J.J. Du

Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slight Pr-doping

(Bi,Pb)2.2Sr2Ca2.2−xPrxCu3Oy silver sheathed tapes with Pr-doping amount of x=0.0, 0.001 and 0.002 have been prepared by powder-in-tube method. X-ray diffraction analyses manifest that the tapes with x=0.001 and 0.002 (doped) have less Pb-rich phase than the tape with x=0.0 (un-doped). Transmission electron microscopy images show that the Pb-rich phase particles of ∼40 nm are embedded in un-doped (Bi,Pb)-2223 grain matrix, while defects induced by Pr-doping with the size smaller than that of the Pb-rich phase particles exist in the doped samples. As compared with the un-doped tape, the magnetic field dependence of critical current densities of the doped tapes are noticeably improved at 77 K, and their irreversibility lines are also shifted to higher temperatures and magnetic fields and activation energies of flux motion increased drastically. The enhanced flux pinning seems to mainly originate from the fine normal-like defects induced by the partial Pr-substitution for Ca.

Influence of Mg deficiency on the properties of MgB2

The samples of Mg deficiency with different Mg nominal content (Mg1-xB2) are prepared by the solid-state reaction method at the sintering temperatures of 900 and 640 degreesC. The effects of Mg deficiency on the structure and superconducting properties are investigated by the Rietveld refinement of X-ray powder diffraction and the resistance measurement, respectively. The results show that the lattice parameters of the samples sintered at different temperatures are of a similar variation, i.e., a decreasing and c increasing with increasing x, respectively. T-c of the samples decreases with increasing x and the maximum T-c variation of DeltaT(c)(T-c(max) - T-c(min)) similar to 2 K is observed for samples sintered at high temperature. It is concluded that the nominal Mg content reduction does not give rise to serious Mg deficiency and slight Mg deficiency does not result in a remarkable change of T-c and normal state properties of the samples

Study on resistivity anisotropy and flux pinning of Bi2-xPbxSr2CaCu2Oy single crystals

Abstract Bi 2− x Pb x Sr 2 CaCu 2 O y ((Bi,Pb)-2212) single crystals with a large doping range up to x =0.8 have been grown by the conventional self-flux method. The resistivity anisotropy of (Bi,Pb)-2212 single crystals and their transport properties under applied magnetic fields with H ∥ c have been systematically studied. In-plane resistivity ( ρ ab ), out-plane resistivity ( ρ c ) and anisotropy ( γ ) of (Bi,Pb)-2212 are significantly reduced by heavy Pb doping. The dissipation behavior of (Bi,Pb)-2212 single crystals in the mixed state can be well described by the thermally activated flux creep model and flux creep is obviously suppressed for heavily Pb-doped single crystals. The origin of the improved flux pinning is suggested to stem mainly from the reduced anisotropy and the improved coupling strength between Cu–O layers due to Pb-doping.

Influence of MgO particle doping on flux pinning in Bi2Sr2CaCu2Oy crystals

Bi2Sr2CaCu2O8+delta (Bi2212) single crystals embedded with MgO particles are successfully synthesized. The critical current density of Bi2212 single crystals with an appropriate amount of MgO doping is noticeably increased at low temperatures below 20 K with applied magnetic fields parallel to the c-axis as compared with undoped samples. Whereas at T greater than or equal to 25 K, there is no extra flux pinning in MgO-doped samples. Microstructure analyses reveal that MgO doping induces a high density of dislocations in the ab plane of Bi2212 matrix, which may be responsible for the enhanced flux pinning at low temperatures

Photoinduced spin-state transition of Co3+ in the layered perovskite manganite thin film

The photoinduced effect, at different temperatures, on layered perovskite manganite La1.2Sr1.8Mn1.8Co0.2O7 thin film deposited on a SrTiO3(100) substrate has been studied using a He–Ne laser (632.8 nm) with the power of output 25 mW at low temperatures. The electrical transport and magnetic properties of the film are investigated systematically in the temperature region from 20 to 300 K. The resistance shows a significant decrease at low temperatures (T<90 K) under laser irradiation, which can be interpreted in terms of the spin-state transition of Co3+ ions. The magnetic field combined with the laser is also used to detect the photoinduced effect of the spin-glass state. The time dependence of the metastable system under several applied fields was also discussed. The system has the ability to keep a persistent memory of the magnetic history imprinted in its zero-field resistance.

Influence of H and Extra La on Magnetocaloric Effect of La

Magnetic properties and magnetocaloric effect were investigated on La0.5+xPr0.5Fe11.4Si1.6Hy (x=0, 0.1, 0.2) melt-spun ribbons. It was found that NaZn13-type phase could be obtained easily owing to that the segregation of α-Fe phase was suppressed obviously by adding appropriate amount of La after a short time annealing for melt-spun ribbons. For the La0.6Pr0.5Fe11.4Si1.6Hy ribbons, the Curie temperature was adjusted dramatically from low temperature of 193 K to room temperature of 321 K due to the interstitial hydrogen adsorption. With La content increasing, Curie temperature was slightly increased; the magnetic hysteresis was reduced, while a large magnetic entropy change remained. The maximum magnetic entropy change (- ΔSM) is 33.8 J/(kg·K) for La0.6Pr0.5Fe11.4Si1.6 while that of La0.6Pr0.5Fe11.4Si1.6Hy is 19.6 J/(kg·K) in a magnetic field change of 5 T. It is shown that rapid quenching to make LaFe13-xSix alloys is more cost effective than arc melting method by reducing annealing time substantially. High magnetic entropy change and low magnetic hysteresis were obtained by adding extra La and the Curie temperature was tuned to near room temperature by interstitial atom H. This work indicated that La0.6Pr0.5Fe11.4Si1.6Hy was a promising magnetic refrigeration material near room temperature.

_{0.5 + x}

We have measured the temperature dependence of internal friction Q−1(T), Youngs modulus E(T), and resistivity ρ(T) in situ along with the measurements of magnetization M(T) on optimally doped manganites La0.67Ca0.33MnOy with different oxygen contents. With the reduction of the oxygen content y, the Q−1 peak in the paramagnetic region, suggested to originate from the formation of the magnetic clusters (MC), shifts to a lower temperature and its magnitude decreases and finally disappears. The results of M(T) and ρ(T) indicates that both the effective magnetic moment μeff and the hopping distance of electrons decrease with the increase of oxygen vacancies, which may suppress the MC. In addition, the analysis of the results of E(T) indicates that the cooperative effects of lattice distortions may favour the formation of MC.

Pr

Abstract MgB 2 /Cu tapes were fabricated by power-in-tube technique using MgB 2 superconducting powder and no post-annealing treatment was performed for the prepared tapes. Both X-ray diffraction and scanning electron microscopy results indicate that there is no obvious texture of grains for the tape samples. However, compared with the bulk sample, evident shifting and broadening of diffraction peaks is observed and T c is reduced for the tape samples. In addition, it is found that there exists a remarkable difference in flux pinning ability for tapes prepared using wires with different diameters. The difference seems to originate from the different strain and microstructure defects of MgB 2 grains caused by cold working.

_{0.5}

Flux pinning and anomalous magnetization peak of heavily overdoped Bi2−xPbxSr2CaCu2−yCryO8+δ [Cr-doping (Bi, Pb)-2212] single crystals are investigated by means of magnetization measurements with the magnetic field parallel to the c axis. Compared with undoped Bi2Sr2CaCu2O8+δ (Bi-2212) and Pb-doping Bi2−xPbxSr2CaCu2O8+δ [(Bi, Pb)-2212] single crystals, the irreversibility line of Cr-doping (Bi, Pb)-2212 single crystals is pronouncedly shifted to high temperatures. The anomalous magnetization peak H2p with maximum peak field near 8000 G is observed. The H2p exhibits strong temperature dependence from 0.19 to 0.93Tc. The temperature dependence of H2p can be explained according to vortex decoupling theory. The improved flux pinning may originate from reduced anisotropy due to Pb doping and a sufficient number of effective pinning centers due to the random Cr substitution on the Cu site.

Fe

The ferromagnetic (FM) manganite La0.8Ca0.2MnO3 is systematically studied by measurements of internal friction Q−1, Youngs modulus E, resistivity ρ and magnetization M versus temperature T. The resistivity ρ(T) exhibits a metal–insulator transition (MIT) at K. The magnetization M(T) shows a paramagnetic–ferromagnetic (PM–FM) transition at K and saturates around 182 K. However, the variation of the internal friction with temperature presents a complicated behaviour. In the FM region, three Q−1 peaks at temperatures 98, 143 and 163 K are observed in the spectra of Q−1(T) at the measuring frequency of 1.8 kHz. The origin of three Q−1 peaks is discussed and attributed to the coexistence of metallic and insulating phases in the FM region. In addition, in the magnetic transitional region, the other two peaks at 182 and 244 K at the measuring frequency of 1.8 kHz are observed. Our results indicate that the internal friction method seems to be a useful technique in studying the coupling between spin, charge and lattice of CMR materials.