The effects of columnar defects on the critical current density (Jc) and electric (E)vs. current density (J) characteristics were investigated for a YBa2Cu3Oy thin film prepared using a pulsed-laser deposition method. The YBa2Cu3Oy thin film was irradiated with 200 MeV Au ions from a direction θ=8.8°off the c-axis. Two peaks in the angular dependence of Jc were observed at the angles of θ=8.8°and θ=90°. These angles correspond to the directions of the columnar defect and intrinsic pinning, respectively. The E-J characteristics at various magnetic field angles can be described by the percolation transition model. The value of pinning parameter m increases at the angles of θ=8.8°and θ=90°. This result indicates that the efficiency of flux pinning is enhanced because fluxoids are effectively pinned by the columnar defects and intrinsic pinning or stacking faults in these directions.
To enhance the critical current density (Jc) in magnetic fields, we performed heavy-ion irradiation to introduce artificial pinning centers into YBCO—coated conductors. It is well known that high-energy heavy-ion irradiation introduces amorphous tracks along ion trajectories. Since the morphology and the size of the defect depend on the electron stopping power, Se, we used three different irradiation conditions, namely, 400 MeV Kr-ions, 450 MeV Xe-ions, and 500 MeV Au-ions, to study its effect. All of them enhance the in-field Jc, though the largest enhancement was observed after irradiation with 450 MeV Xe-ions. We also studied the dependence of Jc enhancement to optimize irradiation fluence. In all cases, the reduction of Jc was observed when the irradiation fluence exceeded 1 x 1012 ions/cm2, which may be attributed to the reduction of superconducting volume as well as the depression of superconductivity. From the angular dependence of Jc measurement, the columnar defects act as unidirectional pinning sites, as expected, but with a very large trapping angle of approximately 45°.
We measured the transport critical current densities in high magnetic fields up to 17 T for Er123 films irradiated by 200MeV Au ion with the matching field of 0.1 T and 1 T along c-axis and 1 T for 23° tilting from c-axis. The angular dependence of critical current density suggests that the c-axis correlated pinning centers is effective even in much higher magnetic field than its matching field. In addition, we found that the peak of the flux pinning force density (Fp) curves related with the matching field of the columnar defects. The temperature scaling behavior of Fp in field along the defect direction was observed only in low field region below the matching field. Therefore, the c-axis correlated pinning is dominant in low field region. In higher magnetic field than the matching field, however, the c-axis correlated pinning works cooperatively with the background random pinning as well.
The effects of growth conditions on one-dimensional nanorods in ErBa2Cu3O7-δ films are discussed in terms of their microstructures and superconducting properties. The Ba(Er0.5Nb0.5)O3 nanorods were observed clearly in transmission electron microscopy images of the films. Nanorod morphologies were controlled by managing the growth temperature (TS) and BNO-doping concentration. The nanorods were thinner and denser at a lower TS under a constant BNO-doping concentration. In case of TS=710℃, the diameter and number density of nanorods with 1.5 wt% BNO-doping were 4–6 nm and 2.6×1011/cm2, respectively. As the BNO-doping concentration increased, nanorods were almost aligned in the c-axis direction and grew in uniform size from the substrate to the film surface. The diameter and number density of the nanorods with 5 wt% BNO-doping were ~10 nm and 2.2×1011/cm2, respectively. These nanorods could effectively act as c-axis correlated pinning centers. Consequently, the pinning effects were controlled by their diameter, number density and growth direction of the nanorods by varying the growth conditions.
We investigated thickness dependence of critical current density(Jc(d)) for BaZrO3-introduced GdBa2Cu3O7-δ(BZO-GdBCO) films fabricated using pulsed laser deposition. Contrary to the typical Jc(d) behavior of REBCO(RE = rare earth elements) thin films, which exhibit decreasing dependency as thickness increases, the BZO-GdBCO films showed an increasing dependency under magnetic fields for the thickness of 0.4-1.2 μm. In addition,increasing dependency was seen for all field directions including both the c-axis and ab-plane and these results suggest that the behaviors can be ascribed to the thermally activated flux creep, irrespectibe of magnetic-field direction and type of pinning site.
Self-organized BaZrO3 (BZO) nanorods in REBa2Cu3Oy superconducting films are known as promising artificial pinning centers. Previously, we have reported high-Jc Sm1+xBa2-xCu3Oy (SmBCO) films fabricated by a low-temperature growth (LTG) technique. The LTG technique expands the range of the deposition temperature in which the REBCO films show c-axis orientation, and makes it possible to control the crystal grain size and number density. In this study, in order to tune the pinning properties, we fabricated SmBCO films with BZO nanorods (LTG-SmBCO + BZO films) using the LTG technique and investigated the influence of the LTG technique on the growth of BZO nanorods and pinning properties in SmBCO films. With decreasing deposition temperature of the upper layer in the LTG-SmBCO + BZO films, the diameter of the films was decreased and the number density of BZO nanorods was increased, producing a high matching field, Bφ. Additionally, remarkable upturn shifts in the irreversibility field line and plateau regions in the magnetic-field dependence of Jc were observed in the range of Bφ/3 < B < Bφ. These results indicate that the nature of the Bose-glass-like state of vortices localized on c-axis-correlated pinning centers emerges in the range of Bφ/3 < B < Bφ.
The magnetic-field angle dependence of critical current densities Jc(H,θ) in commercial YBCO tapes grown by MOCVD and MOD was examined at liquid nitrogen temperatures. We first measured Jc(H,θ) in MOCVD-YBCO tapes at 70 and 77.3 K in fields up to 2 T using both transport and inductive (the third harmonic voltage) methods and compared the results. It was observed that, in low magnetic fields, the transport measurements gave higher Jc than the inductive ones; however, in high fields they agreed well, which is well explained by the effects of weak links due to low-angle grain boundaries. We then investigated Jc(H,θ) in MOCVD- and MOD-YBCO tapes at 77.3 K in fields up to 2 T. All the tapes exhibited peaks at H//ab in the shape of Mount Fuji, which shows that small random pinning plays a major role. However, an anisotropic scaling analysis showed that the flux-pinning mechanisms in those tapes were different, resulting in distinctive angular behaviors of Jc(H,θ). It is suggested that the difference in the flux-pinning mechanisms of the two types of tapes came from the different sizes of point defects originating from the growing processes.
The dependence of the transport and magnetization critical current density (Jc) characteristics on the superconducting layer thickness (d) was investigated for IBAD/CVD-processed, YBCO-coated conductors. It is believed that the decrease in Jc with increasing d in the low magnetic-field region is caused by degradation of the superconducting layer structure. The irreversibility field (Bi) increased in the low electric-field region as the thickness increased, whereas it decreased in the usual electric-field region. The apparent pinning potential (U0*) estimated using magnetic relaxation measurement also showed a complicated dependence on thickness. The results observed were approximately explained by the prediction of the flux creep-flow model.
Most studies on the pinning properties and the relating vortex nature of the RE123 system have been performed at high temperatures around 77 K. However, the pinning characteristics of RE123 at low temperatures have not been well understood yet, while extensive applications using the crycooling system are also strongly expected. In the present study, the Jc-H-T properties have been studied Gd123 single crystal with low level Gd substitution for Ba and Y123 single crystals substituted by small amount of impurity ions, such as Sr and Co, which improve Jc properties accompanying huge peal effect in their Jc-H curves. It was found the increases in Fp as the temperature of the impurity doped Y123 samples decreased were much larger than that of the Gd123 sample, while the Gd123 sample exhibited higher Jc than of the Y123 samples above 77 K. The Jc performance of the Gd123 sample below 50 K was even lower than that of the undoped Y123 sample. This strongly indicates that the RE123 with RE/Ba substitution is not appropriate for low-temperature applications, such as high-field magnets, despite their good Jc performance at high temperatures e.g. 77 K.
Critical current density, Jc, and irreversibility field, Bi, were measured in a magnetic field along the c-axis for (Nd, Eu, Gd)Ba2Cu3Oy (NEG-123) bulk superconductors with the addition of Rare Earth (RE)-211 phase particles in order to correctly understand the flux-pinning mechanism in these superconductors. It was found that the peak value of the critical current density, Jcp, increased and that Bi tends to decrease with the addition of 211 phase particles. That is, a negative correlation exists between Jcp and Bi. The deterioration of Bi is mainly attributed to the deterioration of the upper critical field, Bc2, caused by the proximity effect between substituted regions with lower Bc2 and the surrounding 123 matrix. The peak effect is caused by the order-disorder transition of the flux lines with the assistance of pinning by the nano-lamellas. The observed results can be explained well by the theoretical predictions of the flux creep-flow model.
The temperature changes and magnetic-field trapping behaviors of melt-textured, single-domain Sm-Ba-Cu-O bulk superconductors have been precisely investigated for magnetizing processes by means of quasi-static magnetic fields such as field cooling (FC) and zero field cooling (ZFC). The experiments were conducted in the temperature range of 40-80 K by simultaneously measuring both the temperature and magnetic flux density. According to the balance of heat generation and drainage, the temperature evolution profiles show distinctive behaviors of the invading and exhausting magnetic fluxes into and out of the HTS bulk magnet. The average temperature change during the FC process reached the maximum value of 4.63 K when the sweep rate was 11.3 mT/s and the initial temperature was 45.6 K. In the ZFC process, both the temperature and magnetic flux density kept increasing gradually even after the external magnetic field stopped growing at 5 T in the ascending field process. The highest temperature rise due to the flux motion reached 4.6 K even when the sample was magnetized with at sweeping rate of 11.3 mT/s. As the behavior of temperature rise was different between the ascending and descending field processes, it is suggested that the magnetic fluxes in the processes invade and diffuse under different heating characteristics.
To obtain useful information on how to improve the irreversibility field Hirr and critical current density Jc in RE123 systems, the critical current properties of Nd1+xBa2-xCu3O7+δ(Nd123) have been studied. Various parameters, such as partial pressure of oxygen P(O2) during the growth of single crystals, the influence of post-annealing at 900‣ and quenching under different P(O2), the oxygen annealing under high P(O2) and element-substitution into the crystals, were systematically changed. On special notice is that Hirr of the Nd123 single crystal was remarkably increased by partial substitution of Ca for the Nd-site up to 13.3 T at 77 K.
We will show the superconducting properties of alternately-layered MgB2/Ni thin films inserted as very thin (1 nm) nickel layers between MgB2 layers a few tens of nanometers thick. The MgB2/Ni thin films were prepared on silicon (100) substrates by sequentially switching electron-beam evaporation and coaxial vacuum arc evaporation techniques without post-annealing. In this study, we prepared alternately layered MgB2/Ni thin films with varying Ni-layer spacings. The Ni layer spacings were set to 32, 23 and 16 nm, respectively. The MgB2/Ni thin films were neither inter-diffusion nor chemical reactions between MgB2 and Ni. Clear enhancements of the Jc were observed in the MgB2/Ni thin films when the magnetic fields were applied parallel to the inserted Ni layers, and the peak positions in the Fp–B curves shifted to higher magnetic fields with the decrease of the Ni-layer spacing. These results clearly indicate that the Ni layers inserted in alternately-layered MgB2/Ni thin films work as very effective flux-pinning centers.
We report relationships between the superconducting properties, microstructure and fabrication conditions in high-density MgB2 bulks fabricated using a premix-powder-in-closed-tube (PICT) method. The microstructure of the MgB2 bulks depends on the crystallinity of the initial B powders; use of crystalline B powders reduces the amount of MgB2 and increases re-sidual B and MgO. The heat treatment condition influences the grain sizes of MgB2 as well as the distribution and grain sizes of MgO. A lower temperature and a shorter duration of heat treatment tend to form fine grains of MgB2 and uniform dispersion of fine MgO particles. These microstructural variations influence the distribution of effective flux pinning centers such as grain boundaries and fine MgO particles. Different types of critical current density behavior as a function of magnetic field were meas-ured.