Self-assembling nano-diameter needlelike pinning centers in YBCO, utilizing a foreign element dopant

Although pinning centers created by irradiation presently produce the highest J_c, it is probable that ultimately these will be emulated by chemical pinning centers. The best pinning centers produced by irradiation nevertheless provide guidelines for desirable morphology of chemical pinning structures. The highest J_c produced earlier in textured HTS was obtained using isotropic high-energy ions produced by fission of ²³⁵U. This so-called U/n process produces pinning centers of diameter 4.5 nm, with an effective length of ∼2.7 νm. Maximum J_c occurs for pinning center density of ∼10¹⁰ cm^-3. We use this as a model for desired chemical pinning centers. Our approach to introducing chemical pinning centers has been to produce precipitates within the HTS containing elements not native to the HTS, and to seek needlelike (columnar) deposits of small diameter. We report here on the formation of needlelike or columnar deposits in textured Y123 containing a dopant foreign to Y123. It serves as a demonstration that self-assembling nanometer diameter columns utilizing a dopant foreign to the HTS system are a feasible goal. These deposits, however, do not fully meet the ultimate requirements of pinning centers because the desired deposits should be smaller. The self-assembling columns formed contain titanium, are ∼500 nm in diameter, and up to 10 νm long. The size and morphology of the deposits vary with the mass of admixed Ti dopant. J_c is decreased for small dopant mass. At larger dopant masses needlelike precipitates form, and J_c increases again. A small range of mass of admixed Ti exists in which J_c is enhanced by pinning. In the range of admixed Ti mass studied in these experiments there is a negligible effect on T_c. Magnetization studies of J_c are also reported.