[en] We have studied the effect of flux creep on the AC loss of Ag-Bi2223 tapes by three methods: the transport V-I curve, M-H hysteresis loop and AC susceptibility (χ-T curve). It was found that the dI/dt, dH/dt and f affected the AC loss apparently in addition to the DC field, respectively. Comparison between the data of the three ways shows a satisfactory agreement with each other. Numerical simulation was conducted to understand the experimental data based on flux creep. It was shown that the dependence of AC losses on f, dH/dt and dI/dt could only be understood by flux creep model rather than any static one

[en] Based on systematic measurement of dc susceptibility and parameter and structure observation for YBCO oxide superconductor, a model is proposed in which sandwich of superconducting and normal phases is supposed. If crystal defects in the samples are considered to be the normal phase, the measured small dc susceptibility can be understood

[en] The silver sheathed tapes of Ag-Bi_2-xPb_xSr₂Ca₂-Cu₃O_7-y (Ag-Bi2223) were fabricated by usual process. The limiting factor of the critical current density j_c of the tapes was studied. V-I characteristic was performed for the tapes immersed in liquid nitrogen with and without magnetic fields. Hysteresis loop of V-I curve was observed. A model based on polycrystalline superconductors was proposed to account for this kind of hysteresis. The result shows that in our Ag-Bi2223 tapes the weak link is the key factor limiting the global critical current. The measurement of V-I loop can be an effective method to check whether there exist harmful weak links in polycrystalline samples

[en] We carefully measured the ac susceptibility (acs) for a YBa₂Cu₃O_6.993 single crystal at different dc fields and observed a giant dip effect. The acs in lower dc fields signals the transition of flux pinning mechanism. Meanwhile, the hysteric behavior of acs as the temperature goes down confirms that the solid-liquid phase transition of vortex matter takes place just below the dip temperature

[en] Magnetic properties of the 1222-type structure compounds (Pb,M)(Sr,Pr)₂(Pr,Ce)₂Cu₂O₉ (M = Cu and Cd) and the 2222-type compound Bi₂Sr₂(Pr,Ce)₂Cu₂O₁₀ are reported. The magnetic susceptibility measurements reveal that Pr³⁺ moments are ordered antiferromagnetically around T_N(Pr) of 15-18 K for (Pb_0.5Mo_0.5)(Sr_0.9Pr_0.1)₂(Pr_0.7Ce_0.3)₂Cu₂O₉ (M = Cu and Cd), and at 22-24 K for Bi₂Sr₂(Pr_0.82Ce_0.18)₂Cu₂O₁₀. Except T_N(Pr), there is no other anomalous peak appearing in the susceptibility curve, which is different from the other reported 1222 MSr₂PrCu₂O₁₀ (M = Nb, Ta, Ga, In) systems. In the range of above T_N(Pr) to room temperature, the susceptibility curve can be well fitted by a Curie-Weiss law with an effective magnetic moment μ_eff of 3.12, 2.79 and 3.09 μ_B per Pr ion for (Pb_0.5Cu_0.5)(Sr_0.9Pr_0.1)₂(Pr_0.7Ce_0.3)₂Cu₂O₉, (Pb_0.5Cd_0.5)(Sr_0.9Pr_0.1)₂(Pr_0.7Ce_0.3)₂Cu₂O₉ and Bi₂Sr₂(Pr_0.82Ce_0.18)₂Cu₂O₁₀, respectively. The anomalous Pr magnetism and the suppression of superconductivity by Pr in the above compounds are discussed with the related Pr-containing 1212, 2212, 3212 and 214 compounds. (orig.)

[en] Measurement was made of ac susceptibility (acs) as a function of temperature T for a YBa₂Cu₃O_6.993 single crystal under dc fields (H_dc), and dips in the acs curves were observed in both χ^'(T) and χ^'(H_dc) curves in the same time for the sample. We explain the dips in the experimental χ^'(T,H_dc) curves as bulk pinning effect in higher fields and lower temperatures. However, the dips were weakened and finally replaced by surface barriers. Careful analysis of the data shows that there exists evidence indicating the transition of flux pinning mechanism

[en] We have prepared Na-doped YBa₂Cu₃O_y (YBa_1.9Na_0.1Cu₃O_y+40 mol% Y211) (YBNCO) and Na-free YBa₂Cu₃O_y (YBCO) samples by the melt-textured growth method to study the effect of doped Na ion on flux pinning. The ac susceptibility curves as well as the hysteresis loops were measured for the samples. Then the effective pinning energy (U(T,H_dc,J)), irreversibility line (H_irr(T)) and critical current density (j_c(H_dc)) were determined, where T, H_dc and J are temperature, dc magnetic field and current density, respectively. We found that, with Na doping, the H_irr(T) line shifted to lower temperatures while the J_c(H_dc) and U(T,H_dc,J) became smaller. Hence Na ions play a negative role in the flux pinning of YBCO. The appearance of the second peak in the J_c(H_dc) curves and the enhancement of anisotropy in YBNCO further support this finding

[en] We propose a method to determine critical current density j_c for MgB₂ with flux creep from the real part of AC susceptibility (ACS), which facilitates the determination of j_c at different DC fields B_d in temperature range as wide as 5-38 K. Influence of criterion E_c on j_c was studied by varying amplitude (B_ac) and frequency (f) of AC field. The result shows that it is not proper to obtain temperature dependence of j_c by measuring only the peak temperature of imaginary part of ACS

[en] Flux motion and its influence on the properties of the new discovered superconductor MgB₂ was studied by electric transport resistance as well as by ac susceptibility measurements. Different bias current I was applied perpendicular and parallel to applied dc field B_d up to 7 T. The upper critical field B_c2 and irreversibility field B_irr were determined as functions of temperature T and bias current I. It is found that the larger the bias current I the smaller the B_irr, and B_c2 is observed to be affected by flux motion. The B_irr(T) and B_c2(T) data determined by ac susceptibility measurement confirm the transport result. These results suggest that the influence of flux motion should be taken into account if resistively determined parameters for MgB₂ are considered. The present study also implies that the flux dynamics of MgB₂ is more like as that of high temperature superconductors rather than of low temperature superconductors

[en] Systematic measurement of the AC susceptibility (ACS) has been taken for the Na doped YBa₂Cu₃O_y (YBNCO) crystals as a function of temperature T, DC field B_d, amplitude B_ac and frequency f of AC field. Its critical current density j_c as a function of T, B_d, B_ac and E_c was determined, where E_c is criterion of j_c. Influence of criterion E_c on j_c was studied by varying B_ac and f, which allows one to determine the effective activation energy as a function of j. It is shown that the sweeping rate of the applied field affects the criterion E_c and thus the j_c apparently. Hence the magnetic j_c may be not comparable to each other unless the rate of applied field was indicated clearly