Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

Possible reconstruction of the ferromagnetic state under the influence of superconductivity in epitaxial V/Pd1−x Fe x bilayers

Epitaxial V/Pd_1−x Fe _x (001) bilayers with a V thickness of the order of 40 nm and with a Pd_1−x Fe _x thickness in the range from 0.8 nm to 4.4 nm were prepared by molecular beam epitaxy techniques. The Curie temperature of the Pd_1−x Fe _x layers varies between 90 and 250 K. For a bilayer with a Pd_1−x Fe _x thickness of 1.2 nm the ferromagnetic resonance measurements revealed a decrease of the effective magnetization 4πM _eff of the ferromagnetic layer below the superconducting transition temperature of V. As a possible explanation for this decrease we suggest a spatial modulation of the ferromagnetic order in the Pd_1−x Fe _x layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.     

Electronic structure of new oxygen-free 38-K superconductor Ba1? x K x Fe2As2 in comparison with BaFe2As2 from the first principles

Based on first-principle FLAPW-GGA calculations, we have investigated the electronic structure of the newly discovered oxygen-free 38-K superconductor Ba_1−x K _x Fe₂As₂ in comparison with a parent phase—the tetragonal ternary iron arsenide BaFe₂As₂. The density of states, magnetic properties, near-Fermi band compositions, together with the Sommerfeld coefficients γ and the molar Pauli paramagnetic susceptibility χ have been evaluated. The results obtained allow us to classify these systems as quasi-two-dimensional ionic metals, where the conduction is strongly anisotropic, occurring only in the (Fe-As) layers. According to our calculations, in the case of the hole doping of BaFe₂As₂, the density of states at the Fermi level grows, which may be a factor promoting the occurrence of superconductivity for Ba_1−x K _x Fe₂As₂. On the other hand, Ba_1−x K _x Fe₂As₂ lies at the border of the magnetic instability and the pairing interactions might involve the magnetic or orbital fluctuations. The text was submitted by the authors in English.     

Studies on the valence electronic structure of Fe and Ni in Fe x Ni1−x alloys

K _β-to-K _α X-ray intensity ratios of Fe and Ni in pure metals and in Fe _x Ni_1−x alloys (x=0.20, 0.50, 0.58) exhibiting similar crystalline structure have been measured following excitation by 59.54 keV γ-rays from a ²⁴¹Am point source, to understand as to why the properties of permalloy Fe_0.2Ni_0.8 is distinct from other alloy compositions. It is observed that the valence electronic structure of Fe_0.2Ni_0.8 alloy is totally different from other alloys which may be attributed to its special magnetic properties.     

Effects of La–Zn substitution on microstructure and magnetic properties of strontium ferrite nanofibers

Sr_1−x La _x Zn _x Fe_12−x O₁₉/poly(vinylpyrrolidone) (PVP) (0.0≤x≤0.5) precursor nanofibers were prepared by the sol–gel assisted electrospinning method from starting reagents of metal salts and PVP. Subsequently, the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers with diameters of around 100 nm were obtained by calcination of the precursor at 800 to 1000°C for 2 h. The precursor and resultant Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. The grain sizes of Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are in a nanoscale from 40 to 48 nm corresponding to the calcination temperature from 800 to 1000°C. With La–Zn substitution content increase from 0 to 0.5, the grain size and lattice constants for the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers obtained at 900°C show a steady reduction trend. With variations of the ferrite particle size arising from the La–Zn substitution, the nanofiber morphology changes from the necklace-like structure linking by single elongated plate-like particles to the structure building of multi-particles on the nanofiber cross-section. The specific saturation magnetization of Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers initially increases with the La–Zn content, reaching a maximum value 72 A m² kg⁻¹ at x=0.2, and then decreases with a further La–Zn content increase up to x=0.5, while the coercivity exhibits a continuous reduction from 413 (x=0) to 219 kA m⁻¹ (x=0.5). The mechanism for the La–Zn substitution and the nanofiber magnetic property are analyzed.     

Microstructure and magnetic properties in arrays of ac electrodeposited FexNi1?x nanowires induced by the continuous and pulse electrodeposition

FeNi nanowires were fabricated by ac and pulse electrodeposition into the alumina template matrix. The effects of continuous ac electrodeposition as well as pulse features on the structure and magnetic properties of the nanowire arrays were studied. The microstructures and magnetic properties of the Fe _x Ni_1−x nanowires are seen to be independent of the deposition frequency and off-time between the pulses. The ac electrodeposited Ni nanowires were not formed at more than 400  Hz deposition frequency, while the Fe _x Ni_1−x nanowires, containing a small amount of Fe, formed in the all frequencies. For x less than 50% the coercivity slowly increases but over 50% Fe added to the FeNi alloy increases the coercivity with a higher rate and maximum coercivity was seen for the Fe_0.97Ni_0.03. The Fe and Fe _x Ni_1−x nanowires containing less than 30 at.% Ni was seen to have a bcc structures with (110) preferential direction while Fe _x Ni_1−x nanowires with more than 30 at.% Ni showed (110) bcc (Fe) and/or (111) bcc (FeNi) plus (111) fcc (Ni). A preferential (111) fcc structure was obtained for the Ni nanowires.     

Compositional effect of bcc Co100−x Fe x electrodes on magnetoresistance in AlO x -based magnetic tunnel junctions

The effect of the composition of ferromagnetic bcc Co_100−x Fe _x electrodes on tunneling magnetoresistance (TMR) of Co_100−x Fe _x /AlO _x /Co_100−x Fe _x /IrMn magnetic tunnel junctions was studied. The epitaxial growth of the bottom Co_100−x Fe _x electrode leads to a high-quality electrode and interface, which significantly enhances the TMR ratio and the desired effect for study. Other factors that could also affect TMR, such as interface roughness, tunneling barrier properties, and exchange-bias properties, were kept the same within the uncertainty of the experiment in order to minimize their effects. The observed TMR dependence on composition is attributed to the variation of the s-like electron densities of state of the bcc Co_100−x Fe _x electrodes with different compositions.     

Conditions favoring the polar weak ferromagnetic state in BiFeO<Subscript>3</Subscript>-type multiferroics

The crystal structure and magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A = Ca, Sr, Pb, Ba), Bi_{1 − x} A _x (Fe_{1 − x} Ti _x )O₃, and Bi_{1 − x} A _x (Fe_{1 − x/2}Nb _x/2)O₃ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

Measurement of the spindependent relative conversion coefficients in α-Fe and α-Fe2O3

The relative differences δ _ns (n=1, 2, 3) of the spindependent conversion coefficients were measured for α-Fe and α=Fe₂O₃. In contrast to theoretical predictions of δ_1s≃−10⁻⁵ we found δ_1s≃−1.0(4)x10⁻² for both α-Fe and α-Fe₂O₃. As a possible source for this difference we consider a dynamic coupling with the atomic spin during the conversion process.     

Noncentrosymmetric cubic helical ferromagnets Mn1 ? yFeySi and Fe1 ? xCoxSi

Two systems of noncentrosymmetric cubic helical magnets Mn_{1 − y} Fe _y Si (y = 0.06, 0.08, 0.10) and Fe_{1 − x} Co _x Si (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.50) have been compared. The concentration dependences of the critical temperature and magnetic field have been obtained using small-angle polarized-neutron scattering and analyzed in the framework of the Bak-Jensen model. It has been established that, among the two interactions that play the main role in these systems, i.e., the isotropic symmetric ferromagnetic exchange and the Dzyaloshinskii-Moriya isotropic antisymmetric interaction, the former interaction determines the critical temperature in the Mn_{1 − y} Fe _y Si system and the latter interaction determines this temperature in the Fe_{1 − x} Co _x Si system.     

Ionic conductivity and structural properties of MnO-doped Bi4V2O11 system

Bi₄V₂O₁₁ exists in three phases viz. α, β, and γ. High temperature γ-phase can be stabilized to room temperature owing to its higher conductivity by the partial substitution of metallic cations for vanadium in Bi₄V₂O₁₁. Phase transitions from α → β and β → γ are composition and temperature-dependent. Mn²⁺-doped compounds Bi₄V_2−x Mn _x O_{11− δ} (0 ≤ x ≤ 0.4) have been synthesized by solid state reaction technique and investigated by X-ray diffraction and ionic conductivity measurement. High ionic conducting γ-phase is stabilized for x ≥ 0.2. The ionic conductivity of the series of Bi₄V_2−x Mn _x O_{11− δ} samples has been measured by using ac impedance spectroscopy technique. The conductivity data do show departure from its simple Arrhenius behavior for all of the compositions. The highest conductivity observed for x = 0.2 sample can be attributed to lower activation energy.     

Ion-electron transfer in solid solutions Rb2 ? 2xFe2 ? xPxO4

The rubidium monoferrite RbFeO₂-based solid solutions with the composition Rb_{2 − 2x} Fe_{2 − x} P _x O₄ have been synthesized, and their crystal structure and the temperature and concentration dependences of the total and electron conductivities have been studied. The introduction of P⁵⁺ ions has been found to sharply decrease the electron conductivity that prevails in pure rubidium monoferrite and, at the same time, to increase the ionic conductivity. The latter becomes dominant as the phosphorus concentration increases. The maximum rubidium-cation conductivity of the materials under study is ∼3 × 10⁻² S/cm at 300°C and ∼3 × 10⁻¹ S/cm at 700°C. The results have been compared with the previously obtained data for similar solid solutions based on rubidium monogallate and monoaluminate.     

Ferromagnetism and the metal-insulator transition in the magnetic semiconductor system FexMn1−x S

The results of investigations of the structure, electrical, and magnetic properties in the system of antiferromagnetic semiconductors Fe_xMn_1−x S (0<x⩽0.5) are described. It is established that metal-insulator transitions with respect to concentration and temperature are connected with changes in the magnetic properties of the system. Fiz. Tverd. Tela (St. Petersburg) 40, 276–277 (February 1998)     

Colossal magnetoresistance of FexMn1−xS magnetic semiconductors

The magnetic, electric, magnetoresistive, and structural properties are investigated in the sulfide solid solutions Fe_xMn_1−2x S, which are based on the antiferromagnetic semiconductor α-MnS (the fcc NaCl lattice). Colossal negative magnetoresistance (δ_H∼−83% at 160 K for x ∼ 0.29), comparable to that observed in La-Ca-Mn-O polycrystals and films (δ_H∼−90% at 100 K and 40 kOe), is observed in compounds with intermediate concentrations 0.26<x<0.4, corresponding to the region of incipient ferromagnetism. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 895–899 (25 June 1999)     

Crystallization of amorphous Fe100−xPx (13x24) alloys

A study of the structure change with temperature in amorphous Fe₁₀₀−xP_x (13 x 24) alloys was carried out by measuring magnetization and thermal expansion and also by structural analysis using X-ray diffraction and differential thermal analysis (DTA). The structure of the amorphous alloys relaxes (the decrease of excess free volume) at temperatures 100–150 K below the crystallization temperatures. The alloys with x 15 transform into (α-Fe + amorphous) at about 600 K. The alloys with x15 transform into (α-Fe+amorphous+Fe₃P) at about 600 K. With further heating, the alloys transform into (α-Fe+Fe₃P) both of which are stable phases from the equilibrium phase diagram.     

Magnetic state of nickel-zinc ferrites at high zinc concentrations

Neutron diffraction and magnetic methods are used to investigate ferrites from the system Zn_xFe_1−x [Ni_1−x Fe_1+x ]O₄. In these investigations, no diffraction effects were observed that would indicate ordered positions for the perpendicular projections of spins at 4.2 K over the entire ferrimagnetic range of concentrations x. However, the high-field magnetic susceptibility and intense small-angle scattering of neutrons observed at helium temperatures in samples with x>0.45 are evidence of local angular structures with effective sizes of 1–10 nm. The temperatures at which these local angular structures are disrupted are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 1503–1504 (August 1998)     

X-ray,electrical conductivity,magnetic and infrared studies of the system Co2−x Ge1−x Fe2x O4

X-ray, electrical conductivity, magnetic hysteresis and IR studies for the system Co_2−x Ge_1−x Fe_2x O₄ were carried out. All the compounds, 0⩽x⩽1, showed cubic symmetry. X-ray intensity calculations, magnetic hysteresis measurements and IR studies indicated the presence of Ge⁴⁺ at tetrahedral, Co²⁺ at octahedral and Fe³⁺ at both the sites. The activation energy and threshold frequency decreased with increasing value ofx. The compounds withx⩽0.5 arep-type and those withx⩾0.75 aren-type semiconductors. Magnetic hysteresis indicated that all the compounds are ferrimagnetic except forx=0 which is antiferromagnetic. The shapes of χ/χ _i vsT plots, highH _c values andJ _R/J_s ratios showed that all the compounds exceptx=0 exhibit single-domain behaviour. Curie temperature,T _c increased with increasing Fe³⁺ ions. The probable ionic configuration for the system is suggested as Ge _1−x ⁴⁺ Fe _x ³⁺ [co _2−x ²⁺ Fe _x ³⁺ ]O ₄ ²⁻ .     

Anomalies of the electronic properties of alloys in the presence of variation of the local magnetic moments

The concentration dependences of the electronic properties (residual electrical resistivity, diffusion thermoelectric power, normal Hall effect, and low-temperature specific heat) and the magnetic characteristics (magnetization and paramagnetic susceptibility) of quasibinary (Pd_xPt_1−x )₃Fe, Pt₃Mn_xFe_1−x , (Pd_xAu_1−x )₃Fe, and Sc_xTi_1−x Fe₂ alloys are investigated. A relationship is established between the anomalous behavior of the kinetic properties and the variation of the local magnetic moments. The absence of corresponding anomalies in the concentration dependence of the specific heat indicates that the density of states at the Fermi level does not change significantly and, therefore, that the conventional Mott two-band model cannot be used to describe the anomalies in the properties of the alloys in question. A single interpretation of the sum-total of the experimental results is given on the basis of the theory of local fluctuations of the electron spin density in metal magnets. Fiz. Tverd. Tela (St. Petersburg) 39, 1257–1262 (July 1997)     

Analysis of the electrical and optical properties of VBO3 single crystals and Fe1?xVxBO3 solid solutions on the basis of a many-electron model of energy band structure

A many-electron model of the energy band structure of VBO₃ and of Fe_1−x V_xBO₃ solid solutions is proposed with strong electron correlations taken into account. Experimental optical absorption spectra and data on the resistivity are discussed in the framework of the suggested model. Variation in the magnetic and electronic properties of VBO₃ and Fe_1−x V_xBO₃ under high pressure is predicted. For VBO₃, a Mott-Hubbard (insulator-metal) transition is expected in the high-pressure phase. In Fe_1−x V_xBO₃ solid solutions, a nontrivial variation in the properties is predicted, leading to the appearance of a different magnetic state. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 8, 2004, pp. 1422–1427. Original Russian Text Copyright ? 2004 by Ivanova, Kazak, Markov, Ovchinnikov, Rudenko, Abd-Elmeguid.