Magnetic Fe<Subscript>2</Subscript>P nanowires and Fe<Subscript>2</Subscript>P@C core@shell nanocables

We report the synthesis of one-dimensional (1-D) magnetic Fe₂P nanowires and Fe₂P@C core@shell nanocables by the reactions of triphenylphosphine (PPh₃) with Fe powder (particles) and ferrocene (Fe(₅H₅)₂), respectively, in vacuum-sealed ampoules at 380–400 °C. The synthesis is based on chemical conversion of micrometer or nanometer sized Fe particles into Fe₂P via the extraction of phosphorus from liquid PPh₃ at elevated temperatures. In order to control product diameters, a convenient sudden-temperature-rise strategy is employed, by means of which diameter-uniform Fe₂P@C nanocables are prepared from the molecular precursor Fe(C₅H₅)₂. In contrast, this strategy gives no obvious control over the diameters of the Fe₂P nanowires obtained using elemental Fe as iron precursor. The formation of 1-D Fe₂P nanostructures is ascribed to the cooperative effects of the kinetically induced anisotropic growth and the intrinsically anisotropic nature of hexagonal Fe₂P crystals. The resulting Fe₂P nanowires and Fe₂P@C nanocables display interesting ferromagnetic-paramagnetic transition behaviors with blocking temperatures of 230 and 268 K, respectively, significantly higher than the ferromagnetic transition temperature of bulk Fe₂P (T _C = 217 K).      

The Effect of Substrate Temperature on Magnetic and Electrical Properties of (CeO<Subscript>2</Subscript>)<Subscript>14</Subscript>Fe<Subscript>86</Subscript> Films

(CeO₂)₁₄Fe₈₆ films were fabricated by a radio frequency magnetron sputtering method at different substrate temperature. The results reveal that the films deposited at substrate temperature lower than 773 K exhibit a strong perpendicular anisotropy, and the correlated dynamic permeability spectrum measured over the frequency range of 0.5–7 GHz shows a high resonance frequency. The study on the relation R～T shows that the resistivity of the thin film has a minimum near room temperature and tends to saturation as the temperature approaches zero, exhibiting a behavior reminiscent of Kondo scattering. However, as the substrate temperature increases to 973 K, the films possess an in-plane anisotropy and lower H _c. The resistivity exhibits a transition from metal to insulator characterized by a maximum of resistivity at 220 K.     

Magnetism and Half-Metallicity in the Fe<Subscript>2</Subscript>ZrP Heusler Alloy

The electronic structure and magnetic properties of the Fe₂ZrP full-Heusler compound has been investigated by using ab initio calculations with the full-potential linearized augmented plane wave (FLAPW) method. The exchange-correlation functionals are taken into account within the generalized gradient approximation (GGA). Energetically, the AlCu₂Mn-type structure of the full-Heusler Fe₂ZrP is energetically more preferable than that of the CuHg₂Ti-type structure and it exhibits half-metallic ferrimagnet. The calculated total spin moment is found as 1 μ _B at the equilibrium lattice constant a ₀=5.90Å which remarkably agrees with the Slater-Pauling rule of M _t = Z _t ?24. While the spin-up band is metallic, the spin-down band has a semiconductor behavior with a gap of 0.593 eV and the spin-flip gap is 0.129 eV. The negative formation energy is shown as an evidence of the thermodynamic stability of alloy. The dependencies of the magnetic and electronic properties on the lattice constant are also discussed. The estimated Curie temperature is 752 K in the mean field approximation.     

Al-doped Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles and Their Magnetic Properties

Al-doped Fe₃O₄ nanoparticles were synthesized for the first time via the Composite-Hydroxide-Mediated (CHM) method from Fe₃O₄ and Al₂O₃ without using any capping agent. The synthesis technique was one-step and cost effective. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS). Samples with a tunable size of 500–1500 nm, 200–800 nm, and 100–700 nm could be obtained by adjusting the reaction time and temperature. Magnetic property of the as-synthesized Al-doped Fe₃O₄ nanoparticles was investigated. Magnetic hysteresis loops measured in the field range of −10 kOe<H<10 kOe, indicated the ferromagnetic behavior with coercivity (H _c) of 470 and 110 Oe and remanence magnetization (M _r) of 13 and 6.4 emu/g at the temperature of 5 and 300 K, respectively. The saturation intensity (M _s) was 46.1 emu/g at 5 K, while it was about 43.6 emu/g at 300 K.     

Fast microwave synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/Ag magnetic nanoparticles using Fe<Superscript>2+</Superscript> as precursor

A simple and quick microwave method to prepare high performance magnetite nanoparticles (Fe₃O₄ NPs) directly from Fe has been developed. The as-prepared Fe₃O₄ NPs product was fully characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the as-prepared Fe₃O₄ NPs are quite monodisperse with an average core size of 80 × 5 nm. The microwave synthesis technique can be easily modified to prepare Fe₃O₄/Ag NPs and these NPs possess good magnetic properties. The formation mechanisms of the NPs are also discussed. Our proposed synthesis procedure is quick and simple, and shows potential for large-scale production and applications for catalysis and biomedical/biological uses.     

Hydrogen-induced diffusion phase transformations in Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B and Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript> magnetically hard alloys

The Nd₂Fe₁₄B and Sm₂Fe₁₇ magnetically hard alloys become thermodynamically unstable under the action of hydrogen and suffer hydrogen-induced direct and inverse phase transformations at elevated temperatures. The kinetics of these transformations is investigated. It is shown that they are diffusion-controlled and develop according to the mechanism of nucleation and growth. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 105–111, September–October, 2008.     

Magnetic properties of Nd-Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> nanoparticles

Nb³⁺-substituted garnet nanoparticles Y_3−xNd_xFe₅O₁₂ (x = 0.0, 0.5, 1.0, 1.5, and 2.0) were fabricated by a sol-gel method and their crystalline structures and magnetic properties were investigated by using X-ray diffraction (XRD), thermal analysis (DTA/TG), and vibrating sample magnetometer (VSM). The XRD patterns of Y_3−xNd_xFe₅O₁₂ have only peaks of the garnet structure and the sizes of particles range from 34 to 70 nm. From the results of VSM, it is shown that when the Nd concentration x ( 1.0, the saturation magnetization of Y_3−xNd_xFe₅O₁₂ increases as the Nd concentration (x) is increased, and gets its maximum at x = 1.0, but when x ( 1.0, the saturation magnetization decreases with increasing the Nd concentration (x), this may be due to the distortion of the microstructure of Y_3−xNd_xFe₅O₁₂, which leads to the decrease of the effective moment formed by Fe³⁺. Meanwhile, it is observed that with the enhancement of the surface spin effects, the saturation magnetization rises as the particle size is increased.     

Vortex Flux Dynamics and Harmonic ac Magnetic Response of Ba(Fe<Subscript>0.94</Subscript> Ni<Subscript>0.06</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Bulk Superconductor

Vortex dynamics and nonlinear ac response are studied in a Ba(Fe_0.94 Ni_0.06)₂As₂(T _c = 18.5 K) bulk superconductor in magnetic fields up to 12 T via ac susceptibility measurements of the first ten harmonics. A comprehensive study of the ac magnetic susceptibility and its first ten harmonics finds shifts to higher temperatures with increasing ac measurement frequencies (10 to 10,000 Hz) for a wide range of ac (1, 5, and 10 Oe) and dc fields (0 to 12 T). The characteristic measurement time constant t ₁ is extracted from the exponential fit of the data and linked to vortex relaxation. The Anderson-Kim Arrhenius law is applied to determine flux activation energy E _a /k as a function dc magnetic field. The de-pinning, or irreversibility lines, were determined by a variety of methods and extensively mapped. The ac response shows surprisingly weak higher harmonic components, suggesting weak nonlinear behavior. Our data does not support the Fisher model; we do not see an abrupt vortex glass to vortex liquid transition and the resistivity does not drop to zero, although it appears to approach zero exponentially.     

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.     

Electronic and Magnetic Properties of SnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe₂O₄ spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe₂O₄ inverse spinel phase and near half-metallic character for SnFe₂O₄ mixed spinel phase and metal character for SnFe₂O₄ normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe₂O₄ ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.     

Radiation Synthesis and Magnetic Property Investigations of the Graft Copolymer Poly(Ethylene-g-Acrylic Acid)/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Film

In this work, acrylic acid (AAc) monomer was grafted onto low-density polyethylene (LDPE) films by the direct method to obtain acid (LDPE-grafted poly(acrylic acid) (LDPE-g-PAAc)) graft copolymers. The presence of the grafted PAAc with COOH groups allows coupling with Fe^2+/3+ ions. The stabilization of Fe₃O₄ particles onto the graft copolymers was done by in situ reduction of LDPE-g-PAAc/Fe^2+/3+ with sodium borohydride (NaBH₄) in aqueous solution. The LDPE-g-PAAc graft copolymer and LDPE-g-PAAc/Fe₃O₄ composite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and electron spin resonance (ESR). The synthesized composites exhibit excellent magnetic properties. The results indicated that the magnetic oxide (Fe₃O₄) was embedded and homogenously dispersed into the surfaces of the graft copolymer films as indicated by SEM. The FT-IR analysis clearly suggests that an AAc monomer was effectively grafted onto LDPE. The XRD studies elucidate the change in the crystallinity of the graft copolymers.

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Graphical Abstract The procedures for magnetic film manufacture LDPE-g-PAAc/Fe₃O₄ using gamma irradiation technique

     

Comparative Magnetic and Structural Properties Study of Micro- and Nanopowders of Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B Doped with Ni

Micropowders of melted and heat-treated Nd₁₆(Fe_76?x Ni _x )B₈ alloys system, with x = 0, 10, 20, and 25 (size distribution under 20 μm), were studied and compared with the study of nanopowders obtained, from the previous ones, by surfactant-assisted ball-milling process during 2 h. By XRD, a majority of Nd₂Fe₁₄B hard phase and a minority of α-Fe, Nd_1.1Fe₄B₄ and NdNi₂ phases were detected. The last one increases with Ni content. The crystallite size of the hard phase, in both types of samples, is not affected by the Ni content; however, the grains in micropowders are oblate, with a mean size of 37 nm, while those of the nanopowders are symmetric, with a mean size of 35 nm. Mössbauer spectra were fitted with seven sextets, which correspond to the six ferromagnetic sites of the hard phase and that of the α-Fe, and a doublet corresponding to the paramagnetic Nd_1.1Fe₄B₄ phase. The mean hyperfine magnetic field, for both types of samples, decreases with Ni content. The hysteresis loops of both types of samples show a hard magnetic character, however, the coercive field and the M _r/M _s values for nanopowders are greater than those obtained for micropowders for all the Ni contents. Values of H _c = 2 kOe and M _r/ M _s = 0.54 were obtained for nanopowders with 10 at.% Ni. From the hysteresis loops, which include the initial magnetization curve, Henkel plots for all the samples were obtained. These plots show that for micropowders, the predominant magnetic interaction is of dipolar type, while for nanopowders, the ferromagnetic exchange is the predominant one, which favored the magnetization.     

Effect of Tb substitution on the magnetic properties of exchange-biased Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B/Fe<Subscript>3</Subscript>B

Tb-substituted (Nd,Tb)₂Fe₁₄B/Fe₃B nanocomposite ribbons have been synthesized by melt spinning of Nd₃Tb₁Fe₇₆Cu_0.5Nb₁B_18.5 alloys. Tb substitution has significantly enhanced the value of coercivity and Curie temperature. Highest value of coercivity has been obtained as 4.76 kOe for the sample annealed at 953 K for 10 min. Curie temperature of Tb substituted sample, Nd₃Tb₁Fe₇₆Cu_0.5Nb₁B_18.5 is 549 K while Curie temperature of the sample without Tb, Nd₄Fe₇₆Cu_0.5Nb₁B_18.5 is 535 K. Recoil hysteresis loops measured along the major demagnetization curve are steep having small recoil loop area. Temperature dependence of coercivity, remanent ratio and maximum energy product have been measured for the sample annealed at 893 K and 923 K for 10 min. At 5 K, coercivity and maximum energy product of the sample annealed at 893 K for 10 min are 5.2 kOe and 11.5 MGOe respectively and the sample annealed at 923 K for 10 min are 6 kOe and 13.1 MGOe respectively.     

Hydration-disproportionation-desorption-recombination of the Dy<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B compound

Conclusions We have established the disproportionation of the Dy₂Fe₁₄B compound in hydrogen at a temperature close to 960 K and under an initial hydrogen pressure of 5.2 MPa into dysprosium hydride, iron, and iron boride. Recombination in a vacuum occurs after heating to 980 K in several stages.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 122–124, July–August, 2004.     

Magnetic Properties,Possible Martensitic Transformation,and Elastic Properties of Heusler Alloy Fe<Subscript>2</Subscript>NiSb from First-Principles Calculations

The first-principles plane-wave pseudopotential method has been used to investigate the structural, magnetic, electronic and elastic properties of the Heusler alloy Fe₂NiSb. It is found that the tetragonal phase of Fe₂NiSb is more stable compared with the cubic phase from the energetic point of view. The martensitic transformation from a cubic to a tetragonal structure may happen with decreasing temperature. This phase transition is nearly volume-conserving, suggesting that this alloy is predicted to be a superior shape memory alloy. Fe₂NiSb is found to be a ferromagnet. The saturation magnetization is 5.08 and 4.88 μB/f.u. in the cubic and tetragonal phases, respectively. It is confirmed that Fe₂NiSb displays an excellent ductile behavior by analyzing the ratio of bulk modulus to shear modulus. The calculated densities of states show that the peak splitting at the Fermi level decreases the total energy and is related to the stabilization of the tetragonal phase.     

Synthesis and microwave absorption properties of sandwich-type CNTs/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/RGO composite with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> as a bridge

A novel sandwich-type CNTs/Fe₃O₄/RGO composite with Fe₃O₄ as a bridge was successfully prepared through a simple solvent-thermal and ultrasonic method. The structure and morphology of the composite have been characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. This new structure can effectively prevent the agglomeration of GO and the combination of CNTs/Fe₃O₄ and RGO shows a strong reflection loss (R_L) (?50 dB) at 8.7 GHz with absorber thickness of 2.5 mm. Moreover, compared with CNTs/Fe₃O₄/GO composite, it is found that the thermal treating process is beneficial to enhance the microwave absorption properties, which may be attributed to high conductivity of RGO. On this basis, the microwave absorbing mechanism is systematically discussed. All the data show that the CNTs/Fe₃O₄/RGO composite exhibits excellent microwave absorption properties with light density and is expected to have potential applications in microwave absorption.     

In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

Magnetization Studies of K<Subscript>0.8</Subscript>Fe<Subscript>1.7</Subscript>Se<Subscript>2</Subscript> Single Crystals

Magnetization measurements of the magneto-superconducting K_0.8Fe_1.7Se₂ single crystals (T _C =29.6 K) at various applied fields and temperatures have been performed. In the superconducting range, two unusual phenomena are observed. (i) The presence of paramagnetic Meissner effect up to 20 Oe, which is revealed by the positive field-cooled magnetization curves. (ii) A double peak in the hysteresis loop at low temperatures. The isothermal magnetization measured at 35 K clearly indicates the presence of a tiny amount of a ferromagnetic substance (probably pure Fe). The two peculiar phenomena observed are attributed to this impurity.     

Characterization and electrical properties of semiconducting Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-K<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> glasses

Semiconducting glasses of the Fe₂O₃-Bi₂O₃-K₂B₄O₇ system were prepared by the press-quenching method and their dc conductivity in the temperature range 223–393 K was measured. The glass transition temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The density for these glasses was consistent with the ionic size, atomic weight and amount of different elements in the glasses. Mössbauer results revealed that the relative fraction of Fe increases with increasing Fe₂O₃ content. Electrical conductivity showed a similar composition dependency as the fraction of Fe. The glasses had conductivities ranging from 10 to 10 Scm at temperatures from 223 to 393 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.