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1.
Photoinduced effects in a single crystal of bilayered manganites, La2−2xSr1+2xMn2O7 (x=0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6 eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of 100 ps was observed just below TC=127 K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of 10 ps. This fast relaxing component, whose absolute value has an asymmetric peak at TC, is presumably due to short-range correlation of Jahn-Teller distortion.  相似文献   

2.
The specific heat (C) of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T2 term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).  相似文献   

3.
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   

4.
The magnetic and transport properties of a new cubic KSbO3-type ruthenate, (Ba1−xSrx)2Ru3O9 (x≈0.35), have been investigated. The crystal structure has a singular geometry in which ruthenium atoms form an ideal three-dimensional orthogonal dimer lattice. The magnetic susceptibility is Pauli-paramagnetic but exhibits an anomalous temperature dependence reminiscent of a gap-like behavior. The resistivity exhibits a metallic behavior, except for a rise at low temperature.  相似文献   

5.
Quasi-irreversible increase in the electrical conductivity is observed in single crystals of LaGa1−xMnxO3. The effect lasts for long time at room temperature and can be erased by heating of the crystal above the phase transition temperature. We explain the observed effects in terms of ionization and local lattice distortion processes.  相似文献   

6.
The structural and magnetic properties of Cr1+x(Se1−yTey)2 having a NiAs structure has been studied for (1+x)=1.27, 1.32 and 1.36 and y=0.75 by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The sub-stoichiometry and the disorder on the chalcogenide sub-lattice has been treated by means of the coherent potential approximation (CPA) alloy theory. From total energy calculations a preferential site occupation on the Cr sub-lattice was found together with an antiparallel alignment of the magnetic moments on the two inequivalent Cr layers. The magnetic properties at finite temperature has been studied by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed to determine the critical temperature in good agreement with experiment.  相似文献   

7.
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   

8.
The structure, transport and magnetic properties of (La0.8Sr0.2)1−xMnO3 (0≤x≤0.30) polycrystalline perovskite manganites have been investigated. For all the samples the Curie temperatures, Tc, remain nearly unchanged (329±3 K). Resistivity versus temperature curves for the samples show a double-peak behavior. A significant magnetoresistance (MR) effect and different temperature dependences of the MR ratios of the samples are observed. The shapes of the MR-T curves of the samples can be adjusted by changing x. For the x=0.30 sample, a nearly constant MR ratio of (9.5±0.5)% is obtained over the temperature range from 205 to 328 K.  相似文献   

9.
We have synthesized several polycrystalline samples with nominal compositions La0.7Sr0.3−xHgxMnO3+δ (0≤x≤0.2) by the standard solid-state reaction method. Instead of the sealed quartz tube method widely employed for the Hg-based systems, we adopted open atmosphere synthesis route. All the samples exhibited monophasic nature with rhombohedral structure as revealed by the X-ray diffraction data. A variation in the unit cell volume is observed with x, which is interpreted as a result of extra oxygen in-diffusion and subsequent cation vacancy creation. A broad metallic behavior is seen in the entire temperature range from 300 to 4 K. The samples showed varying amount of colossal magnetoresistance depending upon the temperature and applied magnetic field. The MR value as high as 30% was observed in x=0.2 sample and the MR is persistent over a wide temperature range with a little change in magnitude.  相似文献   

10.
The study of the structural and magnetic phase diagram of the manganites La1−xAgxMnO3 shows similarity with the La1−xSrxMnO3 series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn3+:Mn4+=2) with Tmax=300 K (maximum of the ρ(T) curve in zero field).  相似文献   

11.
The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge2Sb2Te5 have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm2/V s at room temperature).  相似文献   

12.
The structure and magnetic properties of La1−xTbxMn2Si2 (0≤x≤0.3) were studied by X-ray powder diffraction and DC magnetization measurements. All the compounds crystallize in ThCr2Si2-type structure. Substitution of Tb for La led to a linear decrease in the lattice constants and the unit-cell volume. A ferromagnetic phase for x≤0.15, and an antiferromagnetic phase for x=0.3 have been observed at about room temperature, whereas the compounds with x=0.2 and 0.25 exhibit a magnetic phase transition from ferromagnetism to antiferromagnetism.  相似文献   

13.
Metastable ferromagnetic phases, for different compositions in La2MnCo1−xNixO6, are obtained for samples synthesized by a low-temperature method and annealed in air at different temperatures in the range 200-1350 °C. The Tcs of the ferromagnetic phases vary linearly between those of the phases of the end members. Tcs of the different phases of La2MnCo1−xNixO6 can be predicted based on the Tcs and spin states of Mn, Co and Ni in the different phases of the end members, La2MnCoO6 and La2MnNiO6.  相似文献   

14.
In this paper we have studied, by means of high-resolution neutron powder diffraction, the structural and magnetic feature of pure La1.4Sr1.6Mn2O7 and Sr-doped (25%) La1.4Sr1.6Mn2O7.Our data reveal the stabilization of the A-type AFM long-range order for the La1.4Sr1.6Mn2O7 bilayered manganites induced by the partial replacement of the Sr with the smaller Ca, keeping constant the hole doping. This can be in turn due to the change in the orbital character of the eg electrons as a function of Ca-doping.  相似文献   

15.
It is found that the c- Zr1−xErxW2O8−x/2 solid solutions which are well known to have isotropic NTE properties clearly exhibit oxygen-ionic conductance and their ionic conductivities are measured to be about 10−4 S cm−1 at 673 K, which is comparable to that of ceria-based solid electrolytes.  相似文献   

16.
Polycrystalline Sn1−xMnxO2 (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.  相似文献   

17.
Temperature (4.2–260 K) and magnetic field (0–50 kOe) dependencies of the DC electrical resistance, DC magnetization, and AC magnetic susceptibility of (Sm0.65Sr0.35)MnO3 prepared from high purity components have been studied. (Sm0.65Sr0.35)MnO3 undergoes a temperature-induced transition between low-temperature ferromagnetic metallic and high-temperature paramagnetic insulating-like states. A magnetic field strongly affects this transition resulting in a metallic state and “colossal” magnetoresistance in the vicinity of the metal↔insulator transition. Magnetic and electric properties of (Sm0.65Sr0.35)MnO3 are different compared to those reported earlier for similar composition, which is attributable to the purity of the starting materials and/or different process of synthesis. The character of phase transformations observed in (Sm0.65Sr0.35)MnO3 is compared to that reported for Gd5(SixGe4−x) intermetallic alloys with a true first order phase transition.  相似文献   

18.
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.
Bi4Ti3O12 (BiT), Bi3.25La0.75Ti3O12 (BLT), Bi4−x/3Ti3−xNbxO12 (BTN) and Bi3.25−x/3La0.75Ti3−xNbxO12 (BLTN) thin films have been prepared by pulsed laser deposition. BTN and BLTN films exhibit a maximum in the remanent polarization Pr at a Nb content x=0.018. At this Nb content, the BLTN film has a Pr value (25 μC/cm2) that is much higher than that of BiT and a coercive field similar to that of BiT. The polarization of this BLTN film is fatigue-free up to 109 switching cycles. The high fatigue resistance is mainly due to the substitution of Bi3+ ions by La3+ ions at the A site and the enhanced Pr arises largely from the replacement of Ti4+ ions by Nb5+ ions at the B site. The mechanisms behind the effects of the substitution at the two sites are discussed.  相似文献   

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