共查询到20条相似文献,搜索用时 15 毫秒
1.
T.S. TripathiG.C. Tewari A.K. Rastogi C.S. YadavP.L. Paulose 《Journal of magnetism and magnetic materials》2011,323(17):2300-2304
We present a detail study of the effect of excess metal atoms on the magnetic properties of Cu1+xCr2+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 χac 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 2.9μB across all the compositions and it reduces with temperature as M(T)∼A0T3/2+A1T5/2, as expected for the excitation of Bloch's spin waves in a model of the Heisenberg ferromagnet. Our terminal composition Cu1.9Cr2.25Te4 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. 相似文献
2.
1T-V1+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 (Tt is 474 K for V1.04Te2, 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 Tt. χ exhibits a jump at Tt, showing the paramagnetic temperature dependence both below and above Tt. ? measurements show the metallic-like behavior with a slight decrease at Tt. Preliminary electron diffraction examination suggests the formation of a super-structure below Tt. 相似文献
3.
Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr2?xInxSe4(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 temperatureTc (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near Tc. 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 Tc 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. 相似文献
4.
N.E. Rajeevan Ravi KumarD.K. Shukla R.J. ChoudharyP. Thakur A.K. SinghS. Patnaik S.K. AroraI.V. Shvets P.P. Pradyumnan 《Journal of magnetism and magnetic materials》2011,323(13):1760-1765
Thin films of BixCo2−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 (TC∼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-TC. 相似文献
5.
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 Cu0.5Co0.5Cr2?xRhxS4 (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, Eg, and fi, the ionic energy, total energy and ionici parameters respectively. This fact has been used to determine the various bond parameters in the spinels Cu0.5Co0.5Cr2?xRhxS4. 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. 相似文献
6.
Electrical conductivity and Seebeck coefficient for the Bi2−xYxRu2O7 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 Bi2−xYxRu2O7 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 t2g and O2p orbitals, but the mixing of Bi6s, 6p states at EF may not be ruled out. The thermoelectric power factors for the Bi2−xYxRu2O7 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. 相似文献
7.
Osamu Okada Tomonao Miyadai Katsuya Kikuchi 《Journal of magnetism and magnetic materials》1981,22(3):306-310
Precise magnetization measurements and neutron diffraction measurements are carried out for Mg1-pMnpTe2 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 Tf and field-cooling effect is notable below Tf. 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. 相似文献
8.
Polycrystalline La2−xPrxCa2xBa2Cu4+2xOz (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 (Tconset values) with increasing dopant concentration. The mixing of rare earth La3+ and Pr3+/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. 相似文献
9.
W. Kurtz 《Solid State Communications》1982,42(12):875-878
Magnetic data for Rb2Cr1?xCdxCl4 and Rb2Cr1?xMnxCl4 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 Rb2Cr1?xMnxCl4. 相似文献
10.
M. Gogołwicz J. Warczewski T. Mydlarz I. Okońska-Kozłowska 《Journal of magnetism and magnetic materials》1985,50(1):49-52
For the polycrystalline samples of Mn1?xCuxCr2S4 (x = 0.85, 0.90, 0.95) the magnetization was measured in the temperature range between 77 K and the Curie temperature, TC, using a magnetic balance (Faraday's method) and pulsed magnetic fields up to 2.0 T. The magnetic susceptibility was measured between TC 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+xS2?4. 相似文献
11.
The band structure and optical properties of the CdSexTe1−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. 相似文献
12.
The ferroelectric compounds Pb2Na1−xLaxNb5−xFexO15 and Pb0.5(5−x)LaxNb5−xFexO15 (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 Pb2Na1−xLaxNb5−xFexO15, a clear anomaly occurs in the vicinity of x=0.4. 相似文献
13.
The roles of aliovalent CaII-for-YIII substitution and high-pressure-oxygen annealing in the process of ‘superconducterizing’ the Co-based layered copper oxide, CoSr2(Y1−xCax)Cu2O7+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 Tc was obtained for the high-oxygen-pressure treated x=0.3 sample at ∼40 K. 相似文献
14.
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 Fe1?xCuxCr2S4 (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+0.28S2?4 is obtained for x = 0.02. The existence of Cr2+ is concluded. 相似文献
15.
Series of CoxCr1−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. 相似文献
16.
GexSi1−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 GexSi1−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 GexSi1−x alloy NCs including Lamb’s theory, complex-frequency (CF) model, core–shell matrix (CMS) model and spatial coherence effect model. 相似文献
17.
Magnetic moments, magnetic susceptibilities and optical absorption of the system HgxZn1-xCr2Se4 have been measured. A correlation is found between the optical and the magnetic properties. 相似文献
18.
Tomonao Miyadai 《Journal of magnetism and magnetic materials》1979,10(1):29-33
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, Tc remains quite constant. Above Tc, 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 Tc is about 3 μB/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve. 相似文献
19.
SrBi2−xPrxNb2O9 (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 2Ec reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2. 相似文献
20.
A structural study of the CuICu2S system has shown that a large solid solution ranges from pure CuI to Cu3SI with a regular decrease of tehcell parameter (); 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 Cu1+xI1?xSx (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 (Cu1.4I0.6 S0.4) 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 Cu1.4I0.6S0.4. 相似文献