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1.
Crystal structure and magnetic properties of magnetostrictive compounds Tb0.36Dy0.64(Fe0.85Co0.15)2−xBx (0?x?0.15) have been investigated at room temperature. The matrix of these compounds keeps a cubic MgCu2-type structure. Lattice parameter a of the Laves phase decreases to reach a minimum at x=0.10, then increases with increasing boron content. Through analyzing the Mössbauer spectra, the easy magnetization direction (EMD) for all samples is confirmed to lie along 〈111〉 direction at room temperature, suggesting the presence of the giant magnetostriction. The mean hyperfine field Hhf and the deduced iron moment μFe increase with increasing boron content, resulting in the enhancement of both Curie temperature TC and spin reorientation temperature Tr. Although the addition of B enlarges the magnetocrystalline anisotropy constant K1, the composition dependence of the ratio λ/K1 for Tb0.36Dy0.64(Fe0.85Co0.15)2−xBx, however, reaches a maximum value at x=0. 05 under high magnetic fields.  相似文献   

2.
The effect of Ru substitution for Mn in bilayered oxides La1.2Sr1.8(Mn1−yRuy)2O7 (0?y?0.20?y?0.2) was investigated by magnetization measurements and low-temperature Lorentz transmission electron microscopy. It was found that the magnetic anisotropy is controlled by the Ru content y and temperature T. The easy axis changes from 〈1 1 0〉 for the y=0y=0 crystal to the c  -axis for y=0.2y=0.2, and it rotates away from the c-  axis for the y=0.05y=0.05 and y=0.07y=0.07 crystals with decreasing temperature. Furthermore, maze-shaped magnetic domain structures were observed in the (0 0 1) thin crystals with 0.05?y?0.20.05?y?0.2. Changes in domain size and structure indicate that the uniaxial magnetic anisotropy becomes stronger as Ru content y increases.  相似文献   

3.
The magnetic, transport, and optical properties of electron-doped Ca1−xLaxMnO3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0x=0 and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10x=0.10 and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10x=0.10 and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.  相似文献   

4.
A series of the double-doping samples La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3(0?x?0.2)(0?x?0.2) with the Mn3+/Mn4+ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20x=0.20. But the Curie temperature TCTC and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.120.07?x?0.12. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La2/3Sr1/3Mn1−xCuxO3 at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.  相似文献   

5.
A series of the double-doping samples La(2+4x)/3Sr(1−4x)/3Mn1–xCuxO3(0?x?0.2)(0?x?0.2)with the Mn3+/Mn4+ ratio fixed at 2:1 and the single-doping samples La2/3Sr1/3Mn1–xCuxO3(0?x?0.2)(0?x?0.2) have been investigated. For the double-doping samples, though the ratio Mn3+/Mn4+=2:1 has been generally recognized the optimum ratio, the Curie temperature TCTC and metallic–insulator transition temperature Tp1Tp1 are more rapidly decreased by Cu substitution than that corresponding to single-doping samples. And the resistivity ρρ value for the double doping is larger about two or three orders of magnitude than that corresponding to single doping. At the same time, two resistivity peaks and two magnetoresistance (MR) peaks appear. We suggest that for the double-doping samples the A-site cation size 〈rA〉 and the A-site mismatch factor σ2σ2 decreases with increasing doping level, which leads to the system microstructural distortion. This microstructural distortion makes the Mn3+–O–Mn4+ cut off more cluster-spin except for the clusters induced by Cu. These cluster interfaces contribute to ρρ, which exceeds far the contribution of eg electron decreasing with doping increasing in the single doping. At the same time, such interface scattering also gives rise to the appearance of second peak for the double-doping samples. The experimental results shows that double doping could be also a potential way in tuning colossal MR (CMR), which can give a guide for the adequate selection of CMR materials.  相似文献   

6.
Polycrystalline samples with nominal composition of La0.7Ca0.3Mn1−xVxO3 (0?x?0.2) sintered in air were investigated by ac susceptibility, dc magnetization, magnetoresistance (MR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential thermal analysis (DTA) measurements. It is found that V could not substitute for Mn to form La0.7Ca0.3Mn1−xVxO3 phase when the samples are sintered in air. The obtained samples contain several phases such as (La, Ca)Mn1−δO3, LaCa3V3O12, Mn3O4 phases, etc. and constitute multi-phase composites. The Curie temperature TC and spontaneous magnetization of the composites decrease, and the resistance of the composites increases as the V content increases. The addition of V may effectively improve the low-field MR response at low temperatures due to the variation in the microstructure of the composites.  相似文献   

7.
Planar CdBxF2−xp-CdF2–CdBxF2−x sandwich nanostructures prepared on the surface of the n-type CdF2 bulk crystal are studied to register the spin transistor and quantum spin Hall-effects. The current–voltage characteristics of the ultra-shallow p+n junctions verify the CdF2 gap, 7.8 eV, and the quantum subbands of the 2D holes in the p-type CdF2 quantum well confined by the CdBxF2−xδ-barriers. The temperature and magnetic field dependencies of the resistance, specific heat and magnetic susceptibility demonstrate the high temperature superconductor properties for the CdBxF2−xδ-barriers. The value of the superconductor energy gap, 2Δ = 102.06 meV, determined by the tunneling spectroscopy method appears to be in a good agreement with the relationship between the zero-resistance supercurrent in superconductor state and the conductance in normal state, πΔ/e, at the energies of the 2D hole subbands. The results obtained are evidence of the important role of the multiple Andreev reflections in the creation of the high spin polarization of the 2D holes in the edged channels of the sandwich device. The high spin hole polarization in the edged channels is shown to identify the mechanism of the spin transistor and quantum spin Hall-effects induced by varying the top gate voltage, which is revealed by the first observation of the Hall quantum conductance staircase.  相似文献   

8.
We present a systematic investigation on the structural and magnetic properties of Zn1−xCoxO nanoparticles synthesized by an auto-combustion method. The single-phase Zn1−xCoxO crystallize in the wurtzite-type structure with a homogeneity range as large as x≈0.30, which enables the observation of some anomalies. The lattice parameter a and the unit cell volume V increase with the Co content, and anomalies are discernable around x=0.15 on the ax and Vx curves. The magnetization data show no evidence of ferromagnetic (FM) ordering in our samples down to T=5 K, and the magnetization at 5 K and 5 T exhibits a maximum around x=0.125. Based on the detailed analysis of the magnetization data and the donor impurity band exchange model, the anomalies on composition dependence of both the lattice parameters and magnetization can be associated with an occurrence of cation percolation around the threshold xp (≈1.5/Z=0.125 for three-dimensional lattice with coordination number Z=12). Within the framework of the donor impurity band exchange model, the absence of FM in the well-characterized Zn1−xCoxO can be attributed to insufficient donor electron concentration.  相似文献   

9.
A series of Mn-site Co-doping samples La2/3Ca1/3Mn1−xCoxO3 (0?x?0.15) have been prepared. The structure, magnetic and transport properties of this system have been systematically investigated. All the samples showed good single phase, and the lattice parameters decreased with the increase of doping concentration x. Only one paramagnetic–ferromagnetic transition was observed. The Curie temperature TC decreases gradually and the transition width becomes wider with the increase of x. The abnormal transport properties were induced by Co doping, characterized by the double metal–insulator transitions and low-temperature minimum behavior. The present results are discussed and possible explanations were given based on the related theory and previous reported results.  相似文献   

10.
We report here on the fluorination of the perovskite-related phases La1−xSrxFe1−yCoyO3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe3+ ions, which are not possible in oxides containing Fe4+.  相似文献   

11.
Perovskites BaTi1−xFexO3 has been synthesized with the concentration x ranging from 0.01 to 0.02. Their transformation point of ferroelectric to paraelectric and the corresponding latent heat of the phase transformation were observed to decrease with increasing the doping level of Fe3+. Bonded layered composites BaTi1−xFexO3–Tb1−yDyyFe2−z have been fabricated and their magnetoelectric effect has been investigated. The sample containing a layer of perovskite BaTi0.985Fe0.015O3 was found to show the maximum transverse ME voltage coefficient, which is about 1422 mV Oe−1 cm−1 under a magnetic field of 1580 Oe, in these bilayers. Analysis shows that the Fe-doped BaTiO3 with doping level at about 1.5% should have largest piezoelectric coefficient in these ceramics BaTi1−xFexO3.  相似文献   

12.
Nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm−1. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO3. The results can be well attributed to the Mn4+/Mn3+ ratio and to the particle grain size.  相似文献   

13.
In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La2−xSrxCu1−yZnyO4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.  相似文献   

14.
We have investigated the Hall measurement of Gd(Ba2−xNdx)Cu3O7+δ with x=0, 0.05, 0.1, 0.15, and 0.2 in a magnetic field of 0–1 T. The nominal samples has been prepared by the conventional solid-state reaction technique. The iodometric titration experiment was carried out for samples. The Rietveld analysis of the X-ray diffraction patterns indicates that samples are mainly single phase. The normal state Hall coefficient behaves as 1/T in all samples. All samples with nominal compositions show single sign reversal with variation of magnetic field and temperature. The Hall resistivity of the samples with x=0 and 0.1 close to the superconducting transition temperature changes its sign with decreasing temperature and tends to a minimum −Δmax, and then monotonically goes to zero. The absolute value of −Δmax decreases with the increase of magnetic field. This can be qualitatively explained by a model calculation based on the time-dependent Ginsburg–Landau theory.  相似文献   

15.
Hall effect measurements were performed on epitaxial CoxTi1−xO2–δ thin films grown on (0 0 1) LaAlO3 by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in MH loop at room temperature for CoxTi1−xO2–δ thin films for which x?0.02. An anomalous Hall effect was observed for Co0.10Ti0.90O2−δ films grown with the partial pressure of water P(H2O)=4×10−4 Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for TinO2n−1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO2 thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped TinO2n−1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.  相似文献   

16.
The TbxHo0.75−xPr0.25(Fe0.9B0.1)2 (x=0, 0.1, 0.15, 0.2, 0.25, and 0.3) compounds are found to stabilize in a cubic Laves phase structure. The lattice parameter, magnetostriction (at 10 kOe), and Curie temperature are found to increase with increasing Tb content. The compound with x=0.15 exhibits a possible anisotropy compensation between the Tb and (Ho/Pr) sublattices. The easy magnetization direction rotates towards the 〈1 1 1〉 from the 〈1 0 0〉 direction, with increasing Tb content. The splitting of the (4 4 0) peak accompanied by the spontaneous magnetostriction-induced rhombohedral distortion is observed for compounds with x?0.15 and the spontaneous magnetostriction (λ1 1 1) is found to increase with Tb content.  相似文献   

17.
Electrical conductance and X-ray diffraction (XRD) measurements of lanthanum-deficiency La1−xxMnO3 (x=0.05, 0.10 and 0.20) polycrystalline samples were performed to examine the effect of the internal pressure at B-site on the conduction mechanism. The structural study reveals that all samples crystallize in the rhombohedral system. The electronic conduction appears to be thermally activated at high temperature, which indicates the presence of semiconductor behaviour. The increase of the x content converts 3x Mn3+ to 3x Mn4+ ions with smaller ionic radius, which reduces the internal pressure and leads to the increase of the one-electron bandwidth W. This increase causes the appearance of metallic behaviour at low temperature for x=0.10 and 0.20 content.  相似文献   

18.
La1−xCaxMnO3+δ (0.0?x?1.0) samples were prepared and their resistivity and Seebeck coefficients were measured in the high-temperature range. Ca doping changes the ratio of Mn3+/Mn4+ and influences the electronic transport behavior markedly. With the increase of Ca concentration, the samples change from a p-type semiconductor to an n-type one and Seebeck coefficient becomes increasingly negative. Low doping (x=0.2) and high doping (x=0.8) induces the drop of the resistivity compared with undoped LaMnO3+δ and CaMnO3+δ samples due to the rise of carrier concentration. However, the resistivity of moderate-doped samples (x=0.4, 0.6) is larger than low- and high-doped samples because dopant scattering decreases carrier mobility.  相似文献   

19.
We report a successful fabrication of high-Jc GdBa2Cu3O7−δ (GdBCO) films by the metal–organic deposition process on the LaAlO3 (LAO) (0 0 1) substrates. The coating solution was prepared by mixing Gd, Cu fluorine-free sources with Ba trifluoroacetate. Samples were dip-coated, pyrolized within 3 h at the temperature up to 400 °C in a humid oxygen atmosphere, and finally fired at various high temperatures in 100 ppm Ar/O2 atmosphere. The GdBCO films fired at 775 and 800 °C exhibited Jc values of ∼2.1 MA/cm2 at 77 K in a self-field, which are attributable to both high Tc,zero values of ∼89 K and high in-plane textures of 1.3–1.4°. Above 800 °C, however, the superconducting properties of GdBCO films are degraded due to the thermal decomposition of GdBCO film in 100 ppm Ar/O2 atmosphere.  相似文献   

20.
Systematic valence changes in Pr1−xSrxMn1−yInyO3−δ upon cation substitution with Sr2+ and In3+ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn LII,III and Pr MIV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr0.5Sr0.5InO3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.  相似文献   

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