Effect of SiC buffer layer on flux pinning property of MgB2 tapes

In this paper we aimed at investigating the flux pinning property of MgB₂ films on hastelloy tapes which are buffered on various thicknesses of SiC layers. We have observed that the increase in thickness of the SiC buffer layer is very closely related with the systematic improvement of the field dependence of the critical current densities (J_c) of MgB₂ tapes while the values of J_c decreased. According to the analysis of the pinning force density (F_p), there exist two pinning sources both in the pure MgB₂ and in the MgB₂ film with the thinnest SiC buffer layer. On the while, the pinning source observed in the MgB₂ films with thicker SiC buffer layers appears to be different from those previously mentioned. The different pinning behaviors of MgB₂ films may suggest that there be an additional pinning center working on the MgB₂ films with thick SiC buffer layers. The microstructural analyses of MgB₂ films confirmed that intra-granular defects and columnar grain boundaries may be a dominant pinning mechanism in the pure MgB₂ and the MgB₂ film with 170 nm-thick SiC buffer layer. For the MgB₂ films with thicker SiC buffer layers, carbon diffusion into the MgB₂ film, which is defined by the Auger electron spectroscopy, may be the origin of the additional pinning mechanism.     

Effect of grain-boundary flux pinning in MgB2 with columnar structure

We studied the flux pinning properties by grain boundaries in MgB₂ films prepared by using a hybrid physical chemical vapor deposition method on the c-axis oriented sapphire substrates. All the films we report here had the columnar grains with the growth direction perpendicular to the substrates and the grain sizes in the range of a few hundred nanometers. At very low magnetic fields, no discernable grain-boundary (GB) pinning effect was observed in all measuring temperatures, but above those fields, the effect of GB flux pinning was observed as enhanced critical current densities (J_cs) and reduced resistances when an external magnetic field (B) was aligned parallel to the c-axis. We interpret the B dependence of J_c in the terms of flux line lattice shear inside the columnar grains activated by dislocations of Frank–Read source while the flux lines pinned by GB act as anchors for dislocations. Magnetic field dependence of flux pinning force density for B parallel to the c-axis was reasonably explained by the above model.     

Flux pinning in columnar-structured and single crystalline MgB2 films

We have investigated the flux pinning effect of columnar grain boundary in columnar-structured and single crystalline MgB₂ films. The MgB₂ films with columnar structure showed much higher J_c than that of single crystalline thin film, and sample having smaller grain size had a higher J_c in high magnetic fields. At 5 K, the MgB₂ film with grain size of 460 nm showed an abnormal double-peak behavior in pining force density, F_p(B), caused by competition of different types of pinning sites, such as planar defects and point defects. Field dependences of F_p in columnar-structured films suggest that the columnar grain boundary is a strong pinning source in the MgB₂ film and it plays a crucial role in enhancing J_c over a wide range of magnetic fields and temperatures.     

Enhanced critical current density of MgB2 thin films deposited at low temperatures by ZnO seed impurity

We report a facile method to enhance the critical current density (J_c) of superconducting MgB₂ thin films. MgB₂ thin films were deposited on zinc acetate dehydrate (Zn(CH₃COO)₂2H₂O) spin-coated Al₂O₃ (000l) substrates by using a hybrid physical-chemical vapor deposition system at low temperatures. Synthesis of MgB₂ at low temperatures can reduce the substitution of Zn into the Mg site, hence avoiding the reduction of superconducting critical temperature. MgB₂ thin films grown on ZnO-buffered layers showed a significant enhancement of J_c in the magnetic field due to the creation of additional pinning sources, namely point defects and grain boundaries. Broad peaks were observed in the magnetic field dependence of the flux pinning force density, indicating competition of different pinning sources.     

Absence of correlated flux pinning by columnar defects in irradiated epitaxial Bi2Sr2CaCu2O8 thin films

Using heavy-ion irradiation, we produced columnar defects of different density and orientation in epitaxial Bi₂Sr₂CaCu₂O₈ thin films. Although this increases the normal state resistivity and the critical temperature is reduced proportionally to the volume fraction of damaged material, pinning-related quantities like critical current density, activation energy and depinning field are enhanced in external magnetic fields. Transport measurements in dependence of the magnetic field and its orientation consistently indicate two-dimensional pinning of pancake vortices at the columnar defects. We observe the absence of correlated flux pinning by columnar defects and compare to heavy-ion-irradiated single crystals and tapes, where line-like correlations have been observed.     

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (J_c) of MgB₂ thin films via 140-keV Co-ion irradiation. The J_c(H) of the surface-damaged MgB₂ films was remarkably improved in comparison with that of pristine films. The strong enhancement of J_c(H) caused by a surface damage in MgB₂ films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (H_irr) at 5 K for surface-damaged MgB₂ films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve J_c(H) in a MgB₂ superconductor.     

The effect of SiC nanoparticle addition on the flux pinning properties of MgB2

The nanoparticle–MgB₂ composite superconducting sample, (SiC)_4wt.%(MgB₂)_96wt.% ((SiC)₄–MgB₂), was prepared, and the effect of nanoparticle additions on the magnetic flux pinning was investigated. The measurement and comparison of isothermal magnetization M(H), for pure-MgB₂ powder and sintered pellets of (SiC)₄–MgB₂ and pure-MgB₂ were carried out at temperatures between 5 and 50 K in fields up to 8.5 T. The magnetic irreversibility ΔM(H) curves of the (SiC)₄–MgB₂ follow almost identical lines of both pure-MgB₂ powder and sintered bulk MgB₂ in the region above a specific magnetic field (called a merged field), which gradually decreases as the temperature increases. The (SiC)₄–MgB₂ composite superconductor has exhibited the flux pinning effect which comes from both the grain boundaries and the point defect weak pinning centers in the region below the merged field line. This is different from the case of pure-MgB₂ powder and sintered bulk MgB₂ which showed mostly the grain boundaries pinning.     

Hall effect and pinning regimes in thin films doped with nanoparticles

Hall effect and flux pinning in YBa₂Cu₃O_6+x (YBCO) thin films doped with BaZrO₃ (BZO) nanoparticles is investigated. The results show that sign reversal of the Hall coefficient from positive hole-like to negative electron-like occurs in vortex-liquid regime of undoped and BZO-doped YBCO films. With increasing BZO concentration the amplitude of the negative Hall effect is suppressed while the temperature position of the anomalous Hall effect does not depend significantly on doping level. In addition, it is shown that Hall conductivity increases non-monotonically with increasing BZO doping. These results support a model where BZO at low doping concentration induces point pinning centres turning to strong columnar pinning defects in films doped with 4% BZO.     

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

The dissipative mechanism at low current density is compared in three different classes of superconductors. This is achieved by measuring the resistance as a function of temperature and magnetic field in clean polycrystalline samples of NbSe₂, MgB₂ and Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) superconductors. Thermally activated flux flow behaviour is seen in all the three systems and clearly identified in bulk MgB₂. While the activation energy at low fields for MgB₂ is comparable to Bi₂Sr₂Ca₂Cu₃O₁₀, its field dependence follows a parabolic behaviour unlike a power-law dependence seen in Bi₂Sr₂Ca₂Cu₃O₁₀. We analyse our results based on Kramer’s scaling for grain boundary pinning in MgB₂ and NbSe₂.      

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Correlated vortex pinning in Si-nanoparticle doped MgB2

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB₂ tapes have been measured at temperatures T≥28 K in magnetic fields B≤0.9 T. The irreversibility line B_irr(T) for doped tape shows a stepwise variation with a kink around 0.3 T. Such B_irr(T) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B_?) and is very different from a smooth B_irr(T) variation in undoped MgB₂ samples. The microstructure studies of nanoparticle doped MgB₂ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.     

Effects of crossed columnar defects on vortex pinning in Bi2Sr2CaCu2O8

To investigate the existence of a splay effect in Bi₂Sr₂CaCu₂O₈ (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect. Received 27 December 1999     

Concurrent doping effect of Ti and nano-diamond on flux pinning of MgB2

Nano-diamond and titanium concurrently doped MgB₂ nanocomposites have been prepared by solid state reaction method. The effects of carbon and Ti concurrent doping on J_c–H behavior and pinning force scaling features of MgB₂ have been investigated. Although T_c was slightly depressed, J_c of MgB₂ have been significantly improved by the nano-diamond doping, especially in the high field region. In the mean time, the J_c value in low field region is sustained though concurrent Ti doping. Microstructure analysis reveals that when nano-diamond was concurrently doped with titanium in MgB₂, a unique nanocomposite in which TiB₂ forms a thin layer surrounding MgB₂ grains whereas nano-diamond particles were wrapped inside the MgB₂ grains. Besides, nano-diamond doping results in a high density stress field in the MgB₂ samples, which may take responsibility for the Δκ pinning behavior in the carbon-doped MgB₂ system.     

Dissipative flux motion in YBa2Cu3O7−δ films — Investigation by means of transport I-V curves

The nature of dissipative flux motion in epitaxial YBa₂Cu₃O_7– thin films has been investigated by means of resistivity (T, B) and transport current-voltage characteristicsE(j, T, B) covering up to 5 orders of magnitude in voltage for a wide range of temperatures (4.2–85 K) and magnetic fields (0.5–7.0 T). Activation energiesU(T, B, j) have been determined in the framework of the collective flux creep theory. It is shown thatU(T, B, j) decreases with increasing temperature, magnetic field and current density. In order to study the influence of a well-defined defect structure, columnar defects have been generated by irradiation with 340 MeV Xe-ions. From a scaling law relating theI–V curves after irradiation with the curves before irradiation it is found that vortices pinned by the columnar defects do not contribute to dissipation. Due to irradiation the irreversibility line is shifted to higher magnetic fields, indicating its connection to the pinning mechanisms present in the samples.     

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB₂ films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB₂ films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB₂ grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB₂ films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB₂ with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB₂.     

柠檬酸掺杂的MgB2超导体钉扎机理的研究

本文研究了柠檬酸掺杂的MgB₂超导材料的J_c-B行为及其钉扎机理.在纯MgB₂多晶样品中,δT_c钉扎起主要作用,而在掺杂的样品中,则是δl钉扎和δT_c钉扎共同作用,并且δl钉扎机理占主要作用,其贡献比重随着掺杂量的增加而增加.从J_c-B行为和钉扎行为的分析都可以得到 关键词： 柠檬酸 2')" href="#">MgB₂ T_c钉扎')" href="#">δT_c钉扎 l钉扎')" href="#">δl钉扎     

Peak effect at microwave frequencies in swift heavy ion irradiated YBa2Cu3O7-δ thin films

The vortex dynamics at microwave frequencies in YBa₂Cu₃O_7-δ (YBCO) films have been studied. We observe a peak in the microwave (4.88 and 9.55 GHz) surface resistance in some films in magnetic fields up to 0.8 T. This is associated with the ‘peak-effect’ phenomenon and reflects the order-disorder transformation of the flux line lattice near the transition temperature. Introduction of artificial pinning centers like columnar defects created as a result of irradiation with 200 MeV Ag ion (at a fluence of 4×10¹⁰ ions/cm²) leads to the suppression of the peak in films previously exhibiting ‘peak effect’.     

Thermally assisted flux flow in MgB2: strong magnetic field dependence of the activation energy

The origin of the resistive transition broadening of superconducting MgB₂ thin films is investigated. Thermally activated flux flow is found to be responsible for the resistivity in the vicinity of the critical temperature. The activation energy for flux motion is observed to have an extraordinary strong dependence on the magnetic field. The results are discussed.     

Enhanced flux pinning in pulsed laser deposited Y Ba2Cu3O7−δ : BaTiO3 nanocomposite thin films

The effect of incorporation of BaTiO₃(BTO) nanoparticles on the flux pinning properties of pulsed laser deposited YBCO:BTO thin films was studied. Substantial increase in the critical current density (J_C) and the pinning force density (F_p) of the nanocomposite thin films was observed. At 77 K, and zero applied magnetic field, the value of J_C for YBCO and YBCO:BTO (2%) thin films were 2.93 MA/cm² and 6.43 MA/cm², respectively. At the same temperature and an applied magnetic field of 4 T, the value of J_C increases from 3.6×10⁴ A/cm² for YBCO thin film to 2.7×10⁵ A/cm² for YBCO:BTO (2%) nanocomposite thin film. The study of temperature and field dependence of of YBCO and YBCO:BTO thin films indicates similar type of pinning. The lattice mismatch between YBCO and BTO seems to introduce more defects resulting in the improvement of flux pinning properties.     

Angular dependences of critical current density in YBCO thin films with crossed columnar defects

We investigated the dependences of the critical current density J_c on the magnetic field angle θ in YBa₂Cu₃O_7−δ thin films with the crossed configurations of the columnar defects (CDs). To install the crossed CDs, the films were irradiated using the high energetic Xe ions at two angles relative to the c-axis. The additional peak around the c-axis appears in the J_c(θ) for all irradiated films. In lower magnetic fields, the height of the J_c(θ) peak caused by the crossed CDs with the crossing angles θ_i = ±10° was higher than that for the parallel CDs. It is considered that the correlation of the flux pinning by the crossed CDs along the c-axis occurs even in the case of θ_i = ±25°, which was also suggested by the kink behaviors of the scaling parameters of the current–voltage characteristics near 1/3 of the matching field. In higher magnetic fields, on the other hand, the height and width of the J_c(θ) peak for the crossed CD configurations rapidly reduce with increasing the magnetic field compared to the parallel ones. In the crossed CD configurations, the dispersion in the direction of CDs would prevent the correlation of flux pinning along the c-axis in high magnetic fields, which occurs in the parallel CD configurations due to the collective pinning of flux lines including the interstitial flux lines between the directly pinned flux lines by CDs.