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In-field critical current density Jc properties in high-Tc superconductors are significantly affected by the morphology of nano-sized crystalline defects acting as pinning centers (PCs): the optimization in the shape and spatial distribution of PCs is one of important issues on development of REBa2Cu3Oy (REBCO) coated conductors (CCs) for superconducting magnet applications and even on approaching the value of depairing current density Jd, at which Cooper pairs are broken up. The morphology of PCs can be tuned by the energies of ion irradiation to high-Tc superconductors. Columnar defects (CDs), which are produced by swift heavy ion irradiation, work as strong PCs in a certain magnetic field orientation where CDs is parallel to the magnetic field. By contrast, randomly distributed spherical defects (SDs), which are generally formed by proton irradiation, have morphology with no preferential orientation for flux pinning, leading to the isotropic pinning force against any orientation of magnetic field. Furthermore, spherical irradiation defects arranged in serial can be also induced by tuning the electronic stopping power of the heavy-ion beam to high-Tc superconductors. The unique irradiation defects can be expected as PCs combining both flux pinning properties of CDs and randomly distributed SDs. In this work, we systematically investigated flux pinning properties of SDs arranged in serial for REBCO films and CCs with and without Y2O3 as APCs, where the irradiation defects were prepared by 44 MeV Au-ion irradiations parallel to the c-axis. The in-field Jc was enhanced by the introduction of the SDs, the increment in Jc depends on the kind of sample. In particular, the Jc was more pushed up with the fluence for CCs with APCs, which was the same tendency as the introduction of continuous CDs in our previous work. However, the increase in Jc with the fluence was smaller than that for the continuous CDs. The morphology of SDs is more advantageous for suppressing the flux creep than continuous CDs, which provides extra pinning effect. The introduction of the SDs arranged in serial suppressed the flux creep rate for the low fluence, but the flux creep rate increased for a large amount of the fluence. This result suggests that the series SDs behaves as linear defects, promoting the hopping motion of flux lines.
- This work was supported by KAKENHI (JP22K04207) from the Japan Society for the Promotion of Society and was performed under the Common-Use Facility Program of Japan Atomic Energy Agency.
