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1.
A study of the half-metallic character of the semi Heusler alloys Co1−xCuxMnSb (0?x?0.9) is presented. We investigated the saturation magnetization MS at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature TC. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μB for Mn3+ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with MS (5 K)=3.78 μB/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above TC, the χ−1 vs T curves follow very well the Curie–Weiss law. The effective moment μeff and paramagnetic Curie temperature θ are derived. A comparison between the values of MS at 5 K and μeff shows a transition from localized to itinerant spin system in these compounds.  相似文献   

2.
We synthesized the Mn-doped Mg(In2−xMnx)O4 oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn3+ ions to the larger In3+ ions. The undoped MgIn2O4 oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10−3 emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO2, Mn2O3 oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.  相似文献   

3.
Doubly substituted polycrystalline compound bulk samples of BaxAgyCa2.8Co4O9 were prepared via citrate acid sol-gel method followed by spark plasma sintering. The phase composition, orientation, texture and high temperature electrical properties were systematically investigated. The results showed that the orientation and the texture could be modified by altering ratio of Ba to Ag. The resistivity and the Seebeck coefficient of substituted samples were decreased by decreasing Ba/Ag ratio except for that of Ba0.1Ag0.1Ca2.8Co4O9 sample with lowest electrical resistivity (7.2 mΩ cm at 973 K), moderately high Seebeck coefficient (172 μV/K at 973 K) and improved power factor (0.42 mW/mK2 at 973 K).  相似文献   

4.
Nanoscale crystallites of Ag-rich (Ag1−xCuxI, x=0.05, 0.10, 0.15 and 0.25), Cu-rich (Cu1-yAgyI, y=0.05, 0.10, 0.15 and 0.25) and intermediate Ag1-xCuxI (x=0.50) solid solutions and end members AgI, CuI with sizes in the range of 46-13 nm were synthesized by attrition at ambient temperature in a soft mechanochemical reaction (MCR) of Ag, Cu and I. Monophasic γ-AgI (zincblende, ) with disordered Ag+ sublattice and the crystallite size of about ∼31 nm was realized in the case of Ag0.75Cu0.25I (x=0.25) composition. Lattice parameter decreases linearly from 649 to 604 pm with increasing Cu concentration in the AgI-CuI system validating Vegard's law. Smallest size (∼13 nm) agglomerated nanocrystals were realized in the Cu-rich composition Cu0.75Ag0.25I (), while unagglomerated uniform-sized (∼17 nm) and spherical shape nanocrystallites of Ag0.50Cu0.50I () with maximum strain were synthesized for sensor applications using MCR. Differential scanning calorimetry study shows the systematic changes in the phase transition temperature with Cu substitution. Ag-rich composition posses less enthalpy (ΔH (x or Cu=0.05, 0.10, 0.15, 0.25)=6.0, 6.11, 6.6, 6.3 in kJ/mol) and entropy (ΔS (y or Ag=0.05, 0.10, 0.15, 0.25)=14.15, 14.1, 15.03, 13.6 in J/mol K) when compared to undoped AgI () implying greater thermal stability of γ-phase due to Cu-strengthened Ag-I bond. Enhanced entropy () in Cu0.75Ag0.25I (Cu-rich) solid solutions relative to CuI () indicates Ag-induced cation disorder. Fifteen percent Ag-doped CuI (Cu0.85Ag0.15I) nanocrystals apparently behave like microscopic p-n junctions with currents in the range of 10−6-10−8 A characterized by a non-linear I-V curve.  相似文献   

5.
The magnetic domain structures of Fe78.8−xCoxCu0.6Nb2.6Si9B9 (x=0, 20, 40, 60) alloys are investigated by Lorentz microscopy coupled with the focused ion beam method. The specimen prepared using the FIB method is found to have a considerably more uniform thickness compared to that prepared using the ion-milling method. In Fe38.8Co40Cu0.6Nb2.6Si9B9 and Fe18.8Co60Cu0.6Nb2.6Si9B9 alloys, 180° domain walls extending in the direction of the induced magnetic anisotropy are observed. Analysis with Lorentz microscopy reveals that the width of the magnetic domains decreases with an increase in the cobalt content or the induced magnetic anisotropy Ku, that is, the domain width d is proportional to the induced magnetic anisotropy (Ku)−1/4. On the other hand, in the in situ Lorentz microscopy observation as a function of temperature, magnetic ripple structures are found to appear in a localized area due to the fluctuation of magnetization vectors from 423 K. It is observed that the induced magnetic anisotropy caused by the applied magnetic field at 803 K is not suppressed by the magnetic ripple structures observed at 423–443 K.  相似文献   

6.
A study of electronic conductivity using the DC polarization technique has been carried out for AgI and Ag1−x Cu x I (where x=0.05, 0.15, 0.25) solid solutions over a range of temperatures from 300 K to 473 K. A diode-like current-voltage characteristics arises from microscopic p-n junctions an enhanced electronic conductivity of the order of 10−3A is observed for undoped AgI and Cu-doped AgI. Activation energies (E a) for electronic conductivity obtained from log σ−1 cm−1) vs. 1000/T (K−1) were 0.48, 0.6, 0.74 and 1.01 eV for AgI, Ag0.95Cu0.05I, Ag0.85Cu0.15I and Ag0.75Cu0.25I solid solutions respectively. The near-twofold increase in activation energy (1.01 eV) observed upon 25% Cu doping is due to the substantial concentration of current carriers/holes injected by Cu while replacing Ag+ in AgI. Based in part on the paper presented at first National Conference on Nanoscience and Technology, National Chemical Laboratory, Pune, 7–8 March 2005.  相似文献   

7.
Amorphous SiOx thin films with four different oxygen contents (x=1.15, 1.4, 1.5, and 1.7) have been prepared by thermal evaporation of SiO in vacuum and then annealed at 770 or 970 K in argon for various times ?40 min. The influence of annealing conditions and the initial film composition on photoluminescence (PL) from the annealed films has been explored. Intense room temperature PL has been observed from films with x?1.5, visible with a naked eye. It has been shown that PL spectra of most samples consists of two main bands: (i) a ‘green’ band centered at about 2.3 eV, whose position does not change with annealing conditions and (ii) an ‘orange-red’ band whose maximum moves from 2.1 to 1.7 eV with increasing annealing time and temperature and decreasing initial oxygen content. These observations have been explained assuming recombination via defect states in the SiOx matrix for the first band and emission from amorphous Si nanoparticles for the second one.  相似文献   

8.
Positron-lifetime experiments have been carried out on two undoped n-type liquid encapsulated Czochralski (LEC)-grown InP samples with different stoichiometric compositions in the temperature range 10-300 K. For temperatures below 120 K for P-rich InP and 100 K for In-rich InP, the positron average lifetime began to increase rapidly and then leveled off, which was associated with the charge state change of hydrogen indium vacancy complexes from (VInH4)+ to (VInH4)0. This phenomenon was more obvious in P-rich samples that have a higher concentration of VInH4. The transformation temperature of approximately 120 K suggests that the complex VInH4 is a donor defect and that the ionization energy is about 0.01 eV. The ionization of neutral VInH4 accounted for the decrease of the positron average lifetime when the sample was illuminated with a photon energy of 1.32 eV at 70 K. These results provide evidence for hydrogen complex defects in undoped LEC InP.  相似文献   

9.
The magnetic property of double doped manganite Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 with a fixed ratio of Mn3+:Mn4+=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr3+ for Mn3+ ions causes the increase of magnetization and the rise of Tc. As the Cr3+ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn3+ and Cr3+ ions causes a magnetic upturn, namely, the second FM phase transition.  相似文献   

10.
Oxidative (δ>0) nonstoichiometry in the perovskite ‘LaMnO3+δ’ has been known to be manifested not with O interstitials but rather with cation vacancies of equal amounts at the two cation sites, La and Mn, i.e. La1−xMn1−yO3 with x=y. Here, we report the fabrication of samples with record-high cation-vacancy concentrations (x>0.12 or δ>0.4) by means of a variety of high-pressure oxygenation techniques. Linear (negative) dependence of the cell volume on x was observed within the whole x range investigated, down to 56.9 Å3 (per formula unit) for a sample oxygenated at 5 GPa and 1100 °C using Ag2O2 as an excess oxygen source. With increasing degree of cation deficiency in La1−xMn1−xO3, the ferromagnetic transition temperature was found to follow a bell shape with respect to x exhibiting a maximum of ∼250 K about x≈0.1. For moderately oxygenated samples large magnetoresistance effect was evidenced.  相似文献   

11.
Crystal structure of Rb3D(SeO4)2 has been investigated at 25 K (below the transition temperature Tc=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO4 groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO4 groups was identified to be in the state closer to a HSeO4 ion or to a SeO42− ion and the dipole arrangement of SeO4-D-SeO4 dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.  相似文献   

12.
Magnetic and transport properties of (La0.7Pb0.3MnO3)1−xAgx composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La0.7Pb0.3MnO3)1−xAgx composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system.  相似文献   

13.
We examine the ferroelectric-relaxor behavior of (Ba0.65Sr0.35)(Zr0.35Ti0.65)O3 (BSZT) ceramics in the temperature range from 80 to 380 K. A broad dielectric maximum, which shifts to higher temperature with increasing frequency, signifies the relaxor-type behavior of these ceramics. The value of the relaxation parameter γ∼2 estimated from the linear fit of the modified Curie-Weiss law, indicates the relaxor nature of the BSZT ceramics. The dielectric relaxation rate follows the Vogel-Fulcher relation with TVF=107 K, Ea=0.121 eV, and ν0=6.83×1014 Hz, further supports such relaxor nature. The slim P-E hysteresis loop and ‘butterfly’ shape dc bias field dependence of permittivity at T>Tm (Tm, the temperature of permittivity maximum) clearly signifies the occurrence of nanopolar clusters, which is the typical characteristic of ferroelectric relaxor. At 300 K and 10 kHz, the dielectric constant and loss tan δ are ∼1100 and 0.0015, respectively. The high tunability (∼25%) and figure of merit (∼130) at room temperature show that the BSZT ceramics could be a promising candidate for tunable capacitor applications.  相似文献   

14.
Amorphous Ge1−xCrx thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10−3–0.96×102 Ω cm at 300 K, and decrease with Cr concentration. The Ge1−xCrx thin films are p-type. The hole concentrations are 5×1016–7×1021 cm−3 at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm3 at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge1−xCrx thin films are paramagnetic.  相似文献   

15.
We report the iron isotope effect on a transition temperature (Tc) in an optimally-doped (Ba,K)Fe2As2 (Tc = 38 K) and SmFeAsO1−y (Tc = 54 K) superconductors. In order to obtain the reliable isotope shift in Tc, twin samples with different iron isotope mass are synthesized in the same conditions (simultaneously) under high-pressure. We have found that (Ba,K)Fe2As2 shows an inverse iron isotope effect αFe = −0.18 ± 0.03 while SmFeAsO1−y shows a small iron isotope effect αFe = −0.02 ± 0.01, where the isotope exponent α is defined by Tc  Mα (M is the isotopic mass). The results show that αFe changes in the iron-based superconductors depending on the system. The distinct iron isotope effects imply the exotic coupling mechanism in the iron-based superconductors.  相似文献   

16.
The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf1−xTix)Fe2 (0?x?1) intermetallic compounds has been investigated by 57Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf0.4Ti0.6Fe2 compound is completely antiferromagnetic above 200 K.  相似文献   

17.
Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB2 powders. It was found that linked superconducting structure is formed down to the minimum normalized density γc=d/d0≅0.16 (percolation threshold), where d is the density of MgB2 averaged over the sample, d0=2.62 g/cm3 is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and Tc2≅32 K is minimal at γc. The grain boundaries in the porous samples are transparent for the current and Jc∼3×105 A/cm2 in self field at T=20 K in the samples with γ∼0.24.  相似文献   

18.
Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (CoxFe3−xO4) and some indication of γ-Fe3O4 when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at ∼560 and ∼400 cm−1, characteristic of the metal–oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was ∼3 nm with a geometric deviation of ∼0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a=8.388 Å for Co0.61Fe0.39O4, which is near that of CoFe2O4 (a=8.394 Å). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating χ′ and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ∼106 erg/cm3 were determined using the Debye model, whereas anisotropy constants in the order of ∼107 erg/cm3 were calculated assuming Ωτ=1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Ωτ=1 at the temperature peak of χ′ is rigorously incorrect.  相似文献   

19.
It is expected that joint existence of ferromagnetic properties and ferroelectric structural phase transition in diluted magnetic semiconductors IV-VI leads to new possibilities of these materials. Temperature of ferroelectric transition for such crystals can be tuned by the change of Sn/Ge ratio. Magnetic susceptibility, Hall effect, resistivity and thermoelectric power of Ge1−xySnxMnyTe single crystals grown by Bridgeman method (x=0.083-0.115; y=0.025-0.124) were investigated within 4.2-300 K. An existence of FM ordering at TC∼50 K probably due to indirect exchange interaction between Mn ions via degenerated hole gas was revealed. A divergence of magnetic moment temperature dependences at T?TC in field-cooled and zero-field-cooled regimes is obliged to magnetic clusters which are responsible for superparamagnetism at T>TCTf (freezing temperature) and become ferromagnetic at TC arranging spin glass state at T<TfTC. Phase transition of ferroelectric type at T≈46 K was revealed. Anomalous Hall effect which allows to determine magnetic moment was observed.  相似文献   

20.
The oxygen hyperstoichiometry of K2NiF4-type La2Ni0.9Fe0.1O4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10−5-0.7 atm at 923-1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O2)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe3+ and holes localized on nickel cations. Due to low chemical expansion of La2Ni0.9Fe0.1O4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La2NiO4-based phase in reducing atmospheres is essentially unaffected by doping.  相似文献   

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