Tuning critical current density properties of YBa2Cu3O y thin films under longitudinal magnetic field by using heavy-ion irradiation

We attempted to improve the critical current density J _c under a longitudinal magnetic field (LMF) by using columnar defects (CDs) tilted toward the transport current, where the CDs were produced with 80 MeV Xe ions at an incident angle(s) up to φ _i = 87.5° relative to the c-axis of YBa₂Cu₃O _y thin films. The formation of CDs extending throughout the film was confirmed by cross-sectional transmission electron microscopy. The CDs tilted at φ _i = 87.5° weakened the magnetic field dependence of J _c under the LMF, providing a J _c higher than that of the unirradiated film in a high magnetic field. This is the first study to demonstrate that CDs and/or irradiation defects contribute to flux pinning under the LMF in high-T _c superconductors. In addition, the J _c under LMF was sensitive to the tilt angle of the CDs: the J _c was reduced by CDs tilted not only at φ _i = 0°, but also at a slightly larger angle relative to the ab-plane, i.e. φ _i = 85°. The introduction of CDs rather disturbs the LMF effect because of the large volume passing through the film thickness, unless the tilt angles of the CDs are within the narrow angular region effective for the LMF effect.