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.     

LDA + U and GGA + U studies of Al-rich and bulk goethite (α-FeOOH)

The electronic and structural properties of bulk goethite and Al-rich goethite were studied on the basis of spin-polarized DFT at the LDA + U and GGA(PW91) + U levels. Firstly, the periodic model of bulk goethite was optimized varying the value of U_eff. Considering all the results obtained we can conclude that the bulk goethite described at the GGA + U level with U_eff = 6 eV gives us the better agreement with different physical properties. The results of magnetic moments of Fe ions, the DOS analysis and the Bader atomic charges identify goethite as an antiferromagnetic Fe(III) compound. For Al-rich goethite the GGA + U (U_eff = 6 eV) approach was used. The isomorphous substitution of one Fe ion with Al ion produces the reduction of the cell parameters with respect to the bulk goethite. Regarding the magnetic ordering, it was observed that Fe atoms surrounding the Al atom must have the same spin projection, i.e., spin-up or -down. The charge density was changed with the addition of Al ion, producing a depletion where the ion is located and some electron redistribution in the zone of the oxygen atoms surrounding the Al ion. This behavior produces some small magnetization in these O ions.     

The cation inversion and magnetization in nanopowder zinc ferrite obtained by soft mechanochemical processing

Two zinc ferrite nanoparticle materials were prepared by the same method – soft mechanochemical synthesis, but starting from different powder mixtures: (1) Zn(OH)₂/α-Fe₂O₃ and (2) Zn(OH)₂/Fe(OH)₃. In both cases a single phase system was obtained after 18 h of milling. The progress of the synthesis was controlled by X-ray diffractometry (XRD), Raman spectroscopy, TEM and magnetic measurements. Analysis of the XRD patterns by Rietveld refinement allowed determination of the cation inversion degree for both obtained single phase ZnFe₂O₄ samples. The sample obtained from mixture (1) has the cation inversion degree 0.3482 and the sample obtained from mixture (2) 0.400. Magnetization measurements were confirmed that the degrees of the inversion were well estimated. Comparison with published data shows that used method of synthesis gives nano powder samples with extremely high values of saturation magnetizations: sample (1) 78.3 emu g⁻¹ and sample (2) 91.5 emu g⁻¹ at T = 4.5 K.     

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.     

Synthesis,magnetic and Mössbauer study of ε-FexN (2 < x < 3) nanowires

Several micro meter long nanowires of ε-Fe_xN (2 < x < 3) are synthesized through a reduction nitridation method of Fe-NTA precursor formed by a hydrothermal method. The formation of pure iron nitride nanowires is confirmed by XRD. SEM analysis shows the porous nature of the iron nitride nanowires, which will enhance its suitability in catalysis. The field dependent magnetic behavior shows the ferromagnetic nature of the iron nitride nanowires. An appreciably good magnetization value (71 emu/g) and low coercivity (24 Oe) of the system makes it suitable for magnetic recording head applications. The room temperature Mössbauer study of the pristine nitride nanowires shows the existence of two iron sites corresponding to Fe (II) and Fe (III) indicating structural disorder.     

A facile and environmentally friendly NaCl nonaqueous ionic liquid route to prepare crystalline β-CaSiO3 nanowires

We described an environmentally friendly and facile NaCl nonaqueous ionic liquid route for the first time to synthesize β-CaSiO₃ nanowires. β-CaSiO₃ nanowires were prepared by annealing precursor calcium silicate hydrate (CSH) nanostructures in NaCl nonaqueous ionic liquid, in which the precursor CSH nanostructures were first prepared by a facile one-step, solid state reaction, ground with both NaCl and surfactant nonyl phenyl ether (9) (NP-9), and heated at 850 °C for 2 h. β-CaSiO₃ nanowires were characterized by XRD, TEM and HRTEM. The comparative experiments have been conducted systemically to investigate the growth mechanism of β-CaSiO₃ nanowires, and the roles of salt NaCl nonaqueous ionic liquid and NP-9 on the formation of β-CaSiO₃ nanowires. The results demonstrated that both salt NaCl nonaqueous ionic liquid and surfactant NP-9 played key roles on the formation of β-CaSiO₃ nanowires. A rational growth mechanism of β-CaSiO₃ nanowires has been proposed on the basis of experimental results.     

Synthesis and formation mechanism of Cu3V2O7(OH)2·2H2O nanowires

Cu₃V₂O₇(OH)₂·2H₂O nanowires have been synthesized in high yield through a simple and facile low-temperature hydrothermal approach without any template or surfactants. XRD, TG, FE-SEM, TEM and HRTEM were used to characterize the product. The results indicated that the product consisted of wirelike crystals about 80 nm in diameter and length up to several micrometers. The formation of wirelike structure of Cu₃V₂O₇(OH)₂·2H₂O depended crucially on the reaction time and pH value of the precursor suspensions. The optical absorption spectrum indicates that the Cu₃V₂O₇(OH)₂·2H₂O nanowires have a direct band gap of 1.94 eV.     

Magnetic properties and microstructure of radially oriented Sm(Co,Fe,Cu,Zr)z ring magnets

Radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets are prepared by powder metallurgy with appropriate magnetic field molding, sintering process and aging treatment. The results indicate that radially oriented Sm(Co,Fe,Cu,Zr)_z ring magnets have obvious anisotropy of thermal expansion and sintering shrinkage, which easily lead to the splits and deformation of the ring magnets. So, slow heating, vacuum pre-sintering in sintering process and various quenching processes at different steps during quenching are adopted. The magnets have excellent magnetic properties: B_r = 10.8 kGs, H_cj = 27.6 kOe, BH_max = 28.1 MGOe. Besides, there is a uniform magnetization field on the surface of the ring magnets. The average surface magnetization field () is 1.502 kGs. The deviation from average (α) is only 4.2%. The microstructure of the magnets consists of a mixture of homogeneous cellular and lamella structures.     

High-yield synthesis of single-crystalline 3C-SiC nanowires by a facile autoclave route

Single-crystalline 3C-SiC nanowires have been synthesized in large scale through a one-step autoclave route by the reaction of SiCl₄, (C₅H₅)₂Fe and metallic Na at 500 °C. Electron microscopy investigations show that the nanowires have typical diameters of 15-50 nm, lengths up to several tens of micrometers and grow along the [111] direction. The possible growth mechanism of the nanowires is discussed.     

Hybrid nanocomposite of Fe nanoparticles on SiO2 nanowires by sublimation route

A simple route was developed to synthesize the hybrid nanocomposite with Fe nanoparticles (NPs) dispersed on the surface of SiO₂ nanowires (NWs), where SiO₂ NWs with the diameter of 20-40 nm were produced by heating single-crystal silicon wafer, and Fe NPs in the size range of 3-20 nm were generated by heating Fe powders. The nucleation and growth of Fe NPs follows the solid-vapor-solid (S-V-S) mechanism, namely, Fe powders firstly sublime and then Fe atoms deposit on SiO₂ NWs to form Fe NPs.     

A green and facile route to synthesize calcium silicate nanowires

Large-scale single crystalline calcium silicate nanowires have been synthesized via a simple and facile hydrothermal route using nanoscale SiO₂ and CaO powders as the starting materials. Xonotlite [Ca₆(Si₆O₁₇)(OH)₂] nanowires were first achieved after hydrothermal treatment at 220 °C for 12 h. After being calcinated at 800 °C for 1 h, the Ca₆(Si₆O₁₇)(OH)₂ nanowires are completely transformed into β-CaSiO₃ nanowires. The β-CaSiO₃ nanowires have a diameter of 30–150 nm and a length of tens of micrometers. The hydrothermal conditions and the size of the raw materials play important roles on the size of the nanowires. A possible growth mechanism of the nanowires is also proposed.     

Crystal structure and magnetic properties of the compound FeDy6Sb2

The crystal structure of a compound FeDy₆Sb₂ was determined by X-ray powder diffraction using the Rietveld method. The compound crystallizes in the hexagonal, space group P6¯2m (No. 189) with the Fe₂P structure type and lattice parameters a = 0.81449(5) nm, c = 0.41641(3) nm, z = 1 and D_calc = 8.842 g/cm³. The maximum magnetic entropy change ΔS_M for the compound is 3.41 J kg^− 1 K^− 1 near its Curie temperature (143.4 K) on the magnetic field changes of 0-2.0 T.     

Unique properties of polyaniline in the presence of applied magnetic field and ferric chloride

Fe-contained polyaniline (abbreviated as PANI–Fe) was prepared by chemical oxidation of aniline with ammonium peroxydisulfate oxidant in 0.5 mol dm⁻³ HCl and an adequate content of FeCl₃·6H₂O solution in the presence of an applied magnetic field at room temperature. The X-ray photoelectron spectroscopy (XPS) and UV–vis and FTIR spectra suggest that there is an interaction between FeCl₃ and PANI chains, but PANI–Fe backbone is essentially identical with that of parent polyaniline. The electron paramagnetic resonance (EPR) spectrum shows that there were unpaired electrons in PANI–Fe synthesized in the presence of an applied magnetic field, the spin density and the conductivity of which are 7.308 × 10²⁰ spins g⁻¹ and 0.891 S cm⁻¹, respectively. The plot of magnetization (M) vs. the applied magnetic field (H) displays that the PANI–Fe possesses soft ferromagnetic behavior at room temperature. The results of cyclic voltammogram show that the PANI–Fe film is of excellent electrochemical activity.     

Ultrasound assisted template-free synthesis of Cu(OH)2 and hierarchical CuO nanowires from Cu7Cl4(OH)10·H2O

Cu(OH)₂ nanowires have been synthesized by an ultrasound assisted solution route in absence of a template, using Cu₇Cl₄(OH)₁₀·H₂O as a precursor. Hierarchical CuO nanowires were obtained by a simple solid-state thermal transformation of these Cu(OH)₂ nanowires. The products were characterized by XRD, SEM, TEM and HRTEM. The ranges of diameters and lengths of the polycrystalline CuO nanowires are ca. 20-30 nm and several micrometers, respectively. Ultrasonic time is found an important factor to morphology of the CuO products. This could be a potential efficient way for large scale fabrication of CuO nanowires with hierarchical structures. Surface photovoltage spectra of the CuO nanowires in air, NH₃ and CH₂Cl₂ atmospheres were investigated, which demonstrates it a good photoelectric gas sensing material.     

Fabrication and magnetic property of α-Fe2O3 nanoparticles/TiO2 nanowires hybrid structure

α-Fe₂O₃ nanoparticles/TiO₂ nanowires hybrid structure is fabricated by two-step hydrothermal treatment. TiO₂ nanowires are prepared by heating of titanate nanowires, which are obtained by hydrothermal treatment of TiO₂ powder and further repeated HCl treatment. α-Fe₂O₃ nanoparticles are deposited on the surface of TiO₂ nanowires by hydrothermal treatment in Fe(NO₃)₃ solution. The HRTEM images confirm the junctions between α-Fe₂O₃ nanoparticles and TiO₂ nanowires. The formation of hybrid structures has significant influence on the magnetic properties of α-Fe₂O₃. The Morin transition temperature of α-Fe₂O₃ nanoparticles/TiO₂ nanowires hybrid structure is 190 K, which is determined by the sharp peak in the differential ZFC curve. Whereas there is no observable Morin transition for the corresponding isolated α-Fe₂O₃ nanoparticles with similar average particles size of ca. 20 nm.     

Improvement of corrosion resistance of Cu and Nb co-added Nd–Fe–B sintered magnets

Cu and Nb powders are co-added as intergranular modifiers to improve the corrosion resistance of Nd–Fe–B sintered magnets. For the magnet co-added with 0.2 wt.% Cu and 0.8 wt.% Nb, mass loss of accelerated corrosion test in 120 °C, 2 bar and 100% relative humid atmosphere for 96 h drops from 2.47 mg/cm² to 0.49 mg/cm² in comparison with the Cu/Nb free one. The corrosion potential E_corr in 3.5% NaCl aqueous solution increases from −1.115 V to −0.799 V, which indicates the better resistance against electrochemical corrosion. The improved corrosion resistance is ascribed to the enhanced stability of the intergranular phase by forming high electrode potential Cu-containing phase and reduced Nd-rich phase at triple junctions. Besides, the distribution of (Pr, Nd)-rich phases along the grain boundaries becomes more clear and continuous through Cu/Nb co-addition, maintaining fairly good magnetic properties of B_r = 13.6 kGs, H_cj = 11.4 kOe, (BH)_max = 46.2 MGOe. Further investigation demonstrates that Nb is effective to refine the grains of hard magnetic Nd₂Fe₁₄B phase and Cu is beneficial for optimizing the distribution of the intergranular phases.     

Raman optical properties of Cu(OH)2 nanowires

We have for the first time investigated Raman optical properties of polycrystalline Cu(OH)₂ nanowires with an average diameter of ca. 9 nm and lengths of up to hundreds of micrometers that were synthesized by a simple surfactant-free solution-phase route at ambient temperature. The result indicated that the Raman peak of the as-prepared polycrystalline Cu(OH)₂ nanowires was obviously broadened, and red-shifted 16 cm^− 1 compared with that of bulk Cu(OH)₂ crystals. Based on the microstructure analysis and phonon confinement model of a crystal, we have proposed a rational explanation for the red-shift and broadening of Raman peak.     

Critical behaviour and magnetocaloric effect of nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) synthesized by nebulized spray pyrolysis

The structure, critical exponents and magnetocaloric effect (MCE) of Nebulized Spray Pyrolysis (NSP) synthesized nano crystalline La_0.67Ca_0.33Mn_1−xFe_xO₃ (x = 0.05, 0.2) were investigated. The Reitveld refinement of XRD patterns show that the samples adopt an orthorhombic structure with Pnma space group. TEM inspection reveals that the average particle size is about 15 nm and 42 nm for NSP synthesized LCMFe_0.05 and LCMFe_0.2 samples respectively. The temperature and field dependent magnetization studies reveal the superparamagnetic state of La_0.67Ca_0.33Mn_0.95Fe_0.05O₃ and spin-glass-like state of La_0.67Ca_0.33Mn_0.8Fe_0.2O₃. The critical behaviour at the transition region studied using modified Arrott plot provides a second order nature of phase transition for both samples. The magnetocaloric studies show the maximum value of magnetic entropy change (ΔS_max) is in the range 2.3 J kg⁻¹ K⁻¹ at 158 K for LCMFe_0.05 and 0.3 J kg⁻¹ K at 92 K for LCMFe_0.2 respectively at 5 T field. The field dependence of the magnetic entropy changes are also analysed, which show a power law dependence (ΔS_M ∞ Hⁿ, n = 0.72 (2)) at transition temperature, T_C = 162 K for LCMFe_0.05 and n = 1.11(3) at 92 K for LCMFe_0.2.     

One-step synthesis of yttrium orthoferrite nanocrystals via sol–gel auto-combustion and their structural and magnetic characteristics

Single phase yttrium orthoferrite (YFeO₃) nanocrystals have been one-step synthesized by a sol–gel auto-combustion method using nitrates of iron and yttrium as raw materials, citrate acid (CA) and ethylene glycol (EG) as chelating agents and nitrate ammonium as oxidant. The effects of CA/M and the pH value of the solutions on the concentrations of yttrium– and iron–citrate complex species were quantitatively studied to ensure the formation of chemically homogenous gels. The auto-combustion of the dried gels and the microstructural features and magnetic properties of as-synthesized YFeO₃ nanocrystals were characterized and investigated by using XRD, FT-IR, TG-DTA, FESEM and VSM. It has been shown that a hexagonal YFeO₃ can be formed during the auto-combustion of gels with CA/M = 1.5, EG/CA = 1.5, pH = 6.0 and Q   = 35%, the molar ratio of the oxygen available from nitrates (₃NO⁻)$\left({{\text{NO}}_3}^{-}\right)$ to the oxygen required for complete oxidation of fuels to CO₂, H₂O and N₂, while single phase orthorhombic YFeO₃ nanocrystals of ∼90 nm in average diameter may be one-step synthesized from the gels with Q = 80%, and show an enhanced weak ferromagnetism characterized with a RT magnetization of 0.45 emu g⁻¹ at the magnetic field 0.35T, a paramagnetism with the susceptibility χ = 2.27 × 10⁻⁵ emu g⁻¹ Oe⁻¹, and an extremely low coercive field of 245 Oe.     

Structural properties and Mössbauer spectra of metastable NdFe12B6

Metastable ternary NdFe₁₂B₆ compound was prepared by the annealing of the amorphous Nd-Fe-B ribbons and has a main phase of NdFe₁₂B₆ with some impurities of α-Fe and Nd₅Fe₁₈B₁₈. The NdFe₁₂B₆ was found to have a SrNi₁₂B₆ structure with space group R3¯m. Mössbauer spectra of the sample were measured between 300 K and 20 K. The result indicated that an abrupt drop with increasing temperature took place at about 205 K for the hyperfine field at both Fe(18 g) and Fe(18 h) sites. The corresponding hyperfine fields were found nearly constant at ferromagnetic state about 16.9(6) T and 19.3(5) T for Fe(18 g) and Fe(18 h) sites, respectively. Accordingly, the relative size of the Fe moments was deduced.