T. Sueyoshi

Flux pinning properties of ErBa2Cu3Oy thin films with BaZrO3 nanorods

ErBa2Cu3Oy (ErBCO) thin films with BaZrO3 (BZO) nanorods were prepared by a PLD method for an enhancement of the critical current density Jc. The values of Jc for the ErBCO thin film containing 1.5 wt% BZO (Er15) in magnetic fields are higher than those for the ErBCO thin film containing 0.5 wt% BZO (Er05). The peaks of Jc have been observed in the angular dependence of Jc in both the films when the magnetic field is applied parallel to the c-axis. It has been found that the peak is attributed to the flux pinning by BZO nanorods oriented parallel to the c-axis. The vortex glass–liquid transition temperature Tg and the pinning parameter m were derived by fitting observed electric transport properties to the theoretical expression based on the percolation transition model. The value of Tg of Er15 is higher than that of Er05. This result indicates that the vortex glass phase extends to a higher temperature region on increasing the fraction of BZO. The peak of m has been found in the magnetic field dependence. This fact is probably due to matching the density of BZO nanorods with that of fluxoids, which was confirmed by TEM observations.

Critical current density of MgB2 thin film with pinning centres introduced by deposition in oxygen atmosphere

Critical current densities Jc of MgB2 thin films have been measured; these were prepared by an electron beam evaporation method without any post-annealing. In order to improve the characteristics of Jc in high magnetic fields, a MgB2 thin film was deposited in an O2 atmosphere. The values of Jc of the O2-doped film have exceeded those of the non-doped film in magnetic fields applied perpendicular and also parallel to the c-axis. The peak of Jc caused by the c-axis correlated pinning centres has been observed in the angular dependence of Jc. The upper critical field Bc2 and the vortex glass–liquid transition field Bg have been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. It is found that Bc2 and Bg are increased by the introduction of O2 in both the directions of the applied field. This fact indicates that effective pinning centres have been introduced by the deposition of the MgB2 thin film in an O2 atmosphere.

Transport characteristics near the glass–liquid transition temperature before and after heavy-ion irradiation in a YBa2Cu3O7−δ thin film

Abstract We investigated the influence of flux pinning on the behavior of flux lines in a c-axis oriented YBa2Cu3O7−δ thin film, by analyzing the critical scaling of current–voltage characteristics at various temperatures under the magnetic field parallel to the c-axis before and after irradiation. In the irradiation process, splayed columnar defects were produced in the film by tilting 200 MeV Au-ion beams off the c-axis by ±15°. The enhancement of the glass–liquid transition temperature and the increment of the dynamic critical exponent z were observed after the irradiation. The depinning model taking into account a distribution of pinning strength can reasonably describe the origin of the changes of both the glass–liquid transition temperature and z owing to the introduction of defects. This fact indicates that the flux line dynamics is governed by the pinning interaction rather than the intrinsic property of the flux line system.

High critical current density under magnetic fields in as-grown MgB2 thin films deposited by molecular-beam epitaxy

As-grown MgB2 thin films were prepared by a MBE method under the conditions of low temperature, low deposition rate and high vacuum for applications in electric devices. The MgB2 thin films deposited on MgO and Ti buffered ZnO substrates have considerably higher Jc under magnetic fields among MgB2 thin films reported before. The value of Jc for the MgB2 thin film deposited on Ti buffered ZnO has been 5.8 × 105 A cm−2 at 10 K, 5 T in the magnetic field applied parallel to the c axis. In the angular dependence of Jc, the peak of Jc attributable to c-axis-correlated pinning centres has been observed when the magnetic field was applied parallel to the c axis.

Sweep rate dependence of magnetic hysteresis loops and peak effect in high- superconductors

The origin of the peak effect in QMG-processed YBCO was investigated using the sweep rate dependence of magnetic hysteresis loops. A numerical analysis of magnetic hysteresis loops demonstrated that electric field E versus current density J characteristics can be accurately derived from the sweep rate dependence using the Mawatari method rather than the conventional Bean one. The pinning potential energy can be obtained from the slope of the log E versus log J plot accurately. By analysing the experimental data with the peak effect using the Mawatari method, it was found that the pinning potential energy and the critical current density have the peak values at different magnetic fields. This anomalous phenomenon incidental to the peak effect can be explained by the Klein model which is based on the concept of the field-induced flux pinning.

Influence of the columnar defect densities on the current–voltage characteristics in high-Tc superconductors

Abstract We investigated the influence of columnar defect densities on the critical exponents estimated from the critical scaling analysis of the current–voltage characteristics in high- T c superconductors. It was found that the dynamic critical exponent z increased with increasing columnar defect densities, systematically. The peak of z was observed near the magnetic field B / B Φ =1/3. These results are explained by the depinning model considering the distribution of the pinning strength rather than the vortex-glass model.

Angular behaviour of critical current density in YBa2Cu3O y thin films with crossed columnar defects

Columnar defects (CDs) were installed into YBa2Cu3O y thin films at the crossing angle ±θ i relative to the c-axis using 200 MeV Xe ions, in order to clarify the influence of ±θ i on the angular behaviours of critical current density J c. The width of a J c peak occuring at B || c linearly increases with broadening CD-crossing angle, where its shape maintains a single peak in the range of θ i ≤ ±25°. The CDs crossing at ±45°, on the other hand, drastically change the angular behaviour of J c around B || c: the J c is equally elevated over the angular region from −45° to 45° in the lower magnetic field, whereas the rapid reduction of the J c occurs at B || c with increasing magnetic field, leading to the appearance of double peaks near the irradiation angles, ±45°. This crossover phenomenon is attributed to the difference in the pinned states of flux lines between B || c and at the irradiation angles: the CDs crossing at ±45° cannot provide the correlated flux pinning along the c-axis, resulting in the uncorrelated pinned state at B || c.

In situ measurements of transport characteristics in heavy-ion irradiated YBa2Cu3Oy under magnetic field

Abstract We produced splayed columnar defects in a c-axis oriented YBa2Cu3Oy thin film by irradiating 200 MeV Au ions at low temperature (∼80 K ) from two different directions; ±15° with respect to the c-axis. Then the transport critical current density Jc as a function of magnetic field direction θ with respect to c-axis was measured in situ. A symmetric peak for Jc was observed in θ-dependence of Jc around θ=0°.

Critical current density of YBa2Cu3Oy containing inclined columnar defects

Abstract Effects of columnar defects on the critical current density have been investigated by the in situ measurement of current–voltage characteristics for a c -axis oriented YBa 2 Cu 3 O y (YBCO) thin film after the irradiation with 200 MeV Au ions from the direction θ irr =45° off the c -axis. The dependences of the critical current density, J c , on the magnetic field, the magnetic field angle, and the density of columnar defects are discussed. With increasing the density of the columnar defects, B φ , the contribution of the columnar defects to the pinning force per unit volume, ΔF p , increases monotonically. The peak field for ΔF p as a function of the magnetic field seems to be almost independent on B φ .

Critical current density of YBCO films with different configurations of columnar defects in longitudinal magnetic field

Critical current density (J c) properties in longitudinal magnetic fields were investigated for YBa2Cu3O y thin films with columnar defects (CDs), where different configurations of CDs were systematically installed into the films by using heavy-ion irradiation: a parallel configuration of CDs aligned along the c-axis and two bimodal splay configurations composed of CDs crossing at ±θ i relative to the c-axis, where the splay plane defined by the two irradiation angles is perpendicular or parallel to the transport current direction. The unirradiated film under the longitudinal magnetic field shows a J c peak in the magnetic field dependence of J c, which is 1.1 times higher than the self-field J c. For the irradiated films with the parallel CD configuration, on the other hand, the J c is lower than that for the unirradiated film in all magnetic fields and the value of J c decreases with increasing CD density. Such degradation effect by CDs under longitudinal magnetic field was observed even for the bimodal splay configurations. These results are attributed to local meandering of current flow induced by CDs extending through the film thickness, which deteriorates the force-free condition.