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%.     

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.     

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.     

电化学沉积法制备Co_(0.36)Cu_(0.64)合金纳米线及其磁性

利用电化学沉积的方法制备了具有fcc结构的Co0.36Cu0.64合金纳米线阵列,并对样品在400-700℃进行了退火处理.X-射线衍射及磁测量结果显示,随着退火温度的升高CoCu合金出现相分离,伴随着相分离矫顽力显著增大.综合分析得到矫顽力的增加是由于纳米线中Co和cu的相分离使得一些Co的单畴颗粒分散在Cu之间造成的.     

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.     

Structural and magnetic properties of Co and Co71Ni29 nanowire arrays prepared by template electrodeposition

A comparative study on the structural and magnetic properties of Co and Co₇₁Ni₂₉ nanowire arrays prepared by AC electrodeposition in alumina templates has been presented. The Co and Co₇₁Ni₂₉ nanowires observed by SEM and TEM have a 45 nm diameter and exhibit high aspect ratio. Also, the nanowires of both Co and Co₇₁Ni₂₉, determined by XRD, have an identical crystallographic structure. The Co₇₁Ni₂₉ nanowires exist in a cobalt solid solution. Both the as-obtained Co and Co₇₁Ni₂₉ nanowire arrays measured by VSM show obvious magnetic anisotropy, dominated by shape anisotropy. Compared to the Co nanowire arrays, Co₇₁Ni₂₉ nanowire array shows an enhanced coercivity Hc (⊥) and approximate square ratio Mr/Ms(⊥).     

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.     

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.     

Characterization of ordered Cu2O nanowire arrays prepared by heat treated Cu/PAM composite

Cu nanowire arrays were synthesized via a porous alumina membrane (PAM) template with a high aspect ratio, uniform pore size (120–140 nm), and ordered pore arrangement. The Cu₂O nanowire arrays were prepared from the oxidization of Cu metal nanowire arrays. The electrochemical deposition potential of Cu metal nanowires (?180 mV vs. SCE) was determined from X-ray diffraction (XRD) patterns. The microstructure and chemical composition of Cu nanowire arrays were characterized using field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), and X-ray diffraction (XRD). Results indicate that the Cu/Cu₂O nanowire arrays assembled into the nanochannel of the porous alumina template with diameters of 120–140 nm. The valence of copper was controlled by the porous alumina template during the annealing process. Copper nanowires transformed to the Cu₂O phase with the space limitation of the PAM template. Single-crystal Cu₂O nanowire arrays were also obtained under the template embedded.     

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.     

Synthesis of PbTe nanowire arrays using lithographically patterned nanowire electrodeposition

We describe the preparation by electrodeposition of arrays of lead telluride (PbTe) nanowires using the lithographically patterned nanowire electrodeposition (LPNE) method. PbTe nanowires had a rectangular cross-section with adjustable width and height ranging between 60-400 nm (w) and 20-100 nm (h). The characterization of these nanowire arrays using X-ray diffraction, transmission electron microscopy and electron diffraction, scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) is reported. PbTe nanowires were electrodeposited using a cyclic electrodeposition-stripping technique that produced polycrystalline, stoichiometric, face-centered cubic PbTe with a mean grain diameter of 10-20 nm. These nanowires were more than 1 mm in length and two additional processing steps permitted their suspension across 25 microm air gaps microfabricated on these surfaces. The LPNE synthesis of lithographically patterned PbTe nanowires was carried out in unfiltered laboratory air. Nanowires with lengths of 70-100 microm showed an electrical resistivity comparable to bulk PbTe. XPS reveals that exposure of PbTe nanowires to air causes the formation on the nanowire surface of approximately one monolayer of a mixed lead oxide and tellurium oxide within a few minutes.     

Optical and magnetic properties of 30 and 60 nm Ni nanowires

Ni nanowire arrays of high aspect ratio with the diameters of about 30 nm and 60 nm were prepared by DC applied AC electrodeposition. We observe the different preferred orientation and various magnetic behaviors of 30 and 60 nm diameter nanowires. In addition, the coercivity H_c(||), squareness S(||) and the ratio H_c(||)/H_c(⊥) where the applied field is parallel (||) and perpendicular (⊥) to the long axis of nanowires increase with decreasing wire diameter. This is the first time that optical results of Ni nanowires were presented.     

Electrochemical fabrication of ordered Ag2S nanowire arrays

We have successfully fabricated ordered crystalline Ag₂S nanowire arrays by direct current electrodeposition into the nanochannels of porous anodic aluminum oxide templates from a dimethylsulfoxide solution containing AgNO₃ and elemental S. X-ray diffraction and the selected area electron diffraction investigations demonstrate that the Ag₂S nanowires are a uniform actanthite structure. Electromicroscopy results show that the nanowires are quite ordered with diameters of about 40 nm and lengths up to 5 μm. X-ray energy dispersion analysis indicates that the atomic composition of Ag and S is very close to a 2:1 stoichiometry. Furthermore, a possible mechanism for the formation of the Ag₂S nanowires is proposed.     

合成和表征NiFe2O4纳米线阵列

采用真空灌注结合溶胶-凝胶和氧化铝模板法,在多孔氧化铝模板中制备了平均直径为50 nm的NiFe2O4纳米线阵列.X射线衍射结果显示所制备的纳米线是纯相的NiFe2O4纳米线,透射电镜和电子衍射的结果显示已制备的纳米线是多晶的且表面光滑,场发射扫描电镜图片显示纳米线是大面积且平行有序的、纳米线的长度和所用的氧化铝模板的厚度相当.磁测量的结果显示此纳米线阵列有形状各向异性,同块状材料相比矫顽力有所增强.对纳米线的生长机理做了简单的讨论.     

Synthesis and characterisation of γ-Fe2O3 nanowire arrays via a versatile,simple and low-cost method

γ-Fe₂O₃ nanowire arrays embedded in anodic alumina template were fabricated by an improved sol–gel method. The morphologies, structures and magnetic behaviour of the as-prepared products were investigated by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, field emission scanning electron microscopy and magnetic hysteresis analysis. The results show that arrayed γ-Fe₂O₃ polycrystalline nanowires with an average diameter about 40?nm and an average length about 0.5?µm were prepared. A number of superparamagnetic nanoneedles grew along the nanowires. The ordered one-dimensional arrays weaken the superparamagnetic effect. In addition, a possible formation mechanism about nanowires is proposed. The charge factor, gravity effect and molecular heat movement impelled the Fe sols filling into the pores of the template. γ-Fe₂O₃ nanowire arrays look forward to the applications of magnetic recording in the future.     

Preparation and Magnetic Properties of Ni/Pd Multilayer Nanowire Arrays

Ni/Pd multilayer nanowire arrays have been electrodeposited into the nanochannels of porous polycarbonate (PC) template. Current–time profiles obtained during the deposition process were analyzed to understand the mechanism of growth. Synthesized nanowires were fully characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX), and scanning electron microscopy (SEM). The X-ray diffraction pattern showed that Pd and Ni grow in their face-centered-cubic (FCC) structures. Each nanowire had the same length of ≈4 μm and a diameter of 90 nm. The magnetic properties of the prepared nanowires are different under different electrodepositing conditions. A vibration sample magnetometer (VSM) was used to determine the effect of the Pd content on magnetic properties of final products.     

Fabrication and magnetic property of monocrystalline cobalt nanowire array by direct current electrodeposition

Ordered monocrystalline cobalt nanowire array has successfully been synthesized in a porous alumina template by direct current electrodeposition. The as-obtained cobalt nanowires with diameters of 35 nm, 45 nm, 60 nm, and length of 18 μm have been observed by transmission electron microscopy (TEM). A highly preferential orientation of the cobalt nanowires has been obtained by X-ray diffraction (XRD), and the orientation grows better as the hole diameter gradually reduces. M–H hysteresis loops determined by a vibrating sample magnetometer (VSM) indicate that the nanowire arrays obtained possess an obvious magnetic anisotropy.     

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.     

Preparation and characteristics of large-area and high-filling Ag nanowire arrays in OPAA template

Highly ordered porous anodic aluminum oxide (OPAA) on aluminum substrate with an ultrathin barrier layer was prepared by a two-step aluminum anodic oxidation plus a step-by-step voltage decrement process. Ag nanowire arrays have been successfully electrodeposited into the OPAA template through constant current technique. The morphology, composition and structure of the nanowire arrays were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that Ag nanowires possess smooth surface and high aspect ratio up to 330 with a diameter of 70-80 nm. Nearly 100% of the pores were filled by Ag nanowires, which have face center cubic structure and (111) preferential orientation. The branch of originally straight pores at the formation front was demonstrated and the branched pores favor the formation of several nucleation sites in each pore at the beginning of the electrodeposition.     

Synthesis and characterization of single crystalline selenium nanowire arrays

Ordered selenium nanowire arrays with diameters about 40 nm have been fabricated by electrodeposition using anodic porous alumina templates. As determined by X-ray diffraction, Raman spectra, electron diffraction and high-resolution transmission electron microscopy, selenium nanowires have uniform diameters, which are fully controllable. Single crystalline trigonal selenium nanowires have been obtained after postannealing at 180 °C. These nanowires are perfect with a c-axis growth orientation. The optical absorption spectra reveal two types of electron transition activity.