also shows that Fe₇Co₃ nanowire is a well-soft magnetic phase compared with Fe nanowires. It illustrates that Fe₇Co₃ possess higher saturation magnetization.     

Tailoring coercivity and magnetic anisotropy of Co nanowire arrays by microstructure

To tailor coercivity and magnetic anisotropy, we have fabricated Co nanowire arrays in the pores of anodic aluminum oxide templates by electrodeposition. Microstructure measurements performed by X-ray diffraction show that Co nanowire arrays are hexagonal close-packed (HCP) structures with different crystalline textures. A wide range in change of coercivity from 925 to 3310 Oe at 300 K, with a maximum of up to 4050 Oe at 5 K, can be found for nanowire arrays with a diameter of 20 nm. This may be the highest value and the widest range of coercivity reported for Co nanowires prepared by electrodeposition method. This finding could be attributed to the adjustment of the microstructure of the cobalt nanowire arrays prepared in different experimental conditions. We have also investigated the relationship between the crystalline textures and the magnetic properties of Co nanowire arrays using micromagnetic simulation combined with microstructure measurements. The preferred orientation of nanowire arrays, such as (1000) or (0002), is a key factor in determining coercivity. This wide tailoring of coercivity makes possible more promising applications of Co nanowire arrays with fixed diameter and length.     

Effect of the Electrodeposition Frequency,Wave Form,and Thermal Annealing on Magnetic Properties of [Co0.975Cr0.025]0.99Cu0.01 Nanowire Arrays

Highly ordered [Co _0.975Cr _0.025]_0.99Cu _0.01 nanowire arrays were electrodeposited by conducting alternating current (AC) conditions from sulfate-based electrolyte into nanopores of anodic aluminum oxide (AAO) template with 37 nm pore diameter and the interpore distances of almost 50 nm. Fabricated nanowire arrays were characterized using scanning electron microscopy, alternating gradient force magnetometer, and X-ray diffraction. The results illustrated that varying frequency, wave form, and annealing procedure had influence on magnetic properties of as deposited nanowires. The nanowire arrays electrodeposited at different electrodeposition frequencies show remarkably different magnetic behaviors. Due to increasing of the electrodeposition frequency, the rate of ions for reduction was decreased. The nanowires prepared at various wave form illustrated insignificant impact on magnetic properties. X-ray diffraction patterns display that both as-deposited and annealed nanowire arrays expose the same structure. The raised value of coercivity has been determined in annealed nanowire arrays. Magnetization measurements show that the maximum value of coercivity for [Co _0.975]_0.99Cu _0.01 nanowires is observed at high temperature.     

Microstructure and magnetic properties of electrodeposited Co/Cu multilayer nanowire arrays

Highly uniform Co/Cu multilayer nanowire arrays had been electrodeposited into the nanochannels of porous anodic aluminum oxide template. X-ray diffraction pattern showed that Co and Cu grow in their HCP and FCC structures, respectively. Each nanowire had the same length with 20 μm and the diameter with 50 nm. The thickness of Co was 50 nm and Cu layer was about 5 nm. Magnetic measurements of the nanowire arrays showed that the magnetic coercivity for the applied field parallel to the nanowires is larger than that perpendicular to the anowires. The magnetic coercivity of Co multilayer nanowire arrays is smaller than that of the Co/Cu nanowire arrays and the crystal direction of Co layers were not obviously affected by Cu layer. The Co/Cu nanowire arrays exhibited excellent Giant Magneto Resistive ratio of about 75%.     

Enhancement of the coercivity of electrodeposited nickel nanowire arrays

In the present study, the single-crystal Ni nanowire arrays with a preferred growth along the [110] direction have been prepared by the deposition of Ni into the alumina template with nanopores at a current density of 2.0 mA/cm². The single-crystal Ni nanowire arrays show a magnetic anisotropy with the easy axis parallel to the nanowires and an enhanced coercivity as compared with the polycrystalline Ni nanowire arrays. A large coercivity of 1110 Oe together with a high remanence M_r = 0.92M_s is observed for 15-nm diameter single-crystal Ni nanowire arrays. The preferred growth mechanism of the single-crystal nanowires is briefly discussed.     

Template electrodeposition to cobalt-based alloys nanotube arrays

Parallel uniform arrays of amorphous ferromagnetic Co₈₁Ni₁₉ and Co₃₇Fe₆₃ alloy nanotubes with outer diameter around 325-365 nm, wall thickness of 30-60 nm and length of over 40 μm were prepared by a direct current electrodeposition with mercury cathode using porous anodic aluminum oxide membrane as template. The morphology, structure, composition and magnetic property were studied. The results showed that mercury cathode is the key factor to form amorphous alloy nanotubes, and the as-prepared nanotube arrays exhibit obvious uniaxial magnetic anisotropy and the easy magnetization direction is perpendicular to the nanotubes axis. The mechanism of formation of Co based alloy nanotubes was also discussed.     

Fabrication of CoZn alloy nanowire arrays: Significant improvement in magnetic properties by annealing process

Highly ordered arrays of Co_1−xZn_x (0 ≤ x ≤ 0.74) nanowires (NWs) with diameters of ∼35 nm and high length-to-diameter ratios (up to 150) were fabricated by co-electrodeposition of Co and Zn into pores of anodized aluminum oxide (AAO) templates. The Co and Zn contents of the NWs were adjusted by varying the ratio of Zn and Co ion concentrations in the electrolyte. The effect of the Zn content, electrodeposition conditions (frequency and pH) and annealing on the structural and magnetic properties (e.g., coercivity (Hc) and squareness (Sq)) of NW arrays were investigated using X-ray diffraction (XRD), scanning electron microscopy, electron diffraction, and alternating gradient force magnetometer (AGFM). XRD patterns reveal that an increase in the concentration of Zn ions of the electrolyte forces the hcp crystal structure of Co NWs to change into an amorphous phase, resulting in a significant reduction in Hc. It was found that the magnetic properties of NWs can be significantly improved by appropriate annealing process. The highest values for Hc (2050 Oe) and Sq (0.98) were obtained for NWs electrodeposited using 0.95/0.05 Co:Zn concentrations at 200 Hz and annealed at 575 °C. While the pH of electrolyte is found to have no significant effect on the structural and magnetic properties of the NW arrays, the electrodeposition frequency has considerable effects on the magnetic properties of the NW arrays. The changes in magnetic property of NWs are rooted in a competition between shape anisotropy and magnetocrystalline anisotropy in NWs.     

Study on the structures and magnetic properties of Ni, Co-Al2O3 electrodeposited nanowire arrays

We report on the direct electrodeposition of nickel and cobalt nanowire arrays within the nanopores of ordered porous alumina films prepared by a two-step anodization. SEM and TEM images reveal that the pore arrays are regularly arranged throughout the alumina film. X-ray diffraction and TEM analysis show that the nickel and cobalt nanowires are single crystalline with highly preferential orientation. The aspect ratio of nanowires is over 300. M-H hysteresis loops determined by VSM indicate that the nanowire arrays obtained possess obvious magnetic anisotropy. Because of proper square ratio and coercivity the nanowire arrays of nickel seem to be more suitable candidates for perpendicular magnetic recording medium than those of cobalt.     

A study on phase relations and texture of Co1 − xPtx (0.09 < x < 0.86) nanowire arrays

Based on Rietveld refinement of X-ray diffraction patterns, the phase structure and microstructural parameters of Co_1 − xPt_x nanowires are determined for a range of Pt content. The phase structure of the as-deposited Co_1 − xPt_x(0.09 < x < 0.86) nanowire arrays changes progressively from hcp ε-Co to a mixture of the hcp ε-Co and fcc α-Co,Pt solid solution and finally to pure fcc Co,Pt solid solution with Pt content increasing . Moreover, the texture parameter P₍₁₁₁₎ has a maximum value with Pt content of 50% confirmed by the (111) pole figure measurement. It is suggested that this contributes to enhance magnetocrystalline anisotropy, resulting in a relatively high squareness and coercivity for the nanowires.     

Synthesis of high-ordered LiNi0.5Co0.5O2 nanowire arrays by AAO template and its structural properties

High-ordered LiNi_0.5Co_0.5O₂ nanowire arrays were prepared using porous anodic aluminum oxide (AAO) template from sol-gel solution containing Li(OAc), Ni(OAc)₂ and Co(OAc)₂. Electron microscope results showed that uniform length and diameter of LiNi_0.5Co_0.5O₂ nanowires were obtained, and the length and diameter of LiNi_0.5Co_0.5O₂ nanowires are dependent on the pore diameter and the thickness of the applied AAO template. X-ray diffraction and electron diffraction pattern investigations demonstrate that LiNi_0.5Co_0.5O₂ nanowires are a layered structure of LiNi_0.5Co_0.5O₂ crystal. X-ray photoelectron spectroscopy analysis indicates that the most closely resemble stoichiometric layered LiNi_0.5Co_0.5O₂ material has been obtained.     

Electrodeposition and magnetic properties of ternary Fe-Co-Ni alloy nanowire arrays with high squareness ratio

Highly-ordered ternary Fe-Co-Ni alloy nanowire arrays with diameters of about 50 nm have been fabricated by alternating current (AC) electrodeposition into the nanochannels of porous anodic aluminum oxide templates. SEM and TEM results indicate that the alloy nanowires are highly ordered. XRD and HRTEM results show that the ternary FeCoNi alloy nanowires are polycrystalline, with HCP-FCC dual phase structure. Magnetic measurements demonstrate that the ternary alloy nanowire arrays have an obvious magnetic anisotropy with an easy magnetization direction being parallel to the nanowire arrays. Along the easy magnetization direction, the coercivity (H _c ) and squareness ratio (S) increase as the annealing temperature increases, and reach a maximum level (H _c = 1337 Oe, S = 0.96) at 300 °C.     

Superconducting Properties of Pb/Bi Films Quench-Condensed on a Porous Alumina Substrate Filled with Co Nanowires

Superconducting properties of a new ferro-magnet–superconductor hybrid structure have been investigated. Organized arrays of Co nanowires are first electroplated into the columnar pores of anodic aluminum oxide (AAO) membranes. Superconducting Pb/Bi (18 at.%) films are then quench-condensed onto the surface of the AAO membranes filled with Co nanowires. The Co nanowire array produces a magnetic field with a strong spatial variation in the superconducting film. Hysteretic superconducting properties and enhanced critical currents have been observed in applied external magnetic fields, which we explain based on the magnetic domain structure of the Co nanowire arrays.     

NiCo films with perpendicular magnetization anisotropy deposited on dielectric substrate by using polyol process

In this paper, the polyol process, a catalyst free, non-aqueous, and electroless process, is developed to deposit the nanostructured Ni_xCo_100 ? x magnetic films on aluminum nitride (AlN) substrate. Nickel (II) acetate tetrahydrate and cobalt (II) acetate tetrahydrate were reduced by ethylene glycol (EG) at 180 °C, and the reduced Ni and Co nanostructures were deposited on the AlN substrate merged in boiling EG for 60 min. The elongated nanostructures in the films are detected through the scanning electron microscopy (SEM). Interestingly, some of the elongated nanostructures are pointing out of the substrate. It indicates that the component ratio of Ni and Co in the films is different with the starting precursor molar ratio. The film thickness increases from 1 to 1.8 μm when the atomic ratio of Co (at.%) in the film increased from 44.6% to 70.8%. Furthermore, it is found that the crystallite size decreases from 44 to 25 nm with increasing Ni (at.%). In addition, the magnetic properties have been analyzed through vibrating sample magnetometer (VSM) at room temperature. The results show that the films have the perpendicular preferred anisotropy. The anisotropy field (H_K) for the Ni₅₀Co₅₀ is about 4.75 kOe, which is possibly caused by the assembled direction of the elongated nanostructures.     

Giant Magneto-Optical Properties in Hybrid Nanostructures of Diluted Magnetic Semiconductors With Ferromagnetic Co Wires

Giant Zeeman effects of excitons were studied in hybrid nanostructures of diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co, where the DMS-QW wires with the width of 200 nm were sandwiched in the Co wires with the width of 400 nm. When the direction of the Co magnetization is normal to the wire plane, a magnetic field B ^⊥ _Co‐flux perpendicular to the well plane can be applied from the Co wires to all of the DMS-QW wire. In addition, the use of a Co/Pt multilayered film with the perpendicular magnetization at zero external field was examined instead of the pure Co film. The excitonic photoluminescence spectrum from the DMS-QW shows significant broadening in the hybrid structures, indicating the giant Zeeman shift of excitons induced by B ^⊥ _Co‐flux.     

交流电化学沉积FeCo合金纳米线阵列及其磁性能研究

采用二步阳极氧化法制备孔结构高度有序的多孔氧化铝(AAO)模板。在不同Fe2+/Co2+摩尔比的电解液中,利用交流电化学沉积,在模板孔内成功制备了FeCo合金纳米线阵列。分别采用透射电子显微镜(TEM),场发射扫描电子显微镜(FE-SEM),X射线衍射仪(XRD)和震动样品磁强计(VSM)对样品的形貌,结构及磁学性能进行了表征。结果表明,制备的FeCo合金纳米线排列有序,粗细均匀;其直径与模板孔径一致,填充率较高,且具有明显的[110]择优取向。VSM测试结果表明,不同溶液浓度下制备的FeCo合金纳米线阵列均具有良好的垂直磁各向异性,易磁化轴沿着纳米线轴线方向。随着电解液Fe2+/Co2+摩尔比的不同,可在一定范围内对FeCo合金纳米线阵列的磁性进行调控,并对其原因进行了讨论。     

Field emission of Co nanowires in polycarbonate template

The Co nanowire arrays were synthesized by electrodeposition in polycarbonate template (PC) with 4 μm thickness. Electron field emission properties of cobalt nanowires were studied for wires with different aspect ratios, R ranged between 10 and 60, while the diameter of wires was fixed about 50 nm. The field emission properties of the samples showed low turn on electric field (E_to) with values varying between 2.9 and 11.3 V/μm showing a minimum value for R = 20 (E_to < 3 V/μm). On the other hand, the enhancement factor shows a peak for nanowires length about 1 μm. Field emission data using the Fowler-Nordhiem theory showed nearly straight-line nature confirming cold field emission of electrons. The fabricated field emitter arrays of cobalt nanowires in the PC templates opens the possibility of fabricating flexible flat panel displays.     

Self-organized magnetic nanowire arrays based on alumina and titania templates

Densely packed arrays of magnetic nanowires have been synthesized by electrodeposition filling of nanopores in alumina and titania membranes formed by self-assembling during anodization process. Emphasis is made on the control of the production parameters leading to ordering degree and lattice parameter of the array as well as nanowires diameter and length. Structural, morphological and magnetic properties exhibited by nanowire arrays have been studied for several nanowire compositions, different ordering degree and for different nanowire aspect ratios. The magnetic behaviour of nanowires array is governed by the balance between different energy contributions: shape anisotropy of individual nanowires, the magnetostatic interaction of dipolar origin among nanowires, and magnetocrystalline and magnetoelastic anisotropies induced by the pattern templates. These novel nanocomposites, based on ferromagnetic nanowires embedded in anodic nanoporous templates, are becoming promising candidates for technological applications such as functionalised arrays for magnetic sensing, ultrahigh density magnetic storage media or spin-based electronic devices.     

Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

The values of saturation magnetization M _s, exchange coupling constant A, local magnetic anisotropy field H _a, random anisotropy correlation radius R _c, and coercive force H _c were independently measured for multilayer Co/Pd films with nanocrystalline and amorphous Co layers. It is shown that variation of the coercive force H _c(t _Co) as a function of the cobalt layer thickness t _Co is related to changes in characteristics of the magnetic microstructure. The main factor determining changes in the ferromagnetic correlation radius R _f and the average anisotropy 〈K〉 of a magnetic block in multilayer Co/Pd films is variation of exchange coupling constant A(t ^Co).