Annealing Effects in (Hg,Cr)Sr2CuO4+δ: Transport and X-Ray Absorption Studies

Superconducting (Hg_1–x Cr _x )Sr₂CuO₄₊, x 0.36, samples of 1201-type, synthesized in partial vacuum, show T _c ^onset of 58 K, T _c (R = 0) 52 K. It is found that T _c is not affected by the subsequent Ar or O₂ annealing treatment. Remarkably, the annealed samples show significant improvement in the diamagnetic signal. Hg L₃-edge measurements on 1201 samples show divalent state of mercury. The Cu K-edge spectra for the samples, after taking due account of the impurity phases, show noticeable modifications in the split main peak features on Ar or O₂ annealing, suggesting changes in the crystal field asymmetry. In the superconducting (SC) samples, a weak but distinct signature of Cu¹⁺ is seen. The Cu¹⁺ feature is absent in all the non-SC 1201 samples as well as in the impurity phase SrCuO₂ and Sr₂CuO₃ samples. An attempt is made to explain the observed Cu¹⁺ feature in SC (Hg,Cr)-1201 in the light of the available reports.     

Magnetization Study of (Hg,Cr)Sr2CuO4 + δ Superconductor

(Hg,Cr)-Sr-1201 type superconductor of nominal composition Hg_0.7Cr_0.3Sr₂CuO_{4 +} was synthesized by the sealed quartz-tube method. X-ray diffraction analysis of the sample showed it has tetragonal symmetry with lattice parameters a = 3.841(1) Å, and c = 8.664(3) Å. The as-prepared sample is superconducting at 60 K as shown by dc-magnetization and dc-resistivity measurements. The irreversibility line (IL) obtained from the merging point of the zero-field-cooled and the field-cooled magnetization measurements at several applied fields was found to obey the equation H = A(1 – T/T _c) ⁿ and gave a value of 2.55 for the exponent n. Using magnetic hysteresis loops the critical current density of superconducting (Hg,Cr)-Sr-1201 was estimated. The irreversibility line and magnetic hysteresis showed reduced pinning properties of (Hg,Cr)-Sr-1201 as compared to HgBa₂CuO_{4 +} .     

Magnetization and Re LIII-Edge Studies of (Hg,Re)Sr2CuO4 + δ System

Chemically stable nominal compositions of (Hg_1-xRe_x)Sr₂CuO_{4 +} (x = 0.05, 0.10, and 0.20), designated as (Hg,Re)-1201 phase, were synthesized at 920°C in partial vacuum. The samples with x = 0.05 and 0.10 are superconducting (SC) with T_c^onset 40 and 54 K, respectively. On the other hand, for the composition with x = 0.20, the sample is nonsuperconducting down to 5 K. The sample with composition (Hg_0.9Re_0.1)Sr₂CuO_{4 +} on cooling, below 10 K in dc-field of 4 kOe, shows a dramatic decrease (reversal) in diamagnetic signal. Such a reduction in diamagnetic response is also reflected by the sample's supercurrent induced magnetic hysteresis curve generated at 9 K. In the light of the available reports and our Re L_III-edge measurements, we attribute this effect to a field enhanced paramagnetic contribution probably arising mainly from mixed valent (6⁺, 7⁺ dominant) Re ions. The critical current density (J_c), calculated using Bean's model, for (Hg_0.9Re_0.1)Sr₂CuO_{4 +} is 6 × 10³ A/cm² (4.5 K, 5 kOe).     

Susceptibility and X-Ray Absorption Measurements on Superconducting (Hg,Mo)Sr2CuO4+δ System

In continuation of our recent work [1] on superconducting 1201-type (Hg,M)Sr₂CuO₄₊, M = Cr system, the samples of (Hg_1–x Mx)Sr₂CuO₄₊, M = Mo (x = 0.15, 0.25, and 0.30), were synthesised in partial vacuum. The x = 0.15 composition shows a maximum T _c ^onset of 66 K, while x = 0.25 exhibits T _c ^onset of 50 K. For x = 0.30 sample, the observed T _c ^onset of 36 K is discretely different from the reported values of 20 and 58 K. X-ray absorption measurements on (Hg,Mo)-1201 samples show Hg in divalent state and Cu predominantly in Cu²⁺ state. A weak but distinct Cu¹⁺ signature is also observed in the SC (Hg,Mo)-1201 samples, which is absent in the impurity SrCuO₂ and Sr₂CuO₃ phases. On the basis of available reports, it is viewed that Cu¹⁺ can arise because of a small fraction of Cu occupying the Hg site in (Hg,Mo)-1201 system.     

Electrical Properties and Ferromagnetism in (Ga,Cr)As

MBE-grown (Ga,Cr)As has interesting electric and magnetic properties. Ga_1–x Cr _x As with x = 0.1 exhibits short-range ferromagnetic behavior at low temperatures. This is manifest in several anomalous properties: magnetization does not scale with B/T; fitting M(B) requires a model of distributed magnetic cluster or polarons; and inverse susceptibility is nonliner in T (non-Curie–Weiss) at low fields. At room temperature, the conductivity is activated and Hall measurements yield a hole concentration of 10²⁰ cm^–3, indicating that chromium acts as an acceptor similar to Mn in GaAs. For decreasing temperature, the conductivity decreases by eight orders of magnitude and follows exp(1/T ^1/2).     

Experimental Investigation of Ferromagnetism in II–VI Disordered Semiconducting Compounds

Low temperature magnetic properties of p-type bulk (Zn,Mn)Te and n-type (Zn,Mn)O films are presented and discussed in a view of a presence of carrier-induced ferromagnetism. We argue that in insulating p-type systems only a small fraction of Mn moments are coupled in a form of ferromagnetic bubbles. We show that collectively they behave typically for superparamagnets. The metallic n-type layers exhibit a much weaker effect, yet a clear tendency towards ferromagnetism has been observed.     

Sb2O3的Rietveld法定量相分析

用Rietveld法对含有立方和正交两种晶型的Sb2O3样品进行了定量相分析，该样品含立方晶型和正交晶型的Sb2O3分别为(质量分数)97．92％和2．08％。结果表明，用Rietveld法进行的定量相分析是非常有效和精确的。     

Ferromagnetism in (Zn,Cr)Se Layers Grown by Molecular Beam Epitaxy

Magnetization measurements of MBE grown epilayers of (Zn,Cr)Se with relatively large Cr content of 0.014 and 0.021 are presented. We evidence the presence of a strong ferromagnetic coupling between Cr ions, but also suggest a significant clustering due to a pronounced superparamagnetic behavior found in the layers. We estimate the intraparticle Curie temperature to be about 100 K, which combined with other arguments appears to indicate that some magnetic properties of Cr-rich layers might be dominated by the presence of small grains of A^II–Cr₂–B₄ ^VI spinels.     

开放体系下碳热还原氮化法制备(Cr,Ta)_2CN固溶体粉末的研究

以Cr2O3、Ta2O5和纳米碳黑为原料,在开放体系的流动N2气氛条件下,采用碳热还原氮化法制备出了(Cr,Ta)2CN固溶体粉末,利用XRD、SEM、EDS等分析测试手段对制备过程中的物相演变规律和微观形貌变化进行了研究。研究结果表明,碳热还原氮化法制备(Cr,Ta)2CN固溶体粉末过程中物相演变顺序为Cr2O3、Ta2O5、C→Cr2O3、Cr3C2、Cr7C3、Cr Ta O4、C→Cr2O3、Cr7C3、Cr Ta O4→(Cr,Ta)2CN。在N2流量为500 m L/min、烧结温度为1 500℃保温2 h的条件下,可制备出粒度约5μm、游离碳和氧含量分别为0.16%,0.085%的单相(Cr,Ta)2CN固溶体粉末。