Modification of Critical Current Density Anisotropy in High-Tc Superconductors by Using Heavy-Ion Irradiations

The critical current density J(c), which is a maximum value of zero-resistivity current density, is required to exhibit not only larger value but also lower anisotropy in a magnetic field B for applications of high-T-c superconductors. Heavy-ion irradiation introduces nanometer-scale irradiation tracks, i.e., columnar defects (CDs) into high-T-c superconducting materials, which can modify both the absolute value and the anisotropy of J(c) in a controlled manner: the unique structures of CDs, which significantly affect the J(c) properties, are engineered by adjusting the irradiation conditions such as the irradiation energy and the incident direction. This paper reviews the modifications of the J(c) anisotropy in high-T-c superconductors using CDs installed by heavy-ion irradiations. The direction-dispersion of CDs, which is tuned by the combination of the plural irradiation directions, can provide a variety of the magnetic field angular variations of J(c) in high-T-c superconductors: CDs crossing at +/-theta(i) relative to the c-axis of YBa2Cu3Oy films induce a broad peak of J(c) centered at B || c for theta(i) < +/- 45 degrees, whereas the crossing angle of theta(i) >= +/- 45 degrees cause not a J(c) peak centered at B || c but two peaks of J(c) at the irradiation angles. The anisotropy of J(c) can also modified by tuning the continuity of CDs: short segmented CDs formed by heavy-ion irradiation with relatively low energy are more effective to improve J(c) in a wide magnetic field angular region. The modifications of the J(c) anisotropy are discussed on the basis of both structures of CDs and flux line structures depending on the magnetic field directions.