Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x? = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.     

X-ray, IR and bulk magnetic properties of Cu1 + x Mn x Fe2–2x O4 ferrite system

The structural, IR and magnetic properties of the mixed spinel Cu_{1 + x} Mn _x Fe_2-2x O₄ (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) system have been investigated by means of X-ray diffraction, Infrared spectroscopy, magnetization and a.c. susceptibility measurements. X-ray results confirm single-phase spinel structure for all the concentrations. The structure of CuFe₂O₄ is tetragonal and changes to cubic for x = 0.1 to 0.4. For x = 0.5 and 0.6 the structure become tetragonal. The lattice parameter and X-ray density are deduced and their variation with Mn⁴⁺ concentration is studied. The cation distribution derived from the X-ray diffractometry data was found to agree very well with the cation distribution obtained from magnetization measurements. X-ray intensity calculations indicate that Mn⁴⁺ occupy only octahedral [B] sites and Cu²⁺ and Fe³⁺ ions occupy both octahedral [B] and tetrahedral (A) sites. The infrared spectra obtained at room temperature in the range 200–800 cm^–1 showed two absorption bands. The force constants have been obtained from IR data. The variation of the saturation magnetization per formula unit as a function of Mn⁴⁺ content x has been satisfactorily explained on the basis of Neel's collinear spin ordering model for all values of x. The Curie temperature T _C determined from a.c. susceptibility data decreases with increase of Mn⁴⁺ concentration x, suggesting decrease in ferrimagnetic behaviour.     

Electronic structure and optical transitions of Au(20-x)Cu(x) nanoclusters

Gold nanoclusters are promising for photothermal therapy, radiotherapy and cancer cell imaging. We have performed a first-principles study to evaluate the electronic and optical properties of Au7, Au13, Au19, Au20, and Au(20-x)Cu(x) clusters. We have employed the Perdew-Burke-Ernzerhof form of generalized gradient approximation in the frame work of density functional theory. Calculations have been carried out in different configurations. With increasing Cu atoms, the HOMO-LUMO gap of Au(20-x)Cu(x) clusters is decreased due to the increase of electronic states in the HOMO. The results of imaginary part of the dielectric function indicate that the optical transition between HOMO and LUMO has shifted to the low energy range as the Cu atoms increase. The Au and Au(20-x)Cu(x) clusters show tunable optical properties.     

纳米磁性颗粒膜（Fe65Co35）x（SiO2）1-x的微结构和磁性

用射频磁控溅射制备了由FeCo纳米磁性颗粒分布在SiO2绝缘介质中形成的（Fe65Co35）0.4（2SiO2）0.58纳米颗粒膜。通过低温磁场回火得到很好的软磁性,易轴矫顽力低至143Am^-1;并呈现很好的面内单轴磁各向异性,各向异性场为3820Am^-1;电阻率高达13.4mΩ·cm。样品的磁谱（磁导率与频率的关系）显示出很好的高频特性：在工作频率f〈0.7GHz时,复数磁导率的实部μ′〉100,FMR频率达1.8GHz,说明样品有高的截止频率,显示出在高频器件及抗电磁干扰技术方面具有很好的应用前景。用铁磁共振实验证明此样品中好的高频软磁性是由于实现了颗粒间的铁磁性交换耦合所致。     

Magnetic properties of the mixed spinel Co1+x Si x Fe2−2x O4

The structural and magnetic properties of the mixed spinel Co_1+x Si _x Fe_2?2x O₄ system for 0·1≤x≤0·6 have been studied by means of X-ray diffraction, magnetization, and Mössbauer spectroscopy measurements. X-ray intensity calculations indicate that Si⁴⁺ ions occupy only tetrahedral (A) sites replacing Fe³⁺ ions, and the added Co²⁺ ions substitute for (B) site Fe³⁺ ions. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at the A and B sites, forx≤0·3. The Mössbauer intensity data shows that Si possesses a preference for the A site of the spinel. The variation of the saturation magnetic moment per formula unit measured at 300 K with the Si content, is explained on the basis of Neel’s collinear spin ordering model forx≤0·3 which is supported by Mössbauer, and X-ray data. The Curie temperature decreases nearly linearly with increase of the Si content, forx=0·1–0·6.     

Synthesis and structural studies on Ni0.5+x Zn0.5Cu x Fe2−2x O4

The mixed ferrites of Ni–Zn–Cu are synthesized using ceramic double sintering technique. Cu and Ni are substituted in steps of x=0.1 at the interstitial sites of Fe. Powders of mixed ferrites of Ni–Zn–Cu are studied using XRD. The mixed ferrites show a single phase and face center cubic structure for all concentrations. The substitution of Cu and Ni are confirmed from the variation of lattice constant. The cation distribution in the A and B sites of the ferrites is estimated. Mixed ferrites of Ni–Zn–Cu are characterized using a.c. conductivity and magnetic susceptibility methods. The variation of activation energy, magnetic moment and Curie temperature with concentration of Fe ions explains the alterations of the energy levels of d bands. Hopping of charge carriers and the presence of different ionic states of Ni, Cu, and Fe ions are discussed using the FTIR and EPR spectra.     

Co(1-x)Mg(x)MoO4 compounds for pressure indicators

Two Co(1-x)Mg(x)MoO(4) oxide compositions (x=0 and x=0.4) were investigated as potential pressure indicators. The first order phase half-transition induced by pressure application from the β to the α form, i.e. from the high temperature/low pressure form to the high pressure/low temperature form, was studied thanks to the powder diffuse reflection (color) evolution versus the applied pressure. Three key parameters were analyzed: (i) the magnesium content, (ii) the powder grain sizes, (iii) the pressure application mode (uniaxial or isostatical). It was shown these three parameters allow tuning the transition pressure in a wide range from few bars to few kbars.     

Investigation of phase separation in the Ge x Cu1–x Fe2O4 system

The spinel series Ge _x Cu_1–x Fe₂O₄ (x=0.0 to 0.9) has been studied in detail by means of Mössbauer spectroscopy, X-ray diffraction and magnetization measurements at room temperature (298 K). Analysis of X-ray diffraction intensity data and Mössbauer intensity data suggest that this system remains in single phase up tox=0.4 then it phase separates into two different phases forx=0.5 to 0.9. Lattice constants of this system deviate from Vegard's law. Mössbauer spectra for x=0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octrahedral ions (B-sites), while forx=0.5 to 0.9 it gives Mössbauer patterns corresponding to two separate phases. The systematic composition dependence of quadrupole interactions and nuclear hyperfine fields of⁵⁷Fe³⁺ ions also support the concept of phase separation forx=0.5 to 0.9. The observed variation of⁵⁷Fe³⁺ hyperfine field on A- and B-sites withx forx=0.0 to 0.4 can be explained qualitatively on the basis of supertransferred hyperfine interactions.     

(Sn1-x,Co2x)O2纳米粒子的制备及气敏性能研究

采用分析纯SnCl4·5H2O和NH3·H2O为主要原料,控制不同Co2 /Sn4 摩尔比,利用均匀共沉淀法制备了(Sn1-x,Co2x)O2纳米粉体样品,并以白云母为基片利用厚膜工艺制得气敏元件.对样品的结构、结晶性能和活性等分析发现,Co2 以类质同像的方式代替了SnO2晶格中的Sn4 ,并引起了M-O键长、晶胞参数和M-O八面体电价平衡的非均一性,进而提高了粉体活性及n型半导体性能.在不同浓度H2中元件气敏性能测试表明,Co2 的引入提高了SnO2的H2灵敏度.元件的灵敏度与H2浓度之间具有较好的线性关系.实验得到了制备(Sn1-x,Co2x)O2气敏材料的最佳Co2 /Sn4 摩尔比.     

Effects of Cu-doping on the magnetic state of Zn(0.9-x)Fe0.1Cu(x)O

Magnetization measurements were performed on a series of Zn(0.9-x)Fe0.1Cu(x)O samples (0 < x approximately 0.1) prepared using solid state reaction and sol-gel methods. Although Cu is nonmagnetic, we found that increasing Cu content increases the saturation magnetization and enhances the hysteresis losses. Curie behavior of the susceptibility at high temperature indicates the presence of ferromagnetic exchange interaction. Moreover, we found that the exchange interaction and the molecular field coefficient are both ferromagnetic and greatly enhanced with Cu-doping; however, the Arrott-Belov-Kouvel plot did not reveal the presence of spontaneous magnetization down to 4.2 K.     

Magnetic structure and spin-glass state of Fe1-x Zn x Cr2S4 spinels

Fe_1-x Zn _x Cr₂S₄ (0 <x < 1) solid solutions were synthesized and their magnetic properties and structure were investigated. The magnetic phase diagram of the solid-solution system was constructed. The results reveal a ferrimagnetic-spin glass-antiferromagnetic compositional transition in the FeCr₂S₄-ZnCr₂S₄ system.     

Electronic structure and magnetic properties of the Ni0.2Cd0.3Fe(2.5-x)A1(x)O4 (0 < or = x < or = 0.4) ferrite nanoparticles

The structural, magnetic, and electronic structural properties of Ni0.2Cd0.3Fe(2.5-x)Al(x)O4 ferrite nanoparticles were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), DC magnetization, and near-edge X-ray absorption fine-structure spectroscopy (NEXAFS) measurements. Nanoparticles of Ni0.2Cd0.3Fe(2.5x)Al(x)O4 (0 < or = x < or = 0.4) ferrite were synthesized using the sol-gel method. The XRD and TEM measurements showed that all the samples had a single-phase nature with a cubic structure, and had nanocrystalline behavior. From the XRD and TEM analysis, it was found that the particle size increases with Al doping. The DC magnetization measurements revealed that the blocking temperature increases with increased Al doping. It was observed that the magnetic moment decreases with Al doping, which may be due to the dilution of the sublattice by the doping of the Al ions. The NEXAFS measurements performed at room temperature indicated that Fe exists in a mixed-valence state.     

Dispersion of dielectric constant and resistivity of Cu x Zn1−x Fe2O4 samples

Cu _x Zn_1−x Fe₂O₄ samples exhibit dispersion of dielectric constant, tanδ and resistivity in the frequency range of 1 kHz to 50 MHz. The dispersion exhibited is in general accord with Koops’ model. However, the details of the conducting and non-conducting regions must be taken into account when composition tends to change interrelationship between the elementary capacitor resistor circuits. On quenching these samples from 800°C the dielectric constantε ¹ showed an increase for CuFe₂O₄ and Cu_0·8Zn_0·2Fe₂O₄ samples. The dielectric constant of the remaining samples showed no influence on quenching. The compositional variation showed that the dielectric constant has higher value for the ferrite Cu_0·4Zn_0·6Fe₂O₄ The results are explained on the basis of cation transfer.     

Magnetic properties of Ni1−x Cu x Cr2O4(0⩽x⩽1) compounds

The compounds Ni_1−x Cu _x Cr₂O₄ (0⩽x⩽1) have been synthesised by solid-state reaction between basic nickel(II) carbonate, basic copper(II) carbonate and chromium (III) carbonate in required molar ratios at 800±10°C for 20 hr. The reaction products have been characterized by chemical analyses and powder x-ray diffraction patterns. Magnetic susceptibility has been measured in the temperature range of 300–900 K at 10 kOe. All the products show ferrimagnetic behaviour with the ferrimagnetic Curie temperature (T _c) in the range of 50–150 K. The curie temperature increases when copper(II) ion is substituted for nickel(II) ion in NiCr₂O₄. The experimental values of the average effective magneton number (p-0304;) agree with theoretical values.     

A carbon based spintronic material Fe(x)-C(1-x)/Si structure

The decay of spin polarization poses serious problems for spintronic devices. It will be greatly helped by the availability of spintronic materials with a long spin diffusion length. Carbon has small spin-orbital interaction and longer coherent length. This makes carbon suitable material for exploitation in the spintronic materials and devices. A great deal of magnetoresistance (MR) research has been carried out in carbon nanotubes, grapheme and small carbon molecules. However, the MRs of these materials are normally observed at low temperature, making these carbon materials difficult used in information industry. In this paper, we introduce a novel class of carbon based hybrid materials Fe(x)-C(1-x)/Si structure which show larger MR at room temperature. These materials have also some other novel physical properties, such as electromagnetoresistance, switch effect, pressure sensitivity, gas sensitivity and photoconductivity. This kind of carbon based materials has shown early sign of being excellent candidates for spintronic materials operating at room temperature.     

Abnormal dielectric behaviour in the Cd x Cu1-X Fe2O4 system

The an. electrical parameters such as the dielectric constant (₃) and the loss tangent (tan ) for samples slow-cooled and air-quenched at 400, 600 and 800° C of the Cd _x Cu_1-x Fe₂O₄ system have been studied as a function of frequency in the range 50 kHz to 50 MHz. The frequency dependence of the above an. parameters is found to be rather abnormal, giving a peak at a certain frequency for all the samples, irrespective of the thermal history of the ferrites. This has been attibuted to the reduction Cu²⁺ Cu⁺ in the process of sintering of the materials, and therefore to the existence of p-type charge carriers.     

Optical constants of V(1-x)W(x)O(2) Films

The spectral complex optical constants in the visible and the near-infrared region of VO(2) and V(1-x)W(x)O(2) films deposited on glass substrates were determined from observed reflectance and transmittance spectra for which the least-squares method was used. In the metallic phase, the optical properties were characterized by the Drude model in wavelength regions longer than 750 nm.