Effect of Postannealing Process on Bi2Sr2.1Ca0.9Cu2O8+δ Textured Superconductors

Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using the Laser Floating Zone (LFZ) method. They have shown good grain alignment and transport critical current densities (J _c ). After postannealing processes designed to produce the Bi-2212 phase controlled decomposition, J _c values have been increased in an important manner. Maximum values have been achieved when samples were thermally treated at 680 °C for 168 h with improvements around 80 %, compared with the original textured samples. The results clearly indicate that postannealing processes, when adequately controlled, produce the formation of effective pinning centers which are responsible for the increase in the measured J _c values.     

Structural, Transport, and Magnetic Properties of (Ru0.9Nb0.1)Sr2(Gd0.67R0.67Ce0.66)Cu2O z (R = Nd, Gd, and Tb)

Samples with nominal compositions of (Ru_0.9Nb_0.1)Sr₂(Gd_0.67R_0.67Ce_0.66)Cu₂O _z (R = Nd, Gd, Tb) were prepared, and the influence of doped rare-earth element on the structural, magnetic and superconducting properties of these samples has been investigated. Resistivity and room-temperature thermoelectric power measurements showed that the superconductivity is mainly affected by the change in the hole concentration induced by doping of the rare-earth element. All of these samples exhibited weak ferromagnetic behavior at temperatures below approximately 90 K with the branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization. In the ZFC magnetization measurements, diamagnetic transition was observed at temperatures below approximately 15 K for the R=Gd and Nd samples. Magnetization measurements also revealed that the partial substitution of Tb for Gd results in a significant enhancement of weak-ferromagnetic component of the FC magnetization, as well as an increase in the magnetic ordering temperature up to 20 K. A quite opposite behavior was observed in the FC magnetization for the case of the R=Nd sample. The experimental results are discussed in conjunction with the local structural changes in the Ru sublattice, which are induced by doping of the rare-earth element, based on the results of Rietveld refinement of the X-ray diffraction data.     

Structural and Magnetic Properties of YBa2Ru0.85Cu0.15O6 and Sr2Re0.69Ca0.31CuO6 Perovskites

In this present investigation, both YBa₂Ru_0.85Cu_0.15O₆ and Sr₂Re_0.69Ca_0.31CuO₆ perovskite compounds were prepared through high pressure/high temperature (HPHT) synthetic route. The as-prepared samples were structurally characterized by powder X-ray diffraction technique and their magnetic properties were measured. Both compounds crystallized in cubic symmetry with a different space group. The refined lattice parameter for the YBa₂Ru_0.85Cu_0.15O₆ and Sr₂Re_0.69Ca_0.31CuO₆ compounds are found to be a=8.332(2) ? with space group, Fm-3m (225) and a=7.967(4) ? space group Pm-3m (221), respectively. The cuprate ordered perovskite compound shows ferromagnetism with high Curie temperature, T _c at ∼450 K whereas the YBa₂Ru_0.85Cu_0.15O₆ compound reveals mixed magnetic natures below ∼170 K which is complicated by the presence of magnetic order in the Ru and Cu sublattices. Below 170 K, the compound shows two antiferromagnetic-like transitions (in ZFC mode) at ∼135 K and ∼60 K. With a further decrease of temperature (below 60 K), the susceptibility crosses negative (diamagnetic) signal at ∼35 K and shows maximum negative susceptibility at ∼11 K. By decreasing the temperature below 11 K, the negative susceptibility decreases and shows positive magnetic susceptibility at 2 K. In the fc mode curve, the sample shows broad antiferromagnetic like transition at ∼55 K. At low temperature (below 30 K), an increase in susceptibility signal is seen in the fc magnetic susceptibility curve. The magnetic parameters, such as Weiss temperature, θ _w and the effective paramagnetic moment, p_eff obtained from the linear region of χ ⁻¹(T) plots are found to be −307.25 K and 3.74 μ _B/Ru, respectively, for the YBa₂Ru_0.85Cu_0.15O₆ compound. The negative sign of θ _w reveals the antiferromagnetic correlations of the compound. While these studied compounds are already known to the literature, the synthesis method (HPHT) employed for the present YBa₂Ru_0.85Cu_0.15O₆ compound is a new approach and quite different from the routine conventional solid state synthesis approach.     

Magnetic Ordering and Exchange Interaction of Laves Compounds Sm0.9Pr0.1（Fe1—xCox）2

Structure,magnetic properties and magnetostriction of Sm0.9Pr0.1(Fe1-xCox)2 compounds have been investi-gated by means of X-ray diffraction,a.c. initial susceptibility, extracting sample magnetometer,Mossbauer spec-troscopy and standard strain gauge techniques.The lattice parameter a of the MgCu2-type Laves compounds Sm0.9Pr0.1(Fe1-xCox)2 decreases nonlinearly with increasing Co concentration,deviating from the Vegardˊs law.Curie temperature Tc increases initially from 668 K for x=0 to 694 K for x=0.2 and then decreases to 200 K for x=1.0.The saturation magnetization Ms at temperatures 1.5K, 77K and 300K have the same variation tendency as the composition dependence of Curie temperature,in consistence with rigid-band model.The easy magnetization direction(EMD) od Sm0.9Pr0.1(Fe1-xCox)2 lies along [111] direction in the range x≤0.6,and changes to [110] for x=0.8 ,while Sm0.9Pr0.1(Fe1-xCox)2 stays in the paramagnetic state at room temperature.The composition dependence of the average hyperfine field,Hhf,demonstrates a similar variation tendency as that of the saturation magnetiza-tion Ms and Curie temperature Tc .The spontaneous magnetostricton λ111 increases with increasing Co content.The saturation magnetostriction λs decreases monotonically with increasing x,which is caused by the increase of magnetostriction constant λ100 with opposite sign to that of λ111.A two-sublattice model has been proposed to understand the intermediate region between the [111]and [110] spin configurations ,which can also be used to explain the temperature dependence of magnetization.     

Li2O助烧的Gd0.1Ce0.9O1.95烧结过程及其电导性能研究

氧化铈基电解质是中低温固体氧化物燃料电池最常用的材料之一。本研究利用密度泛函理论计算方法阐述了Li₂O和CeO₂分子之间的相互作用, 计算结果表明Li₂O对CeO₂具有助烧作用。在此基础上, 在氧化铈基电解质Gd_0.1Ce_0.9O_1.95(GDC)中掺入不同比例(0~5mol%)Li₂O, 通过烧结曲线测试及扫描电镜分析了其实际烧结过程, 并对其电化学性能进行了研究。实验结果表明, 添加Li₂O后, GDC的烧结开始收缩温度明显向低温偏移, 随掺杂量的增加, 最大收缩速率的温度也逐渐降低, 其中掺入2.5mol%Li₂O-GDC在650 ℃就开始迅速收缩, 900 ℃时相对致密度在99%以上; 添加Li₂O后, GDC总电导率提高, 同时电池开路电压没有降低。因此, Li₂O是一种很好的燃料电池氧化铈基电解质的助烧剂, 具有很好的应用前景。     

The Effect of Sintering Temperature in Bi1.7Pb0.2Sb0.1Sr2Ca2Cu3O y Superconductors

In this study, the effects of sintering temperature on the Bi-2223 phase formation and the influence of minor phases on the intergranular properties of Sb substituted Bi-2223 samples were investigated. The samples were prepared by solid-state reaction method with different sintering temperatures ranging from 800 to 855 °C. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and density measurements. We observed the optimal temperature of the Sb + Pb substituted Bi-2223 system as 845 °C.     

Formation of high-T c Bi1.6Pb0.4Sr2Ca2(Cu0.9Ti0.1)3O x

Influences of sintering conditions onT _c are studied for Bi_1.6Pb_0.4Sr₂Ca₂(Cu_0.9Ti_0.1)₃O _x . HighT _c values are obtained at 1123±5 K sintering temperature and over 50 h sintering time. In order to explain theT _c changes, morphological discussion is attempted.     

Low-Temperature Neutron Diffraction and Magnetic Properties of La1.2Sr0.9Ca0.9Mn2O7

The crystal structure and magnetic properties of a typical polycrystalline double-layered manganite, La_1.2Sr_0.9Ca_0.9Mn₂O₇, were examined using neutron diffraction and magnetization techniques. Neutron diffraction was performed at low and room temperatures and the crystal structure was refined using the Rietveld method based on the space group I4/Mmm. Low temperature neutron diffraction and magnetization measurements clearly revealed antiferromagnetic ordering. A neel transition was observed at approximately 150 K.     

PZT/Y0.9Ca0.1Ba2Cu4O8及Ag2O掺杂的复合氧化物陶瓷的NTC效应

报道了压电材料ＰＺＴ中掺杂超发Ｙ０．９Ｃａ０．１Ｂａ２Ｃｕ４Ｏ８,而超导粉中又能Ａｇ２Ｏ的掺杂,这种复合氧化物陶瓷的制备方法及电导率的测试结果,表明材料具有很强的ＮＴＣ效应。     

Effects of Substituting Ni and Zn for Cu in the Electron Doped Sr0.9La0.1CuO2 Superconductor

We used X-ray diffraction, energy dispersive spectroscopy and low field magnetization to characterize Sr_0.9La_0.1Cu_1–x R _x O₂ samples for R=Zn and Ni. We found that both Ni and Zn are substituted at the Cu-site. The value of T _c was almost independent of composition until x0.03 for R=Zn while it was reduced to 30 K at x0.02 for R=Ni. This trend of T _c reduction is very similar to the pair breaking-effect in conventional superconductors and electron-doped (Nd, Ce)₂CuO_4– but the rate of reduction of T _c for R=Ni is larger in Sr_0.9La_0.1CuO₂ than in (Nd, Ce)₂CuO_4–. These results are discussed along with the recent experimental observation of gap symmetry in electron-doped superconductors.     

The Characteristics of the Superconducting and Magnetic Phases in the Polycrystalline Samples of Ruthenocuprates of Nominal Compositions RuSr2GdCu2O8, Ru0.98Sr2GdCu2O8 and Ru0.5Sr2GdCu2.5O8−δ

The temperature dependencies of the resistivity for the superconducting ruthenocuprates of nominal compositions RuSr₂GdCu₂O₈, Ru_0.98Sr₂GdCu₂O₈ and Ru_0.5Sr₂GdCu_2.5O_8?δ were examined for the magnetic field dependent characteristics of the superconducting transitions. The effect of the insignificant diminishing of the Ru/Cu ratio in parent RuSr₂GdCu₂O₈ was confirmed as relevant for the stabilisation of the superconducting phase. Noted differences in the compared characteristics are interpreted for possible inhomogeneous nucleation of the superconducting phase in the parent ruthenocuprate. The phase anisotropy in RuSr₂GdCu₂O₈ and Ru_0.98Sr₂GdCu₂O₈, in presence of the compounds Ru magnetism, appears to be a cause of a significant softening of the H _c2(T) phase line. An anomalous lowering of the magnetoresistivity was observed in the approx. 10 K range above the onset of the superconducting transition, which may suggest the presence of enhanced superconducting fluctuations in the samples. The positive magnetic field shift of the temperatures, which limit the magnetoresistivity and the specific heat signatures of the magnetic ordered state of the Ru sublattice, suggests probing the influence of the ferromagnetic Ru interactions in an effective metallic-like conduction channel present in the samples. Superconducting characteristics of the Ru_0.5Sr₂GdCu_2.5O_8?δ reveal a significant contribution of the Gd paramagnetic signal at low temperatures, interpreted for the presence of a significant anisotropy of the superconducting phase. It is concluded that the Ru–Cu substituted phases of ruthenocuprates may present an opportunity to investigate the effectively anisotropic superconducting phase despite its comparatively high T _c in the compounds related to the 123-type cuprate superconductor.     

Electron paramagnetic resonance studies in the oxides Bi2Sr2Ca1−x Gd x Cu2O y

We have recently reported our results on magnetic susceptibility and microwave absorption studies on Bi₂Sr₂Ca_1−x Gd _x Cu₂O _y and Bi₂Sr₂Ca_1−x Y _x Cu₂O _y . In the present work two important observations of these investigation, viz, the dependence ofT _c on Gdconcentration and the absence of EPR-signals of Gd and Cu, are considered in greater detail. It is suggested that the microwave absorption by fluxon lattice might be playing one of the important and effective pathways for electron spin relaxation resulting in nonobservation of EPR signals.     

Structural and Magnetic Characteristics of Gadolinium Manganite Modified with Barium Gd0.9Ba0.1MnO3

Journal of Superconductivity and Novel Magnetism - The synthesis conditions for the gadolinium manganite doped with barium (x = 0.1) are determined. The sequence and typical features of phase...     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (～300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to ～7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

La0.9Sr0.1MnO3/Fe2O3复合体系的异常磁电阻效应

采用软化学方法制备了不同粒径的La0.9Sr0.1MnO3和软磁材料γ-Fe2O3,并将两者按不同比例进行复合,分别用扫描电子显微镜和X射线技术观察了样品的形貌特征和相结构,发现掺杂后的复合体系的电阻率要远大于未掺杂样品的电阻率.掺入γ-Fe2O3后,样品表现出异常磁电阻效应,即加外磁场后样品的电阻率明显大于未加磁场时样品的电阻率.同时,掺杂样品磁电阻的磁场灵敏度显著提高,在室温附近观察到了磁电阻极大值.