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.     

Optimized Sol–Gel Chemical Route Using Vacuum Suction for Fabrication of Densely Packed NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanowires

Highly ordered nanowire arrays of NiFe₂O₄ spinel with a high aspect ratio were fabricated by sol–gel method associated with anodic aluminum oxide (AAO) templates. The preparation of nanowires was carried out by sol–gel method using nickel nitrate, ferric nitrate and citric acid. The molar proportion of nickel nitrate to ferric nitrate and citric acid was 1:2:3. The suction with 0.1 mbar vacuum was used to draw the gel into the AAO nanochannels. The results showed that the lowest annealing temperature is around 600?°C to obtain the single-phase nanostructured NiFe₂O₄. The NiFe₂O₄ nanowires were also uniform and parallel. TEM pictures determined the diameter size of the nanowires of about 100 nm. The magnetic results also showed that the wires have an easy axis of magnetization along their length and they are fully saturated in a field of 7 kOe. It seems that this material could be a good candidate for high perpendicular magnetic storage devices.     

Magnetic and structural properties of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript> on MgO(110) substrates

Fe _x Pt_100−x (70.1 ≤ x ≤ 83.4) thin films with ordered Fe₃Pt phase were grown successfully onto MgO(110) substrates by electron beam evaporation. The unit cell of ordered Fe₃Pt phase is elongated along c-axis direction and the thin films become more chemically ordered with decreasing Fe content. The magnetization of thin films shows a decrease when Fe content is around 79 at.%. The relationship between magnetic anisotropy and structural parameters suggests that the change of magnetic anisotropy in ordered Fe₃Pt thin films with different compositions most likely stems from the magnetocrystalline origin.     

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.     

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.     

Magnetoimpedance Effect on Single NiFe Nanowire

In this study, Ni₈₂Fe₁₈ nanowires were deposited on Au-coated porous anodic alumina oxide (AAO) template by electrochemically. Scanning electron microscopy showed that wires have diameters of about 250–310 nm and length of 50–60 μm. NiFe nanowire arrays embedded in AAO were mechanically polished until nanowires appeared. Later, electrical contacts were made to a single nanowire using a focused ion beam (FIB) system. Magnetoimpedance (MI) properties of single nanowire were investigated, and nearly 3.5 % MI was observed at 4 GHz driving current frequency.     

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(⊥).     

Dielectric and ferromagnetic properties of BaTiO<Subscript>3</Subscript>/Ni<Subscript>0.93</Subscript>Co<Subscript>0.02</Subscript>Cu<Subscript>0.05</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> composites

xBaTiO₃ + (1 − x)Ni_0.93Co_0.02Cu_0.05Fe₂O₄ (x = 0.5, 0.6, 0.7, 0.8) composites with ferroelectric–ferromagnetic characteristics were synthesized by the ceramic sintering technique. The presence of constituent phases in the composites was confirmed by X-ray diffraction studies. The average grain size was calculated by using a scanning electron micrograph. The dielectric characteristics were studied in the 100 kHz to 15 MHz. The dielectric constant changed higher with ferroelectric content increasing; and it was constant in this frequency range. The relation of dielectric constant with temperature was researched at 1, 10, 100 kHz. The Curie temperature would be higher with frequency increasing. The hysteresis behavior was studied to understand the magnetic properties such as saturation magnetization (M _s). The composites were a typical soft magnetic character with low coercive force. Both the ferroelectric and ferromagnetic phases preserve their basic properties in the bulk composite, thus these composites are good candidates as magnetoelectric materials.     

Sol–gel template synthesis of LiV<Subscript>3</Subscript>O<Subscript>8</Subscript> nanowires

The LiV₃O₈ nanowires are fabricated by using sol–gel process with porous anodic aluminum oxide (AAO) as the template. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) characterizations show that the synthesized LiV₃O₈ nanowires are monodispersed and paralleled to one another. Selected area electron diffraction (SAED) pattern, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) investigations jointly demonstrate that the synthesized nanowires are most consisted of monoclinic phase LiV₃O₈. Since the LiV₃O₈ nanowires can be mass-produced by using this method, it is expected to find promising application as a new cathode material in lithium ion battery.     

Enhanced electrochemical reduction of hydrogen peroxide by Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowire electrode

Crystalline Co₃O₄ nanowire arrays with different morphologies grown on Ni foam were investigated by varying the reaction temperature, the concentration of precursors, and reaction time. The Co₃O₄ nanowires synthesized under typical reaction condition had a diameter range of approximately 500–900 nm with a length of 17 µm. Electrochemical reduction of hydrogen peroxide (H₂O₂) of the optimized Co₃O₄ nanowire electrode was studied by cyclic voltammetry. A high current density of 101.8 mA cm^?2 was obtained at ?0.4 V in a solution of 0.4 M H₂O₂ and 3.0 M NaOH at room temperature compared to 85.8 mA cm^?2 at ?0.35 V of the Co₃O₄ nanoparticle electrode. Results clearly indicated that the Ni foam supported Co₃O₄ nanowire electrode exhibited superior catalytic activity and mass transport kinetics for H₂O₂ electrochemical reduction.     

Synthesis and photocatalytic properties of BiOCl nanowire arrays

Bismuth oxychloride (BiOCl) nanowire arrays have been successfully prepared employing the Anodic Aluminum Oxide (AAO) template assisted sol-gel method. Nanowires of 100 nm diameter and length 2-6 μm, assembled in the porous of AAO templates, were formed. XRD and HRTEM results show that the nanowires are pure BiOCl polycrystal phase without Bi₂O₃ or BiCl₃. The photocatalytic activity of BiOCl nanowire arrays was investigated by the degradation of Rhodamine B dye solution under UV irradiation.     

Effect of NaOH Concentration and Adding Sequences on Structural and Magnetic Properties of Ni<Subscript>0.4</Subscript>Co<Subscript>0.6</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanopowders Prepared Via Co-precipitation Route

Ni_0.4Co_0.6Fe₂O₄ nanopowders were prepared via the co-precipitation route followed by annealing treatment. The structural and magnetic properties of the as-synthesized samples were determined by XRD, FT-IR, TG-DSC, and PPMS measurements, respectively. The XRD patterns indicated a single-phase cubic spinel structure for all the Ni-Co ferrite samples, regardless of adding sequences of the reactants or NaOH concentration. The analysis of the XRD patterns revealed that the enhancement in lattice constant with increasing NaOH concentration is related to the prevention of oxidization of Co²⁺ ions in the Ni-Co ferrite lattice. The FT-IR spectra indicated that samples prepared in the B process have fewer impurities than those prepared in the A process. The enhancement in saturation magnetization with the increase in sodium hydroxide concentration could be attributed to the strengthening of super-exchange interaction between A and B sublattices, due to replacements of Co³⁺ ions (magnetic moment of 0 μ _B) by Co²⁺ ions (magnetic moment of 3 μ _B) at B sublattices. The obvious increase in the coercivity field with the increase in concentration of NaOH solutions can be interpreted in terms of enhancement of magneto-crystalline anisotropy that originated from gradual substitutions of Co³⁺ ions with Co²⁺ ions at the octahedral sites.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Electrochemical capacitance study on Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowires for super capacitors application

One-dimensional Co₃O₄ nanowires have been prepared by utilizing the ordered mesoporous silica material SBA-15 as template. The results of transmission electron microscope (TEM) and N₂ adsorption–desorption characterizations show that the Co₃O₄ nanowires possess a uniform size and a large Brunauer-Emmett-Teller (BET) surface area. Its electrochemical performance was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in various concentration of KOH solution. This nanomaterial shows a small resistance, a high specific capacitance (SC), and a strong cyclic stability. The maximal SC value of 373 F·g⁻¹ was obtained in 6 M electrolyte under the scan speed of 3 mV·s⁻¹ at the first CV cycle. After 500 CV cycles, the SC value is about 90% of the original value. It is considered that the short path of ion transfer given by nanomaterial brought on the great pseudo capacitance performance.     

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.     

Inverted surface hysteresis loops in heterogeneous (nanocrystalline/amorphous) Fe<Subscript>81</Subscript>Nb<Subscript>7</Subscript>B<Subscript>12</Subscript> alloys

Inverted hysteresis loops were observed for the first time in the near-surface layers of heterogeneous (nanocrystalline/amorphous) Fe₈₁Nb₇B₁₂ alloys. In particular, a negative residual magnetization is retained when a positive magnetic field applied in the sample plane is decreased to zero. The inverted hysteresis is qualitatively explained within the framework of a two-phase model, according to which the heterogeneous alloys contain two dissimilar phases exhibiting uniaxial magnetic anisotropy and featuring antiferromagnetic exchange interaction.     

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 of superparamagnetic and flexible γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowire arrays in an anodic aluminum oxide template

Superparamagnetic and flexible Fe₂O₃ nanowire arrays were fabricated by the controlled electrostatic assembly of iron oxide nanoparticles and poly(dimethyldiallylammonium chloride) (PDADMAC) in an anodic aluminum oxide (AAO) template. The micrograph of iron oxide nanowire arrays was characterized by field emission scanning electron microscopy. The magnetic hysteresis loops obtained by a vibrating sample magnetometer confirm that the nanowire arrays have superparamagnetic properties. The filling ratio of iron oxide nanoparticles and polymers in the AAO template was affected by four factors, including the concentration of iron oxide nanoparticles, the pore diameter of the AAO template, the charge ratio of iron oxide nanoparticles and PDADMAC, and the molecular weight of polyacrylic acid. The effect of the AAO template on the diameter and length of the nanowire arrays was also analyzed. In addition, the nanowire arrays were shown to be flexible because of the presence of polymers. These nanowire arrays with superparamagnetic and flexural properties have potential applications in sensor probes.     

Lightweight glass/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-polyaniline composite hollow spheres with conductive and magnetic properties

Lightweight composite hollow spheres with conductive and magnetic properties were prepared by using Hollow Glass Spheres (HGS) as substrate. The morphology, composition, conductive, and magnetic properties of the resultant products were characterized by SEM, EDX, XRD, FTIR spectra, conductivity measurement, and vibrating-sample magnetometry. Polyaniline (PANI) were in situ polymerized on HGS with increasing ratios of PANI to HGS, resulting in the enhanced conductivity of HGS/PANI composites from 1.3 × 10⁻² S/cm to 4.4 × 10⁻² S/cm. Lightweight glass/Fe₃O₄-PANI composite hollow spheres (HGS/Fe₃O₄-PANI) with conductivity of 5.4 × 10⁻³ S/cm and magnetization of 9.25 emu/g were prepared by deposition of Fe₃O₄ nanoparticles onto HGS via electrostatic adsorption first, and then polymerization of aniline onto HGS/Fe₃O₄. The glass/PANI-Fe₃O₄ composite hollow spheres (HGS/PANI-Fe₃O₄) composed of Fe₃O₄ as the outmost layer and PANI as the inner layer were prepared for comparison. The conductivity and magnetization of HGS/PANI-Fe₃O₄ were 1.1 × 10⁻⁴ S/cm and 2.61 emu/g, respectively.     

Magnetic force microscopy and simulation studies on Co<Subscript>50</Subscript>Fe<Subscript>50</Subscript> elliptical nanomagnets

We studied the magnetization reversal mechanism of single-layered Co₅₀Fe₅₀ nanomagnets by measuring the magnetization reversal and using the micromagnetic simulations. The magnetization reversal strongly depends on the thickness of the nanomagnets. In the remanent state, the magnetic force microscopy studies and the simulation data showed the formation of single and vortex states depending on the thickness of nanomagnets.