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1.
2.
The thermal expansion of Lu2Fe17−xSix solid solutions has been measured by X-ray powder diffraction. The magnetic ordering in all compounds within the homogeneity range (x3.4) is accompanied by a large spontaneous volume magnetostriction, distributed anisotropically over the principal axes of the hexagonal crystal structure. The volume effect ωs in the ground state reaches 14.7×10−3 in Lu2Fe17 and decreases monotonously to 8.9×10−3 for x=3.4, following the reduction of magnetic moment. Despite a still large ωs, the Invar behavior observed in Lu2Fe17 changes to a positive thermal expansion for x>1 due to an increasing Curie temperature.  相似文献   

3.
We have previously reported the magnetic properties of the Er(Co,Mn)O3 solid solution, in which Mn atoms have been partially substituted by Co. Several magnetic entities coexist in this system (Er3+, Mn3+, Mn4+, Co2+, Co3+), resulting into a subtle interplay between different interactions and leading to unusual phenomena such as: a spin reversal due to antiferromagnetic exchange between erbium and transition-metal spins, a step-like transition due to reorientation of magnetic domains, a relaxation mechanism connected to the rotation energy of the domains, the intersection of the increasing and decreasing branches in the magnetization loops, etc. These results have been obtained in highly dense ceramic samples for compositions near ErCo0.50Mn0.50O3. In order to better understand the different mechanisms, we have started a program on crystal growth. High-quality single crystals of orthorhombic structure (S.G. Pbnm) of Er(Co,Mn)O3 manganite were grown in a floating-zone oven under oxygen atmosphere. Although the zero-field-cooled/field cooled (ZFC/FC) curves confirm the spin reversal observed in bulk samples, the remarkable effects seen on hysteresis loops for bulk samples were not present in single crystals, probably due to oxygenation conditions. On the other hand, a very large anisotropy was observed between two perpendicular orientations of the single crystal.  相似文献   

4.
The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La0.7Sr0.3MnO3 and La0.5Sr0.5CoO3 has been investigated. It is found that substitution of 10% Ru at the Mn site of La0.7Sr0.3MnO3 decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔTc80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La0.5Sr0.5CoO3. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru+4/Ru+5. The eg and t2g parentage of Ru+5 is similar to Mn+4 and therefore, Ru+5 ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co+3: t2g5eg1), forcing a double exchange FM state to anti-FM state.  相似文献   

5.
The temperature-composition (Tx) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U1−xMxPd2Al3 (M=Y,Th) are described. The Tx phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y3+ and Th4+, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.  相似文献   

6.
Structure and magnetic properties of the Zr1−xMnxCo2+δ alloys were studied for 0 x <0.7, δ=0, 0.45. The cubic C15 Laves phase structure shows Mn solubility up to x≈0.4. The other Laves phase with the hexagonal C36 structure found for x0.5 apparently has a small region of Mn solubility in the vicinity of Zr0.4Mn0.6Co2. Though the parent Mn-free compounds are known to be paramagnetic, the Mn-substituted alloys show ferromagnetic behavior with the Curie temperatures up to 625 K and the room-temperature saturation magnetization of about 100 emu/g. The onset of ferromagnetism with the Mn substitution for Zr may be caused by polarization of itinerant 3d electrons, like it was earlier supposed for the off-stoichiometric ZrCo2+δ. The universal composition dependencies of the intrinsic magnetic properties for different δ can be obtained, if plotted against the amount of zirconium atoms missing in its sublattice. The room-temperature anisotropy with the noticeable anisotropy field of 24 kOe and the 1 1 0 easy magnetization direction laying in a basal plane was found in the hexagonal Zr0.5Mn0.5Co2.  相似文献   

7.
Cation deficient spinels NixMn3−x3δ/4O4+δ (0≤x≤1) have been prepared by thermal decomposition of mixed oxalates Nix/3Mn(3−x)/3(C2O4nH2O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H2, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni2+ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO1−δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni3+ and excess of Mn4+.  相似文献   

8.
Materials from the Mn(0.5−x)CaxTi2(PO4)3 (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, 31P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn2+/Ca2+ ions occupy only the M1 sites in the Ti2(PO4)3 framework. The divalent cations are ordered in one of two M1 sites, except for the Mn0.50Ti2(PO4)3 phase, where a small departure from the ideal order is observed by XRD and 31P MAS NMR. The electrochemical behaviour of Mn0.50Ti2(PO4)3 and Mn(0.5−x)CaxTi2(PO4)3 phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti2(PO4)3 framework. In the 0≤y≤2 range, the OCV curves of Li//LiyMn0.50Ti2(PO4)3 cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li(1+y)Ti2(PO4)3 and Li//LiyMn0.50Ti2(PO4)3 shows that the intercalation occurs first in the unoccupied M1 site of Mn0.50Ti2(PO4)3 at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn0.50Ti2(PO4)3 on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn0.50Ti2(PO4)3, the stability of Mn2+ either in an octahedral or tetrahedral environment facilitates cationic migration.  相似文献   

9.
The Co-sublattice anisotropy in Lu2Co17 consists of four competitive contributions from Co atoms at crystallographically different sites in the Th2Ni17-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu2Co17−xSix (x=0−3.4) grown by the Czochralski method. The SRT in Lu2Co17 was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at TSR1≈680 K and “easy-cone”–“easy-axis” at TSR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu2Co16Si (TSR1≈75 K and TSR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu2Co17−xSix compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.  相似文献   

10.
We have made high-temperature (250 K<T<800 K) DC susceptibility measurements in the compounds RuSr2Eu2−xCexCu2O10 for x=0.6,0.8, and 1.0 in order to determine the Ru effective magnetic moment. After carefully subtracting all contributions to the magnetic susceptibility except that of the Ru ions, we have been able to fit the Ru susceptibility with a law χRu0+CRu/(T−ΘRu). We have found that the Ru effective moment falls between the values expected for Ru5+ in spin states and . We have also found a dependence of μeff(Ru) and ΘRu with the Ce content x.  相似文献   

11.
The polarized Raman spectra of Nd1+xBa2−xCu3O7−δ (−0.023≤x≤0.107) and Pr1+xBa2−xCu3O7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) Ag mode softens by 6 cm−1 in Nd 1:2:3 and 4 cm−1 in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.  相似文献   

12.
Layer-type oxide NaxMx/2IITi1−x/2IVO2 (M=Co, Ni; 0.60≤x≤1.0) has been prepared by solid state reactions. In both series, two structural variants of type -NaFeO2 (O3) and β-RbScO2 (P2) have been obtained consecutively as x decreases with a borderline composition around xc0.7. With the decrease of x, the ionic conductivity has been found to increase up to 8.4×10−2 S cm−1 at 770 K (Na0.67Co0.33Ti0.67O2). Compositions of P2 have been found to exhibit the conductivity values two to five times greater than those of O3, primarily due to the larger rectangular threshold available for the diffusion of Na+ ions. Such a structural effect has also been considered to depend on the polarizability of alkali ion. HT-XRD and 23Na-NMR data of Na0.67Co0.33Ti0.67O2 strongly suggest that the diffusion of Na+ ion is deeply related with the local distortion of trigonal prismatic sites, leading to the change of activation energy around 430 K.  相似文献   

13.
The metal-insulator transition in the solid solution Bi2Sr2Ca1−xYxCu2O8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La2CuO4; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L3 edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi2O3 in agreement with the charge transfer between [CuO2] and [BiO] planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.  相似文献   

14.
FMR measurements have been carried out on several members of the Ln1−xSrxCoO3 (Ln = Rare earth) system. The results show that geff in these systems is around 1.25 independent of x as well as the rare earth ion. It is suggested that this unusual value of geff is due to the localized intermediate-spin Co3+ ions (t52g e1g) located at the top of the π* band.  相似文献   

15.
A series of substituted lead iron niobate compounds with the general formula Pb2+(1−x)AZx(Fe{(1−(2−Z)x)/2}Nb{(1+(2−Z)x)/2})O3 (0<x<0.6 and A=La3+, K+ or Sr2+) were prepared by a modified solid-state synthesis. The relative concentrations of Fe3+ and Nb5+ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, TM, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures TN1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at TN2. TN1 linearly increased with the increasing concentration of Fe3+ ion at the B-site of ABO3-type substituted compounds. TM is shown to be directly dependent on the concentration of the ferroactive Nb5+ ions at the B-site and Pb2+ ions at the A-site.  相似文献   

16.
The LaGa1−xyCoxMgyO3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa1−xyCoxMgyO3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10−6 K−1 at 300–1100 K. Doping La(Ga,Co)O3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa1−xyCoxMgyO3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.  相似文献   

17.
New Scheelite-related solid solutions of the compositions Nax/2Bi1−x/2MoxV1−xO4 (0≤x≤1) and Bi1−x/3 MoxV1−xO4(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO4 type ferroelastic BiVO4. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Nax/2Bi1−x/2MoxV1−xO4, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi1−x/3 MoxV1−xO4 where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.  相似文献   

18.
Structural, magnetic, magnetoresistance and magnetocaloric studies on La2/3Sr1/3Mn1−xCoxO3 compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.  相似文献   

19.
The magnetic measurements performed on xFe2O3(1-x)[B2O3 · PbO] glasses show that for x 5 mol.% Fe2O3 the thermal variation of reciprocal susceptibility obeys a Curie behaviour. For higher iron content, at T 50 K, a nonlinear variation, typical for systems with random distribution of exchange interactions is observed. At greater temperatures than 50 K a Curie-Weiss behaviour is shown. The composition dependence of the Curie constants is analysed in correlation with the number of Fe3+ and Fe2+ ions as determined from Mössbauer effect measurements. A comparison with the data obtained in case of xFe2O3(1-x)[3B2O3 · PbO] glasses is made.  相似文献   

20.
The crystal structure and properties of La1 − xSrxCoO3 − y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co4+ (cobalt ions with one positive effective charge, CoCo.) and oxygen vacancies (Vo..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.  相似文献   

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