THE PUZZLE OF TWO DIFFERENT SUB-MICROMETER TUNGSTEN-RICH DEPOSITS IN BULK YBCO: ONE ACTS AS PINNING CENTERS AND THE OTHER DOES NOT

Two types of large grain YBCO samples doped with tungsten oxide, one with platinum and the other without, were produced using a slow cooling process. Observations of the trapped magnetic flux density showed that the flux density of the W-doped, Pt-free samples did not change with W doping levels of up to 2.1 mol%. In contrast, the (W+Pt)-doped samples resulted in a monotonic improvement in trapped magnetic flux density as a function of W doping. Microstructure studies indicate that both types of samples contain profuse sub-micrometer deposits of a W-rich compound. The Pt-free samples contain (W{sub 0.4}Y{sub 0.6})BaO{sub 3} deposits while the (W+Pt)-doped samples contain deposits of a (W{sub 0.5}Pt{sub 0.5})YBa{sub 2}O{sub 6} compound. Both types of deposits are of essentially the same size and have comparable number density. The results are strikingly similar to an earlier experiment in which uranium doped, Pt-free, large grain YBCO also did not show any improvement in trapped magnetic flux density. The U-doped, Pt-free samples contain profuse sub-micrometer deposits of a (U{sub 0.4}Y{sub 0.6})BaO{sub 3} compound, which have been shown to be ferromagnetic. The inability of both the (W{sub 0.4}Y{sub 0.6})BaO{sub 3} and (U{sub 0.4}Y{sub 0.6})BaO{sub 3} sub-micrometer deposits to act as pinning centersmore » in self-field, suggest that this behavior is systematic.« less