Magnetization Processes of (La0.7Pb0.3MnO3)1?x(SiO2)x Composites

The ceramic composites, (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x , with diluted magnetic properties are prepared using solid-sate sintering route. Magnetization processes of (La_0.7Pb_0.3MnO₃)_1−x (SiO₂) _x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution induced by the increase of SiO₂ content. Clearly, irreversible behavior is observed in the zero-field cooling and the field cooling (ZFC–FC) curves at a low field of 100 Oe. Saturation magnetization decreases as x increases while ferromagnetic transition temperature remains around 346 K for all composites. All the composites exhibit ferromagnetic hysteresis behavior which can be modeled by the law of the approach to saturation in the form M=M _S(1−a/H). The term a/H expresses the deviation of magnetization from saturation. The smaller factor a for La_0.7Pb_0.3MnO₃-rich samples results in sharper square curve which should be associated with the long-range spin order of ferromagnetic coupling.     

Magnetic Phase Diagram of Ca1?xMnxO

Alternating current susceptibility and direct current magnetization have been studied for polycrystalline Ca_1–x Mn _x O. On increasing the Mn content, magnetic ordering changes from spin glass behavior for 0.25 x 0.4 to antiferromagnetic order. The paramagnetic/antiferromagnetic transition is of second order for 0.5 x 0.65 and of first order for x 0.7. For low Mn concentrations, the high-temperature alternating current susceptibility can be described by a diluted Heisenberg magnet model developed for diluted magnetic semiconductors.     

Superconductivity of YBa2?xNdxCu3Oy Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2–x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x0.30. The onset transition temperature is insensitive to the Nd content x in the region of x0.40. All are higher than 95 K. The zero resistance transition temperatures , however, exhibits two-step variation with the increase of x. For x0.25, are all above 92 K. The highest of 94 K was obtained for x=0.25. For x0.3 drops sharply to about 84 K. Finally falls to 30 K and is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

Thermal conductivity of Ca1 ? xHoxF2 + x optical ceramics

The thermal conductivity of Ca_{1 − x} Ho _x F_{2 + x} (x ≤ 0.03) optical ceramics has been studied experimentally in the temperature range 50–300 K. With increasing holmium content, the thermal conductivity of the ceramics decreases, especially at low temperatures: from 10.2 to 2.3 W/(m K) at 300 K and from 250 to 4.5 W/(m K) at 50 K.     

Relation of Structure and Superconductivity in Self-compensated Y1?xCaxBa2Cu3?xAlxOz System

The co-doped compounds of Y_1−x Ca _x Ba₂Cu_3−x Al _x O _z , with x from 0.1 to 0.4, were synthesized through a solid-state reaction method. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement. The lattice constant a decreases while b increases with the addition of x. The difference between a and b diminishes gradually. Careful study of the crystalline structure shows that the critical temperature (T _c ) changes monotonically with some local structural parameters, such as the difference between Ba and Cu(2) atoms’ Z coordinates, the bond length of Cu(2)–O(4), and the bond angle of Cu(2)–O(2)–Cu(2), which are all closely related to the interaction between the perovskite block and the rock salt block in the unit cell. The results indicated that the influence of the crystalline structure on superconductivity is important and independent of the carrier concentration.     

The Effects of Substitution of Sn for Cu in Bi1.75Pb0.25Sr2CaCu2.3?xSnxSnxOy

The influence of Sn doping on superconductivity in the Bi-based 2212 phase is studied in this paper. For the samples R–T relations and magnetic hysteresis loops were measured. X-ray powder diffraction analysis was also performed. For Bi_1.75Pb_0.25Sr₂CaCu_2.3–x Sn _x O _y , the experimental results show that by adding the proper amount of Sn the superconductivity of the samples can be improved. As x = 0.15, the critical temperature T _c, the critical current density J _c, and the magnetic pinning force density F reach a maximum. At T = 11 K, the critical state parameters H _c1, H _c2, , , and are calculated and compared with the results reported by other researchers. The experimental results also show that the Sn doping is able to speed up the growth of the 2223 phase. In brief, Sn doping is an effective way of improving the superconductivity in Bi-based superconductors.     

Electrical Characterization of p-Type Pb1?xEuxTe

The transport properties of p-type Pb_1–x Eu _x Te epitaxial layers were studied as a function of Eu content, temperature, and magnetic field. The low-temperature hole mobility is drastically reduced when the Eu concentration is increased from 0 to 6%, while the hole concentration remains almost constant. A metal-insulator transition was observed for x 0.04, which is probably induced by the disorder caused by the introduction of Eu. For temperatures down to 10 K, only positive magnetoresistance has been observed at low magnetic fields. An anomalous behavior of the resistivity as a function of temperature has been detected for a Eu content about 5%, which is attributed to the resonance between the localized Eu 4f level and the valence band maximum.     

Electrical properties and microstructure of CdxHg1 ? x ? yCrySe crystals

Crystals of the Cd _x Hg_{1 − x − y} Cr _y Se (x = 0.4, y = 0.1) quaternary solid solution have been grown by the Bridgman method, and their microstructure and electrical properties have been studied. The crystals are shown to contain various types of inclusions in the form of filaments and triangles.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

Molecular beam epitaxy of Pb1 ? xEuxTe and Pb1 ? xSnxTe layers and related periodic structures

Layers and periodic Bragg structures based on Pb_{1 − x} Eu _x Te (0 < x < 1) and Pb_{1 − x} Sn _x Te (0 < x < 0.1) ternary solid solutions have been grown on (111) BaF₂ and (111) Si substrates by molecular beam epitaxy. X-ray diffraction measurements show that the layers in three-period EuTe/Pb_0.94Eu_0.06Te structures maintain the [111] crystallographic orientation normal to the substrate plane. In the interface plane, the [110] directions of the layers and substrate are parallel to each other. The full width at half maximum of the rocking curve of each layer of both the binary and ternary compounds is about 20′. The large optical contrast (40%) in such structures allowed us to reach 99.9% mid-IR reflectivity.     

Characterizations of InzGa1?z As1?x?yN xSbyP-i-N structures grown on GaAs by molecular beam epitaxy

The GaAs-based double-heterojunction P-i-N structures using In_zGa_1–zAs_1–x–yN_xSb_y as the i-layer is investigated for the first time using solid source molecular beam epitaxy. High quality coherent bulk InGaAsN_3.45%Sb (0.5 m) with a lattice-mismatch of 2.6 × 10^–3 can be achieved due to the surfactant properties of antimony, while the bulk InGaAsN_2% at 0.5 m with 1.06× 10^–3 mismatch is fully relaxed. Rapid thermal annealing (RTA) resulted in 25 meV low temperature (LT) photoluminescence (PL) full-width half-maximums (FWHM) for the bulk InGaAsNSb layers. InGaAsNSb experiences 30% less blueshifting with subjected under the same annealing conditions as InGaAsN with similar N content. Room temperature (RT) absorption measurements are in the range of 0.9–1.4 eV. The absorption edge of 1.41 m is achieved for sample D4.     

Electrical properties of Zn2(TiaSnb)1 ? xZrxO4 solid solutions

The electrical conductivity, band gap, dielectric permittivity, and molar polarizability of Zn₂(Ti _a Sn _b )_{1 − x} Zr _x O₄ solid solutions have been determined. All of the synthesized samples are dielectrics with semiconducting behavior of conductivity. The phase diagram of the Zn₂TiO₄-Zn₂SnO₄-Zn₂ZrO₄ system is presented.     

Crystal structure and magnetic properties of high-coercivity Sr1 ? xPrxFe12 ? xZnxO19 solid solutions

Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ ferrites with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 have been prepared by solid-state reactions between praseodymium, iron, and zinc oxides and strontium carbonate in air at 1470 K. According to X-ray diffraction results, the samples with x ≤ 0.2 were single-phase and those with 0.3 ≤ x ≤ 0.5 contained, in addition to the magnetoplumbite phase, small amounts of α-Fe₂O₃, ZnFe₂O₄, and PrFeO₃. The mixed-phase samples further fired twice at 1470 K for 4 and 2 h contained no impurity phases at x = 0.3 and contained only α-Fe₂O₃ at x = 0.4 and 0.5. In the composition range 0 ≤ x ≤ 0.3, the a and c cell parameters, unit-cell volume V, and X-ray density ρ_x of the magnetoplumbite phase vary linearly according to the relations a(?) = 5.8869 − 0.0162x, c(?) = 23.027 + 0.449 x, V(?³)= 691.10 + 9.65x, and ρ_x(g/cm³) = 5.102 + 0.230 x. The highest degree of combined heterovalent substitution of Pr³⁺ for Sr²⁺ and Zn²⁺ for Fe³⁺ in the SrFe₁₂O₁₉ ferrite (formation of Sr_1−x Pr _x Fe_{12 − x} Zn _x O₁₉ solid solutions) at 1470 K is x = 0.32−0.36. The saturation magnetization per formula unit (n _s) of the x = 0.1 ferrite exceeds that of SrFe₁₂O₁₉ by 1.7% at 6 K and by 15.2% at 308 K. The 308-K n _s and coercive force (_σ H _c) of the x = 0.2 ferrite exceed those of SrFe₁₂O₁₉ by 7.6 and 8.5%, respectively.     

Neutron structural investigations of Y1?xCaxBa2Cu3?yCoyO7±δ

The results of a systematic powder neutron study on Y_1–x CaxBa₂Cu_3–y Co _y O_7± for A (x=y=0), B (x=0;y=0.2), C (x= 0;y=0.4), D (x=y=0.2), and E (x=y=0.4) are investigated with a view to understanding the relation between the structural parameters and superconductivity. Rietveld refinements of the structures show that: (a) Co substitutes at the chain Cu(1) sites only, except for sample E, where the presence of a minor amount of Co at the planar Cu(2) site cannot be ruled out; (b) Co substitution reduced thec-parameter, which is reduced even further upon substitution of Ca at the Y-sites; (c) the occupancy factors of the chain O(1) site indicate an average coordination of 4.8 and 5.1 for Co for samples B and C, but only 4 for D and E; (d) Cu/Co(1) atoms for B and C display large thermal parameters, suggesting a displacement from their ideal centrosymmetric location; (e) the apical Cu(1)-O(4) bond lengthens upon substitution of Co in samples B and C but undergoes shortening upon substitution of Ca in the case of samples D and E. The apical Cu(2)-O(4) bond, on the other hand, shows just the opposite trend; (f) samples D and E show a reduction in the separation of CuO₂ layers and their oxygen content; and (e) the bond valence of the Cu(2) ion shows the lowest value of 2.127 for the nonsuperconducting sample C.     

Laser induced photoluminescence and morphological characterization of Cd(1?x)?yZnxMnyS nanocrystals

Doped II–VI chalcogenide semiconductor nanostructures have recently attracted a lot of attention due to the possibility of their application in various modern era devices. In the present study, Cd_(1−x)−y Zn _x Mn _y S {(0 ≤ x ≤ 0.5); (0.0001 ≤ y ≤ 0.1)} nanocrystals (NC) have been synthesized by facile wet chemical technique. Morphological and structural analyses of these synthesized quaternary NC have been done using X-ray diffraction (XRD) and transmission electron microscope (TEM) studies. Room temperature photoluminescence (PL) has been investigated using high peak power pulsed N₂-laser. Important optical parameter; excited state lifetime values have been calculated from the recorded multi-exponential decay curves. These fast and efficient nanophosphors have wide applications in opto-electronic industry as futuristic displays, lasers, nanoelectronics and nanosensors.     

Structure, Phonon Vibration, and Spin Correlation of LaBa2Cu3?xCoxOy

Polycrystalline samples LaBa₂Cu_3–xCo _x O _y (0 x 1.0) were synthesized by solid state reaction method. The structure, phonon vibration, conduction, and spin correlation were investigated by means of X-ray diffraction, infrared spectra, resistivity, and electron spin resonance. It is found that there are orthorhombic–tetragonal and tetragonal–orthorhombic structural transitions with Co doping, and the conduction behavior changes from metallic to semiconducting. With the increase of Co content, the Cu(1)—O(1) phonon mode around 531 cm^–1 softens, the Cu(2)—O(2) phonon mode around 657 cm^–1 hardens, and the Cu(1)—O(4) mode around 583 cm^–1 is nearly unchanged. The Cu²⁺ spins tend to localize with Co doping. The changes in structure, phonon vibration, and spin correlation with Co doping are analyzed and discussed.     

Experimental determination of the spatial distribution of electron defects in La2 ? xSrxCuO4 and Nd2 ? xCexCuO4 crystal lattices

The data of M?ssbauer emission spectroscopy on ⁶⁷Cu(⁶⁷Zn) and ⁶⁷Ga(⁶⁷Zn) isotopes show that holes appearing as a result of the Sr²⁺ substitution for La³⁺ in the La_{2 − x} Sr _x CuO₄ crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce⁴⁺ substitution for Nd³⁺ in the Nd_{2 − x} Ce _x CuO₄ crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La_{2 − x} Sr _x CuO₄ and Nd_{2 − x} Ce _x CuO₄ crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).     

Synthesis, and study of magnetic properties, of Bi1?xCdxFeO3

Ceramic Bi_1−x Cd _x FeO₃ (x = 0.0, 0.05, and 0.1) samples were prepared by a citrate-gel method. The as-prepared compounds were calcined at 600 °C for 3 h to obtain nearly single-phase materials. The crystal structure, examined by X-ray powder diffraction (XRD) and Rietveld analysis, confirmed that the samples crystallize in a rhombohedral (space group, R-3c no. 161) structure. Magnetic measurements were carried out on the resultant powders from 300 to ~2.5 K. Magnetic hysteresis loops showed a significant increase in magnetization as a result of Cd doping in BiFeO₃.     

GaxIn1 ? xBiyAszSb1 ? y ? z/InSb and InBiyAszSb1 ? y ? z/InSb heterostructures grown in a temperature gradient

Heterophase equilibria in the Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} -InSb and InBi _y As _z Sb_{1 − y − z} -InSb systems have been analyzed within the simple solution approximation. Ga _x In_{1 − x} Bi _y As _z Sb_{1 − y − z} /InSb and InBi _y As _z Sb_{1 − y − z} /InSb heterostructures have been grown in a temperature gradient, and their composition and structural perfection have been assessed.     

X-ray diffraction study of multicomponent oxides in the Zn2 ? x(TiaZrb)1 ? xFe2xO4 system

The multicomponent refractory oxide system Zn_{2 − x} (Ti _a Zr _b )_{1 − x} Fe_2x O₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 2: 1, 3: 1, 4: 1; x = 0–1.0; Δx = 0.05) has been studied by X-ray diffraction, using samples prepared by melting appropriate metal oxide mixtures in a low-temperature hydrogen-oxygen plasma. Two phases, both with wide homogeneity ranges, have been identified: α-phase, with a cubic inverse spinel structure, and β-phase, with a tetragonal inverse spinel structure. The phase boundaries in the system have been determined. Structural data are presented for about 100 solid solutions.