Modification of Structure and Properties of Superconducting YBaCuO Crystals by Combined Irradiations
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Modification of Structure and Properties of Superconducting YBaCuO Crystals by Combined Irradiations Elvira M. Ibragimova* and Marquis A. Kirk** * Institute of Nuclear Physics, Ulugbek, 702132, Tashkent, UZBEKISTAN ** Material Science Division, Argonne National Laboratory, Argonne, IL, USA ABSTRACT Effects of electron irradiations in combination with 60Co gamma-quanta on magnetic properties of YBa2Cu3O7-x crystals have been studied to determine the contribution from oxygen defects to the pinning, and the condition of improving the critical parameter, Jc , of the crystals. Irradiation with 300 keV electrons followed by gamma-irradiation makes strong oxygen disorder, causing degradation of SC-transition and appearance of diamagnetic transition at 200 K, but no flux pinning in H>1 Tesla at 77 K. However, it is shown that oxygen defects do contribute substantially to the flux pinning at cation-anion defect clusters in CuO-planes created by moderate fluences of 1 MeV-electrons. A model is suggested for different oxygen and copper defects induced by the irradiations. INTRODUCTION Promising industrial application of high temperature superconductors in radiation fields as magnetic systems for plasma confinement, nuclear fusion reactors and accelerators requires study of effects of different types of high energy particles and electromagnetic radiation on the critical parameters Tc , Hc and Jc of the materials. YBa2Cu3O7-x superconductors have been studied under irradiations because it’s structure contains 1020-1021 cm-3 native oxygen vacancies in CuO-chains, donating holes into superconducting CuO planes, and the number and distribution of the vacancies strongly influence the critical parameters of superconductivity [1]. Many experiments and defect modeling showed that the decrease in Tc and Jc is proportional to the number of displaced atoms (dpa) and depends on the crystal orientation during measurement (along or across c-axis of YBaCuO) [1,2]. However, besides the radiation induced degradation of superconductivity, two remarkable radiation induced effects were observed: a steplike drop of resistivity at 290 K well above Tc (90 K) [3], and flux pinning at strong magnetic fields, causing the magnetisation and Jc enhancement after moderate doses of irradiations with neutrons and ions [2,4]. Both effects are due to some radiation induced modification of structure and superconducting properties of YBaCuO. As fast ions or neutrons produce complicated dimensional defects (cascades and columns), their microstructure was studied by TEM successfully to understand some aspects of magnetic vortex pinning by these irradiation defects [5]. Contribution from point defects to pinning was separated by experiments on annealing of proton and neutron irradiated crystals [2]. Using a variable energy of electron beam, the energy threshold > 400 keV for the pinning by point defects in the copper sublattice was determined in [6]. Nevertheless, the mechanism of strong flux pinning at H >1 T is still unclear. Possible contribution from oxygen defe
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