G. Liang

Magnetic properties of the Sm1−xCexMn2Ge2 system

Abstract Magnetic properties of polycrystalline Sm 1− x Ce x Mn 2 Ge 2 (0 ⩽ x ⩽ 1) compounds have been studied by magnetic susceptibility measurements. A magnetic phase diagram has been constructed. The SmMn 2 Ge 2 sample shows re-entrant ferromagnetism with a ferromagnetic (FM) transition at T 1 = 341 K, a FM to antiferromagnetic (AF) transition at T 2 = 144 K, an AF to FM transition at T 3 = 105 K, and then a Sm-ordering-related FM transition at T Sm ≈ 20 K. As x increases, (i) T 1 and T 2 decrease, (ii) T 3 increases initially and then decreases beyond x = 0.011, and (iii) T Sm remains unchanged up to x = 0.1 and then decreases rapidly. The AF phase is destroyed at x = 0.011. The AF region width ΔT AF (= T 2 − T 3 ) varies with R MnMn a at a rate about ten times as fast as that in Sm 1− x Gd x Mn 2 Ge 2 . Our results suggest that T 3 can be affected by both the intralayer MnMn distance and the spin disorder in the rare-earth layers.

Lattice, Ce-L3valence, transport, and magnetic results on mixed-valent/Kondo system Ce1−xLaxMn2Si2

Lattice, Ce-L3 edge, resistivity, and magnetic susceptibility measurements have been carried out on the polycrystalline Ce1−xLaxMn2Si2 system (0⩽x⩽1) to study the interplay between Mn 3d-host magnetism and Kondo-type Ce-spin fluctuations. As x increases, the system varies gradually from a Ce mixed-valent system with 3d-host antiferromagnetism (at x=0) to a nearly trivalent system with strong 3d-host ferromagnetism (near x=1). In the antiferromagnetic (AF) phase region (0⩽x⩽0.5), the Neel temperature TN decreases with the increase of x, manifesting the weakening of the host AF field. Impurity Kondo behavior is observed at x=0.05, indicating that the low-temperature coherence state in CeMn2Si2 can be destroyed by “Kondo holes” which are created by very small substitution of La for Ce. The susceptibility and resistivity results suggest that the impurity Kondo effect in the 0.1⩽x⩽0.5 samples is partially suppressed by a nonvanishing ferromagnetic (FM) field component; whereas the phononlike behavior of the re...

Negative effects of crystalline-SiC doping on the critical current density in Ti-sheathed MgB2(SiC)y superconducting wires

Ti-sheathed MgB2 wires doped with nanosize crystalline-SiC up to a concentration of 15?wt% SiC have been fabricated, and the effects of the SiC doping on the critical current density (Jc) and other superconducting properties studied. In contrast with the previously reported results that nano-SiC doping with a doping range below 16?wt% usually enhances Jc, particularly at higher fields, our measurements show that SiC doping decreases Jc over almost the whole field range from 0 to 7.3?T at all temperatures. Furthermore, it is found that the degradation of Jc becomes stronger at higher SiC doping levels, which is also in sharp contrast with the reported results that Jc is usually optimized at doping levels near 10?wt% SiC. Our results indicate that these negative effects on Jc could be attributed to the absence of significant effective pinning centres (mainly Mg2Si) due to the high chemical stability of the crystalline-SiC particles.

Development of Ti-sheathed MgB2 wires with high critical current density

Working towards developing lightweight superconducting magnets for future space and other applications, we have successfully fabricated mono-core Ti-sheathed MgB2 wires by the powder-in-tube method. The wires were characterized by magnetization, electrical resistivity, x-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry measurements. The results indicate that the Ti sheath does not react with the magnesium and boron, and the present wire rolling process can produce MgB2 wires with a superconducting volume fraction of at least 64% in the core. Using the Bean model, it was found that at 5 K, the magnetic critical current densities, Jc, measured in magnetic fields of 0, 5, and 8?T are about 4.2 ? 105, 3.6 ? 104, and 1.4 ? 104?A?cm?2, respectively. At 20?K and 0?T, the magnetic Jc is about 2.4 ? 105?A?cm?2. These results show that at zero and low fields, the values of the magnetic Jc for Ti-sheathed MgB2 wires are comparable with the best results available for the Fe-sheathed MgB2 wires. At high fields, however, the Jc for Ti-sheathed MgB2 wires appears higher than that for the Fe-sheathed MgB2 wires.

Magnetic phase diagram of Ce(Mn1−xCux)2Si2

The magnetic structure of mixed valence-Kondo-heavy fermion series Ce(Mn1−xCux)2Si2 (0⩽x⩽1) is reported. The magnetic susceptibility results indicate that for 0⩽x⩽0.3, Mn moments are antiferromagnetically (AFM) ordered and the Neel temperature TN decreases rapidly with decreasing x. For 0.4⩽x⩽0.7, ferromagnetic (FM) phase is observed below 140K and both the ordering temperature Tc and Curie–Weiss temperature θp decrease with increasing x, showing the weakening of in-plane FM magnetic coupling between Mn moments due to the substitution of Mn by nonmagnetic Cu. For 0.8⩽x⩽1, samples exhibit only paramagnetic behavior. The types of magnetic order in this series are basically consistent with the criterion that FM (AFM) phase is favored when intralayer Mn–Mn spacing RMn–Mna is greater (smaller) than a critical value of 2.865A. A magnetic phase diagram is constructed for this compound series.

Effects of the Size of the Doped SiC Nanoparticles on the Critical Current Density of the Ti-Sheathed MgB

The effects of the grain size of the doped SiC nanoparticles on the critical current density (<i>J</i>_c) of the Ti-sheathed MgB₂ superconducting wires were studied. The concentration of the SiC dopant was 10% and the sizes of the SiC particles were 20 nm, 45 nm, and 123 nm. Contrary to the <i>J</i>_c results reported on the SiC-doped Fe-sheathed MgB₂ wires, we found that the <i>J</i>_c for the Ti-sheathed MgB₂ wires decreased with the size of the SiC particles. We also found that the <i>J</i>_c is greater than that of the undoped MgB₂ wires only for the wires with 123 nm SiC size. This unusual dependence of <i>J</i>_c on the size of the SiC dopant is discussed in association with the results from the magnetization, electrical resistivity, x-ray diffraction, and transmission electron microscopy measurements.

_{2}

The effects of sintering temperature on the superconducting properties of Ti‐sheathed MgB2 wires have been studied. The wires were fabricated by in situ powder‐in‐tube (PIT) method and characterized by x‐ray diffraction, magnetization, scanning electron microscopy and transport measurements. Samples were sintered for 30 minutes at the following temperatures: 650° C, 700° C, 750° C, 800° C, and 850° C. It is found that the cores of these wires are almost in pure MgB2 superconducting phase, indicating that the Ti‐sheath does not react with Mg or B. The superconducting transition temperature Tc decreases from 36 K to 34.2 K with the decrease of the sintering temperature from 850° C to 650° C. At 5 K and 20 K, critical current density Jc peaks up for samples sintered at 800° C, which is in sharp contrast with previously reported result that for Fe‐sheathed MgB2 wires, Jc peaks up at sintering temperature of 650° C.

Superconducting Wires

Single crystals Nd2-xCexCuO4-δ with x=0.0 and 0.2 are studied by polarized X-ray absorption measurements at Cu K-edge. The positions and intensities of the 1s→ 4pσ transition features in the K-edge spectra are analyzed with a modified cluster model and functional curve fitting procedure. All of the experimental results including the energies of the spectral features, the energy separations between features and the variation of feature intensities, can be well explained by the cluster model parameters obtained from the curve fitting. Particularly, it is found that the charge transfer energy gap Δ in the Nd2-xCexCuO4-δ system decreases from about 1.4 eV at x=0 to 0.5 eV at x = 0.2. Our analysis in this paper provides a method for quantitative X-ray absorption spectroscopy (XAS) study of the charge transfer gap for electron-doped cuprate superconductors and related materials.