GMR in multilayered nanowires electrodeposited in track-etched polyester and polycarbonate membranes

Commercially available track-etched polyester membranes were used as templates to electrodeposit Co–Ni–Cu/Cu multilayered nanowires, giving room-temperature current perpendicular to plane (CPP) giant magnetoresistance (GMR) values of up to ∼12%. In contrast to similar nanowires electrodeposited in track-etched polycarbonate membranes, the GMR obtained in multilayered nanowires electrodeposited in the polyester membranes increased with decreasing Cu-layer thickness t_Cu, for t_Cu in the 2–7 nm range, indicating a lack of ferromagnetic coupling through pinholes, etc. Transmission electron micrographs showed clear evidence for smooth, parallel layer interfaces in the nanowires.     

Synthesis and magnetic properties of Mn doped ZnO nanowires

Mn doped ZnO nanowires have been synthesized using a simple autocombustion method. The as-synthesized Mn doped ZnO nanowires were characterized by X-ray diffraction and transmission electron microscopy. An increase in the hexagonal lattice parameters of ZnO is observed on increasing the Mn concentration. Optical absorption studies show an increment in the band gap with increasing Mn content, and also give evidence for the presence of Mn²⁺ ions in tetrahedral sites. All Zn_1−xMn_xO (0≤x≤0.25) samples are paramagnetic at room temperature. However, a large increase in the magnetization is observed below 50 K. This behavior, along with the negative value of the Weiss constant obtained from the linear fit to the susceptibility data below room temperature, indicate ferrimagnetic behavior. The origin of ferrimagnetism is likely to be either the intrinsic characteristics of the Mn doped samples, or due to some spinel-type impurity phases present in the samples that could not be detected.     

Synthesis and magnetic properties of Mn doped CuO nanowires

Study of diluted magnetic semiconductor nanowires is one of the important topics in materials science. By using Mn-Cu alloy as the starting material, Mn doped CuO nanowire arrays have been synthesized in air at the temperature of 550 °C. X-ray diffraction measurements and scanning electron microscopic study shows that the nanowires were grown on Cu₂O substrate. Transmission electron microscopic study shows the single crystal property of the nanowires. Magnetic measurements show ferromagnetic property in the Mn doped CuO nanowires with the critical temperature higher than 80 K.     

Arrays of Ni nanowires in alumina membranes: magnetic properties and spatial ordering

Magnetic characteristics of arrays of Ni nanowires embedded in porous alumina are reviewed as a function of their spatial ordering. The different steps for the controlled production of highly-ordered nanowires is firstly described. Nanopores are formed into an hexagonal symmetry arrangement by self-organized process during anodization of pure Al. Parameters of the anodization allow us to control their diameter, hexagonal lattice parameter and size of crystalline domains. Subsequently, Ni nanowires are grown inside the pores by electrodeposition. Control of the pores filling and of geometrical ordering characteristics has been performed by SEM, HRSEM, RBS and AFM techniques. The magnetic characterisation of the arrays has been achieved by SQUID and VSM magnetometers, while information on the magnetic state of individual nanowires is obtained by MFM. Experimental studies are presented, particularly coercivity and remanence, for arrays with different degree of ordering (crystalline domains up to around 1 m), and for ratio diameter to lattice parameter (diameter ranging between 20 and 180 nm, and distance between 35 and 500 nm). FMR studies have allows us to obtain complementary information of the anisotropy and magnetic characteristics. A modelling of multipolar interacting nanowires is introduced to account for the influence of short and long range ordering degree of the arrays.Received: 24 November 2003, Published online: 15 June 2004PACS: 75.60.Jk Magnetization reversal mechanisms - 81.15.Pq Electrodeposition, electroplating     

Electronic and magnetic properties of single-wall GeC nanotubes filled with iron nanowires

Under GGA, the structural, electronic and magnetic properties of single-wall (8, 8) GeC nanotubes filled with iron Fe_n nanowires (n = 5, 9, 13 and 21) have been investigated systematically using the first-principles PAW potential within DFT. We find that the initial shapes of the Fe₅@(8, 8), Fe₉@(8, 8) and Fe₁₃@(8, 8) systems are preserved without any visible changes after optimization. But for the Fe₂₁@(8, 8) system, the initial shapes are distorted largely for both nanowire and nanotube. The binding processes of Fe_n@(8, 8) systems are exothermic, and Fe₅@(8, 8) system is the most stable structure. The pristine (8, 8) GeCNT is nonmagnetic and direct semiconductor with a wide band gap of about 2.65 eV. Projected densities of states onto different shell Fe atoms show that the separation between the bonding and antibonding d states is reduced as going from the core Fe atom to the outermost shell Fe atom. The spin polarization of the Fe_n@(8, 8) systems and free-standing nanowires are higher than that in bulk Fe. And the spin polarization generally decreases with the number n of the Fe atoms increasing for both the Fe_n@(8, 8) systems and free-standing nanowires. Both the largest spin polarization value itself and not more decrease with respect to value of free-standing Fe₅ nanowire suggest the Fe₅@(8, 8) system could be of interest for the use in electron spin injection. The magnetism is mainly confined within the inner Fe nanowire for these combined systems. More importantly, the Fe₅ nanowire encapsulated inside (8, 8) GeCNT is under the protection of the GeCNT to prevent from oxidation, thus may stably exist in atmosphere for long time and can be expected to have potential applications in building nanodevices.     

Synthesis and properties of magnetic fluid based on iron nanoparticles prepared by a vapor-phase condensation process

Magnetic fluid containing metallic iron nanoparticles was successfully fabricated in this work. The iron nanoparticles were synthesized by chemical vapor condensation process and then dispersed in water-base solution (pH 11) with oleic acid as surfactant. More than 80% of iron nanoparticles were fully dispersed in the fluid and remained stable without any further oxidation over 200 h. Both the iron nanoparticles and the subsequent magnetic fluid exhibited typical ferromagnetic behavior.     

The influence of the orientation of polyethyleneterephtalate films irradiated by heavy ions on polymer structure and track-etched membrane pores

It is shown that the orientation of polymer macromolecules after irradiation by heavy ions allows one to produce a polyethyleneterephtalate film with increased mechanical strength and chemical resistance. Chemical etching of such a film results in the formation of elliptical shape pores oriented in the direction of film elongation.     

Dynamics of iron and europium in Nafion polymer membranes

Mossbauer spectral studies of Fe²⁺ and Eu³⁺ in Nafion polymer membranes in the temperature range between 90K and 250K have been performed. Above a critical temperature (∼180K) the spectra show a sharp decrease in the f-factor. Both the iron and europium spectra contain elastic narrow and quasielastic broad absorption lines, which are typical of bounded diffusion phenomena observed in biopolymers. The results are interpreted in terms of bounded diffusive motion of the metal ion using a model based on overdamped harmonically bound Brownian motion. The temperature dependence of the f-factor changes with the amount of water contained in the polymer.     

Electrochemical growth of copper single crystals in pores of polymer ion-track membranes

Copper single-crystals are grown by a galvanic method, using etched ion tracks in a polymer foil as templates. The copper deposition is carried out by reversible pulse electrolysis in an ultrasonic field. The method applied for this purpose permits fabrication of stable standing cylindrical single crystals with diameters in the nanometer and micrometer range with high aspect ratio and density of 10⁵–10⁷ per cm² on a large area. The experimental results obtained in this way are compared with the results obtained by direct current plating under ultrasonic treatment and sole reversible electrolysis. The effects of all these deposition processes on the structure of copper claddings are shown. Received: 12 May 1999 / Accepted: 14 May 1999 / Published online: 24 June 1999     

Microstructural and magnetic properties of CoPt nanowires

In this work, the magnetic and microstructural properties of CoPt nanowires are presented as a function of the electrolyte pH and current density during electrodeposition into anodized alumina templates. CoPt nanowires of high aspect ratio have been prepared using electrolyte pH values in the range from 2 to 6. The as-made samples exhibit a face centered cubic (fcc) structure with soft magnetic properties which transform into the face centered tetragonal (fct) L10 phase after thermal treatment. Different pH values of the electrolyte during electrodeposition lead to significantly different microstructures and, therefore, different magnetic properties. The CoPt nanowires prepared at high pH value are composed of fcc nanorods of about 25 nm in length. Thermal annealing of these samples leads to a preferred (0 0 1) orientation (along the direction perpendicular to the direction of nanowires) which increases with annealing time. On the other hand, the CoPt nanowires prepared at lower pH value are composed of uniform fcc nanograins with the size ∼2−3 nm. Magnetization curves for the later sample are virtually identical in both directions indicating an isotropic behavior.     

Synthesis,structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe₃O₄ oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.     

Electrical and magnetic properties of Ni-Cu-Si heterojunction prepared by the liquid phase epitaxy technique

Ni-Cu-Si heterojunction was prepared by the liquid phase epitaxy (LPE) technique. Two growth solutions containing Indium (In) with Cu pieces and In with Ni pieces were employed during the fabrication process. The as-formed junction was directly characterized by different techniques including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thin film X-ray diffraction (XRD) measurements. The current-voltage (I-V) characteristics of the Au/Ni/Cu/Si diode were found to be nonlinear, asymmetric, having a good rectification behavior with a very small leakage current of 0.003 μA at a reverse bias voltage of 2.0 V. The value of turn on voltage was located at 0.2 V. The magnetic properties were also evaluated at room temperature with a vibrating sample magnetometer. Systematic study of junction fabrication and characterization of such a heterosystem, comparison of the behavior of flat silicon and nanoporous silicon as substrates are presented and thoroughly discussed.     

Influence of ageing on the phase composition,structure, and magnetic properties of nanopowders of oxide ferrimagnets

This paper reports on the observation of the aging of cubic oxide ferrimagnet nanopowders prepared by mechanochemical synthesis, which manifests itself in a variation of the phase composition, structural parameters, and main magnetic properties.

     

Synthesis and magnetic properties of multilayer Ni/Cu and NiFe/Cu nanowires

Highly ordered composite nanowires with multilayer Ni/Cu and NiFe/Cu have been fabricated by pulsed electrodeposition into nanoporous alumina membrane. The diameter of wires can be easily varied by pore size of alumina, ranging from 30 to 100 nm. The applied potential and the duration of each potential square pulse determine the thickness of the metal layers. The nanowires have been characterized by transmission electron microscopy (TEM), magnetic force microscopy (MFM), and vibrating sample magnetometer (VSM) measurements. The MFM images indicate that every ferromagnetic layer separated by Cu layer was present as single isolated domain-like magnet. This technique has potential use in the measurement and application of magnetic nanodevices.     

Cr掺杂ZnO纳米线电子性质和磁性质

本文采用第一性原理密度泛函理论系统的研究了Cr原子单掺杂和双掺杂两种尺寸ZnO纳米线的电子性质和磁性质.所有掺杂纳米线的形成能都比纯纳米线的形成能低,表明掺杂增强了纳米线的稳定性.研究发现Cr原子趋于替代纳米线表面的Zn原子.所有掺杂纳米线都显示了金属性.纳米线的总磁矩主要来源于Cr原子3d轨道的贡献.由于杂化,相邻的O原子和Zn原子也产生了少量自旋.在超原胞内,Cr和O原子磁矩反平行排列,表明它们之间是反铁磁耦合.表面双掺杂纳米线铁磁态能量比反铁磁态能量低149 meV,表明Cr掺杂ZnO纳米线可能获得室温铁磁性.     

Synthesis and electrical properties of ZnO nanowires

Vertically aligned ZnO nanowires were synthesized on the p(+) silicon chip by modifying the CVD process with a vapor trapping design. Scanning electron microscopy was used to investigate the morphology of as-obtained nanowires. X-ray diffraction showed that the obtained nanowires were ZnO crystalline. The rectifying characteristics of the p-n heterojunction composed of ZnO nanowires and a p(+) silicon chip were observed. The positive turn-on voltage was 0.5V and the reverse saturation current was 0.01mA. These vertically aligned ZnO nanowires showed a low field emission threshold of 4V/microm at a current density of 0.1microA/cm(2). The dependence of emission current density on the electric field followed Fowler-Nordheim relationship.     

Synthesis and photoluminescence properties of silver nanowires

Silver nanowires of 50–190 nm in diameters along with silver nanoparticles in the size range of 60–200 nm in prismatic and hexagonal shapes are synthesized through chemical process. The lengths of the silver nanowires lie between 40 and 1000 μm. The characterizations of the synthesized samples are done by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–visible absorption spectroscopy. The syntheses have been done by using two processes. In the first process, relatively thicker and longer silver nanowires are synthesized by a soft template liquid phase method at a reaction temperature of 70 °C with methanol as solvent. In the second process, thinner silver nanowires along with silver nanoparticles are prepared through a polymer mediated polyol process at a reaction temperature of 210 °C with ethylene glycol as solvent. The variations of photoluminescence (PL) emission from the silver nanocluster dispersed in methanol as well as in ethylene glycol are recorded at room temperature under excitation wavelengths lying in between 300 and 414 nm. The blue–green PL emission is observed from the prepared samples and these emissions are assigned to radiative recombination of Fermi level electrons and sp- or d-band holes.     

Size-dependent chemical transformation,structural phase change,and optical properties of nanowires

Nanowires offer a unique approach for the bottom-up assembly of electronic and photonic devices with the potential of integrating photonics with existing technologies. The anisotropic geometry and mesoscopic length scales of nanowires also make them very interesting systems to study a variety of size-dependent phenomenon where finite-size effects become important. We will discuss the intriguing size-dependent properties of nanowire systems with diameters in the 5–300?nm range, where finite-size and interfacial phenomena become more important than quantum mechanical effects. The ability to synthesize and manipulate nanostructures by chemical methods allows tremendous versatility in creating new systems with well-controlled geometries, dimensions, and functionality, which can then be used for understanding novel processes in finite-sized systems and devices.