RF SURFACE-RESISTANCE OF A MAGNETICALLY ALIGNED SINTERED PELLET OF YBA2CU3O7

rf (radio frequency) properties of polycrystalline ceramic material are substantially inferior to those of the best microtwinned crystals. At 6 GHz and 77 K, the best surface resistance values for ceramics are ∼20 mΩ, compared to <0.5 mΩ for high-quality crystals. The microwave resistance is observed to increase at higher rf fields for ceramics, e.g., two orders of magnitude between 0.1 and 10 Oe, but remains low for crystals. A possible reason for the inferior properties in randomly oriented polycrystalline ceramics is related to the anisotropy of the superconducting properties; crystals aligned unfavorably with the sampling rf field could be responsible for the high resistance. Another possibility is related to the difficulty of carrying current across the grain boundaries due to weak links arising from second phases, impurities, cracks, etc., at the boundaries. To elucidate the contribution from these two potential problem sources, we have measured the rf properties at 6 GHz of an oriented polycrystalline ceramic pellet prepared from a suspension of high purity powder in a 4-T magnetic field. Samples were characterized by x-ray diffraction, light microscopy, transmission electron microscopy, x-ray rocking curves, and x-ray pole figure studies, indicating a high degree of alignment, although not as complete as in epitaxial thin films. At liquid He temperature, the surface resistance is 28 times lower when the c axis is perpendicular to the plane in which rf currents flow than when the c axis is in the plane. At 77 K, the surface resistance is 3 mΩ, a significant improvement over the properties of the best randomly oriented material reported. At 4 K the resistance improves to 0.3 mΩ. These resistances were all measured with rf fields below 0.1 Oe, but increased by one order of magnitude when the rf field was increased to 10 Oe, as in the polycrystalline material. Our results indicate that while the low-field rf behavior is strongly improved by orientation, the high-field behavior is dominated by the poor current carrying capacity across weak links.