Magnetic Flux Pinning in YBa 2 Cu 3 O 7 Films
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MAGNETIC FLUX PINNING IN YBa2Cu3O7 FILMS H. R. KERCHNER,* R. FEENSTRA,* J. 0. THOMSON**, J. R. THOMPSON,** D. K. CHRISTEN,* S. T. SEKULA,* and L. A. BOATNER,* *Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37931-6061 **Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831-6061, and University of Tennessee, Knoxville, TN 37996-1200. ABSTRACT The magnetic hysteresis was studied for YBa2Cu3O7 films of different epitaxial orientations on single-crystal SrTiO3 and KTaO3 substrates. Deposition by coevaporation of Y, BaF2, and Cu was followed by annealing in flowing, wet oxygen at 800-850'C. The critical current density Jc deduced from magnetic hysteresis loops was compared with fourterminal transport-current results. Thermally activated magnetic flux flow was manifest in the audio-frequency data. Jc fits a scaling law similar to one used for describing Nb3Sn and other A15 materials. INTRODUCTION Numerous applications envisioned for the new high-temperature superconductors 2 requires these materials to carry sizable electrical currents, of the order of 105-107 A/cm with little or no loss. An essential feature of such materials is the presence of crystalline defects that can pin magnetic flux lines. We carried out a magnetic hysteresis study of YBa2Cu3O7 thin films in an effort to clarify the presence and nature of such pinning defects. The critical current densities Jc were deduced from magnetic measurements. Our film deposition methods are described in a previously published article [1]. The starting materials, Y, Cu, and BaF2 were vaporized by electron-beam heating and deposited onto single-crystal SrTiO3 and KTaO3 disks, 6-mm in diameter. Deposition was followed by furnace anneal in flowing wet oxygen and finally by cooling in dry oxygen. Magnetic hysteresis was measured by using a vibrating sample magnetometer (VSM) and in a mutual inductance bridge (MIB). The VSM provided a continuous reading of a film's magnetic moment while the applied magnetic field B was slowly swept from B=0 to 8 T and back to zero. The critical current density Jc (A/cm2 ) was then deduced from the difference Am between the magnetic moments (G-cm 3 ) while B was increasing and decreasing, Jc = 15 Am/Na 3 t.
(1)
Here a is the substrate radius and t is the measured film thickness, both in cm. The MIB, previously balanced with no sample in place, yields the ac magnetic moment mac of the film induced by an audio-frequency, applied ac field, Bac, parallel to a much larger applied dc field B. With increasing Bac, mac initially increases linearly and then saturates at mac = Am at sufficiently large Bac. CRITICAL CURRENT DENSITY The critical current densities Jc of the various samples depend strongly on applied magnetic field and temperature. As seen in Fig. 1, the ac magnetic hysteresis results depend on applied field qualitatively the same as four-terminal critical current measurements on another film grown using a nearly identical technique. With both types of measurements, Mat. Res. Soc. Symp. Proc. Vol. 169
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