Magnetic properties of stuffed copper chromium tellurides Cu1+xCr2+yTe4 (x=0-1, y<0.3)

We present a detail study of the effect of excess metal atoms on the magnetic properties of Cu_1+xCr_2+yTe₄ at 2-400 K. With the increase in x=0-1 and y<0.3, these compounds retain metallic behavior, while ferromagnetic ordering temperature reduces from 325 to 160 K. Our low field susceptibility χ_ac measurements reveal a second transition on cooling below the ferromagnetic ordering; the transition at around 160-180 K intensifies with the excess amount of copper and chromium atoms. The value of spontaneous magnetization at 2 K remains between 2.6 and 2.9μ_B across all the compositions and it reduces with temperature as M(T)∼A₀T^3/2+A₁T^5/2, as expected for the excitation of Bloch's spin waves in a model of the Heisenberg ferromagnet. Our terminal composition Cu_1.9Cr_2.25Te₄ showed only second transition at 160 K with short range magnetic order much above the transition temperature and in the absence of the specific heat jump at this temperature. The magnetic properties are explained as a result of random magnetic anisotropy in the excess-metal compositions induced by the interstitial atomic defects in their parent spinel structure. The large stuffing of cations has been made possible in the telluride compounds because of the large size of tellurium and also by the covalent bonding that stabilizes the defect structure.     

Phase transition in V1+xTe2 (0.04⩽x⩽0.11)

1T-V_1+xTe₂ was synthesized in a composition range of 0.04?x?0.11. The reversible first order transition was observed by DTA (DSC), powder X-ray diffraction, magnetic susceptibility (χ), and d.c. electrical resistivity (?) measurements. Transition temperature (T_t is 474 K for V_1.04Te₂, and decreases with increasing x. Heat of transition, ΔH was estimated to be as high as 500 cal mol^?1 from the endothermical peak in DSC. The expansion of the c-axis is observed at T_t. χ exhibits a jump at T_t, showing the paramagnetic temperature dependence both below and above T_t. ? measurements show the metallic-like behavior with a slight decrease at T_t. Preliminary electron diffraction examination suggests the formation of a super-structure below T_t.     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Magnetoelectric behavior of ferrimagnetic BixCo2−xMnO4 (x=0, 0.1 and 0.3) thin films

Thin films of Bi_xCo_2−xMnO₄ (x=0, 0.1 and 0.3) were grown on quartz, LaAlO₃ (LAO) and YBa₂Cu₃O₇ (YBCO) buffer layer coated LAO substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) and Raman scattering measurements showed that the thin films exhibit single phase polycrystalline cubic spinel structure on all the substrates. Near edge X-ray absorption fine structure (NEXAFS) studies confirmed the octahedral occupancy of the substituted Bi³⁺ ions. Temperature dependent zero-field cooled (ZFC) magnetization measurements show the ferrimagnetic (FM) behavior (T_C∼186 K) and magnetization undergoes a crossover from positive to negative, owing to the opposite contributions of magnetic moments from Co and Mn ions. A weak ferroelectric property exhibited by the films above room temperature was evidenced through the capacitance-voltage (C-V) and dielectric measurements. Magnetoelectric coupling was found to be maximum just below FM-T_C.     

Chemical shifts of copper and cobalt K absorption discontinuities in the spinels CuCr2X4 (X =O,S, Se,Te), CoCr2X4 (X =O,S) and Cu0.5Co0.5Cr2−xRhxS4 (x = 0, 1, 2)

The present paper reports the chemical shifts of the copper and cobalt K absorption discontinuities in the spinels CuCr₂X₄ (X = O, S, Se, Te), CoCr₂X₄ (X = O, S) and Cu_0.5Co_0.5Cr_2?xRh_xS₄ (x = 0, 1, 2). The magnitudes of the chemical shifts show that copper is monovalent (except in CuCr₂O₄) and cobalt is bivalent in all these spinels. The valence structure for the spinels containing copper shows that the conductivity of such spinels (except CuCr₂O₄) is due to the holes in the anion p-orbitals. Levine's theory of ionicity has been extended to spinels which has made it possible to calculate the various bond parameters for the spinels CuCr₂X₄ and CoCr₂X₄. It has been found that a linear relation exists between the X-ray chemical shift ΔE and C, E_g, and f_i, the ionic energy, total energy and ionici parameters respectively. This fact has been used to determine the various bond parameters in the spinels Cu_0.5Co_0.5Cr_2?xRh_xS₄. It is found that in these spinels a natural balance of ionicity at the A site is maintained, when chromium is gradually replaced by rhodium.     

Thermoelectric properties of the Bi2−xYxRu2O7 (x=0-2) pyrochlores

Electrical conductivity and Seebeck coefficient for the Bi_2−xY_xRu₂O₇ pyrochlores with x=0.0,0.5,1.0,1.5,2.0 were measured in the temperature range of 473-1073 K in air. With increasing Bi content, the temperature dependence of the electrical conductivity changed from semiconducting to metallic. The signs of the Seebeck coefficient were positive in the measured temperature range for all the samples, indicating that the major carriers were holes. The temperature dependence of the Seebeck coefficient for the Y₂Ru₂O₇ indicated the thermal activation-type behavior of the holes, while that for the Bi_2−xY_xRu₂O₇ with x=0.0-1.5 indicated the itinerant behavior of the holes. The change in the conduction behavior from semiconductor to metal with increasing Bi content is consistent with the increase in the overlap between the Ru4d t_2g and O2p orbitals, but the mixing of Bi6s, 6p states at E_F may not be ruled out. The thermoelectric power factors for the Bi_2−xY_xRu₂O₇ with x=1.5 and 2.0 were lower than 10⁻⁵ W m⁻¹ K⁻² and those with x=0.0,0.5,1.0 were around 1-3×10⁻⁵ W m⁻¹ K⁻².     

Magnetic properties of Mg1-pMnpTe2 near p = 0.5

Precise magnetization measurements and neutron diffraction measurements are carried out for Mg_1-pMn_pTe₂ near the critical concentration p = 0.5 for the antiferromagnetic ordering. A magnetization property which is similar to micto-magnetism is observed for p = 0.35 and p = 0.45 samples. The susceptibility shows an anomaly at T_f and field-cooling effect is notable below T_f. Neutron diffraction experiments show that most of the manganese moments are ordered antiferromagnetically for p = 0.53 while small fractions of Mn²⁺ are ordered for p = 0.45 and p = 0.35.     

Critical current density and flux pinning in La2−xPrxCa2x Ba2Cu4+2xOz (x=0.1-0.5) superconductors

Polycrystalline La_2−xPr_xCa_2xBa₂Cu_4+2xO_z (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (T_c^onset values) with increasing dopant concentration. The mixing of rare earth La³⁺ and Pr^3+/4+ ions at rare earth site (La³⁺) along with substitution of divalent Ca²⁺ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.     

Magnetic properties of Cr-rich crystals of Rb2Cr1−xMxCl4(M = Mn,Cd)

Magnetic data for Rb₂Cr_1?xCd_xCl₄ and Rb₂Cr_1?xMn_xCl₄ are reported. Curie temperatures can be determined quite accurately in these systems with the help of magnetometers if small fields are applied. The magnetic moment per mol Cr(II) is independent of x for small concentrations of cadmium. A roughly linear decrease of that moment is observed in the case of an admixture of manganese. This gives evidence for an antiferromagnetic interaction between Cr(2+) and Mn(2+) in the mixed crystal system Rb₂Cr_1?xMn_xCl₄.     

On the mixed valence of the chromium ions in the noncollinear ferrimagnetic spinels Mn1−xCuxCr2S4 (Where x = 0.85, 0.90, 0.95)

For the polycrystalline samples of Mn_1?xCu_xCr₂S₄ (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, T_C, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between T_C and about 600 K. The Curie temperatures were obtained using the kink point method.In the temperature range between 4.2 and 77 K the magnetization was measured in stationary magnetic fields up to 14 T. The data indicate a noncollinear ferrimagnetic structure. The compounds under investigation can be treated as CuCr₂S₄ slightly doped with Mn, with a valence distribution Mn²⁺_1?xCu¹⁺_xCr³⁺_2?xCr⁴⁺_xS^2?₄.     

Electronic structure and optical properties of CdSexTe1−x mixed crystals

The band structure and optical properties of the CdSe_xTe_1−x ternary mixed crystals have been studied using the pseudopotential formalism under an improved virtual crystal approximation approach. Quantities such as, energy gaps, band-gap bowing parameters, electron effective mass and dielectric constants are calculated. Our results agree well with the available data in the literature. The composition dependence of all studied quantities has been expressed by quadratic polynomial forms.     

Raman characterization of Pb2Na1−xLaxNb5−xFexO15 and Pb0.5(5−x)LaxNb5−xFexO15 (0≤x≤1) solid solutions

The ferroelectric compounds Pb₂Na_1−xLa_xNb_5−xFe_xO₁₅ and Pb_0.5(5−x)La_xNb_5−xFe_xO₁₅ (0≤x≤1) with the tungsten bronze type structure have been investigated using Raman spectroscopy. The evolution of the spectra as a function of composition at room temperature is reported. In the frequency range 200-1000 cm⁻¹ three main A₁ phonons around 240 (υ₁), 630 (υ₂) and 816 (υ₃) cm⁻¹ were observed. The broadening of the Raman lines for high values of x originates from a significant structural disorder. This is in good agreement with the relaxor character of these compositions. The lowest-frequency part of the spectra, below 180 cm⁻¹, reveals a structural change in the studied solid solutions. The behaviour of the Raman shift of the υ₁ mode confirms that in Pb₂Na_1−xLa_xNb_5−xFe_xO₁₅, a clear anomaly occurs in the vicinity of x=0.4.     

Superconductivity in CoSr2(Y1−xCax)Cu2O7+δ

The roles of aliovalent Ca^II-for-Y^III substitution and high-pressure-oxygen annealing in the process of ‘superconducterizing’ the Co-based layered copper oxide, CoSr₂(Y_1−xCa_x)Cu₂O_7+δ (Co-1212), were investigated. The as-air-synthesized samples up to x=0.4 were found essentially oxygen stoichiometric (−0.03≤δ≤0.00). These samples, however, were not superconducting, suggesting that the holes created by the divalent-for-trivalent cation substitution are trapped on Co in the charge reservoir. Ultra-high-pressure heat treatment carried out at 5 GPa and 500 °C for 30 min in the presence of Ag₂O₂ as an excess oxygen source induced bulk superconductivity in these samples. The highest T_c was obtained for the high-oxygen-pressure treated x=0.3 sample at ∼40 K.     

Electron hopping in Fe1−xCuxCr2S4 (0<x<0.5)

A theoretical expression for the line shape of the Mössbauer spectra in the presence of electron hopping between Fe²⁺ and Fe³⁺ is obtained by using a simple stochastic model. Analyses based upon this expression show that the origin of the complicated Mössbauer spectra observed in the magnetic semiconductors Fe_1?xCu_xCr₂S₄ (0<x<0.5) at 77 K is electron hopping between Fe⁺² and Fe³⁺ This hopping occurs at a rate of a few MHz. Quantitative estimates are given for some parameters; the isomer shifts, the internal magnetic fields, the quadrupole splittings and the proportions of Fe²⁺ and Fe³⁺. The valence distribution in this system is determined from the results. For example, the distribution Fe²⁺_0.69Fe³+_0.29Cu¹⁺_0.02Cr³⁺_1.72Cr²⁺_0.28S^2?₄ is obtained for x = 0.02. The existence of Cr²⁺ is concluded.     

Effect of Cr on the magnetic properties of evaporated CoxCr1−x/Si(1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si(1 0 0) and glass substrates. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Alternating gradient field magnetometer (AGFM) measurements provided saturation magnetization values ranging from 220 to 1200 emu/cm³. Values of squareness exceeding 0.8 have been measured. Coercive field may reach values up to 700 Oe, depending on the percentage of chromium, as well as the substrate nature and the direction of the applied magnetic field. The saturation magnetization value decreases as the Cr content increases. In order to study their dynamical magnetic properties, Brillouin Light Scattering (BLS) measurements have been performed on these samples. Stiffness constant value and anisotropy magnetic field were adjusted to fit the experimental BLS spectra. These results are analyzed and correlated.     

Electronic states and phonon properties of GexSi1−x nanostructures

Ge_xSi_1−x nanostructures that can be manipulated through size reduction, geometry variation, and alloying, are considered as one of the key developments for next generation technologies, due to their easy processing, unique properties, and compatibility with the existent silicon-based microelectronic industry. In this review, we have thoroughly discussed the major advances in electronic structures and phonon properties of Ge_xSi_1−x nanocrystals (NCs). Experimental and theoretical characterization related to several main factors, for example, size, composition, strain, temperature, and interface and surface were presented with special emphasis in low-frequency Raman scattering. Current difficulties in explaining the Raman spectra are the assignment of the low-frequency modes because of the complexity of the environment around the NCs, thus different theoretical models are introduced in detail to deal with different properties of Ge_xSi_1−x alloy NCs including Lamb’s theory, complex-frequency (CF) model, core–shell matrix (CMS) model and spatial coherence effect model.     

Magnetic properties of system HgxZn1-xCr2Se4

Magnetic moments, magnetic susceptibilities and optical absorption of the system Hg_xZn_1-xCr₂Se₄ have been measured. A correlation is found between the optical and the magnetic properties.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

Dielectric properties of SrBi2−xPrxNb2O9 ceramics (x=0, 0.04 and 0.2)

SrBi_2−xPr_xNb₂O₉ (x=0, 0.04 and 0.2) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase layered perovskite structure ferroelectrics were obtained. A relaxor behavior of frequency dispersion was observed among Pr-doped SrBi₂Nb₂O₉. The degree of frequency dispersion ΔT increased from 0 for x=0-7 °C for x=0.2, and the extent of relaxor behavior γ increased from 0.94 for x=0-1.45 for x=0.2. The substitution of Pr ions for Bi³⁺ ions in the Bi₂O₂ layers resulted in a shift of the Curie point to lower temperatures and a decrease in remanent polarization. In addition, the coercive field 2E_c reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2.     

Study of the CulCu2S system. conductivity of the solid solution Cu1+xI1−xSx(x<0.5)

A structural study of the CuICu₂S system has shown that a large solid solution ranges from pure CuI to Cu₃SI with a regular decrease of tehcell parameter ($\text{a=6.045 to 5.901}\text{A}\text{?}$); for richer Cu₂S concentrations, two-phase systems appear with the formation of a new Cu₇S₃I compound, and another solid solution exists for higher Cu₃S fractions (0.85<x<1). The ionic conductivity of the solid solution Cu_1+xI_1?xS_x (x<0.5) has been measured as a function of temperature between 54 and 307°C. The conductivity increases from 10^?7 ω^?1 cm^?1 (CuI) to 10^?3 ω^?1 cm^?1 (Cu_1.4I_0.6 S_0.4) at 25°C and then decreases until 10^?4 ω^?1 cm^?1 (Cu₃SI). In this last domain, no further phase transition occurs below the melting point and the low temperature γ phase can be considered as a stabilization of the high-conducting α phase. The variations of the conductivity are directly connected to those of the activation energy that decreases to a minimum value of 2.5 kcal mole^?1 for Cu_1.4I_0.6S_0.4.