Unusual Change in Columnar Defect Morphology in YBCO upon Annealing
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Unusual Change in Columnar Defect Morphology in YBCO upon Annealing Y. Yan*, M. A. Kirk*, A. Petrean** and L. Paulius** *Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 **Department of Physics, Western Michigan University, Kalamazoo, MI 49008 ABSTRACT We show evidence from transmission electron microscopy of an unusual change in columnar defect morphology in YBCO upon annealing to 600°C. The disappearance of the nanotwinned, but not the larger scaled thermal twinned, structure is found. A removal of the large fluctuations in the diameter of the amorphous column, preserving a narrow continuous column, is surprisingly discovered. Correlations with magnetization measurements demonstrate the greater efficiency of vortex pinning at 77 K by the annealed defect structure. INTRODUCTION High-energy heavy-ion irradiation is a well-established method for the controlled introduction of magnetic vortex pinning defects in high TC superconductors. Each ion produces a columnar track of amorphous material with fluctuating diameter depending on the incident ion Z and energy [1]. Additionally, in YBCO, these columnar defects can produce local oxygen reordered regions [2], which at higher columnar defect density can become connected in a nanotwin network [3]. Thermal stability of columnar defects has been investigated in BSCCO [4], but not in YBCO. This paper will report an annealing study of columnar defects in YBCO, correlating changes in microstructure with those in the critical current density. A more complete report on these experiments, especially focussing on measurements of the superconducting properties, is in preparation [5]. EXPERIMENTAL PROCEDURES Samples used in this study were single crystals of YBCO grown by a self-flux method [6], and subsequently annealed for 10 days in flowing oxygen at 420°C yielding a TC of 93.5 K. Several crystals were irradiated parallel to the c-axis with 3.9 GeV 197Au29+ ions at the National Superconducting Cyclotron Laboratory (Michigan State University). The calculated (TRIM) ion range of 100µm easily exceeded the typical crystal thickness of 30µm, insuring that the ions pass through the entire crystal. The ion fluence was 5x1010 cm-2, which was confirmed by the track density measured by TEM. At this fluence the density of columnar defects equals the magnetic vortex density at an applied field of 1 T, also known as the matching field, Bø. The annealing process consisted of heating the crystal for one hour in flowing oxygen at succeeding temperatures of 400, 450, 500,550,575, and 600°C. Due to oxygen loss during the last 2-3 anneals, an additional anneal of 5 days at 420°C in flowing oxygen was performed to fully reoxygenate the crystal. An unirradiated crystal also went through this annealing schedule. Magnetization measurements were made following each anneal. TEM measurements (which require destroying the crystal) were performed on the irradiated and annealed crystal following the final anneal and magnetization measurement. A second irradiated crystal was divided i
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