Magnetization studies of the effects of oxygen deficiency [delta] on the superconductive properties of aligned YBaℓCuℓOℓℓ[delta] materials

Author

Jorge G. Ossandon

Date of Award

12-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R. Thompson

Abstract

Magnetically aligned samples of sintered YBa₂Cu₃O_7-δ were used to test the effects of oxygen-deficiency 5 (with δ ≤ 0.2) on the superconductive magnetization M, critical current density J_c, irreversibility field B_irr, upper critical field H_c2, coherence length ξ, condensation energy F_c, London penetration depth λ and related properties as functions of temperature T and applied magnetic field H || c. In selected cases, studies were also made with H || ab. The open porosity and granularity of the material allowed rapid and homogeneous oxygenation. The 0-content was monitored in situ by Thermo-Gravimetric Analysis. We found no significant enhancement of intragrain J_c with chain-site O-defects. With few exceptions, maximum J_c occurred at full oxygenation. This implies that chain-site O-defects are not strong or effective pinning centers over most of the field-temperature regime investigated. Except for T_c, which was practically independent of δ within the interval 0 ≤ δ ≤ 0.11 (so called "T_c-plateau"), most properties such as J_c, F_c, H_c2, B_irr(T), λ and ξ were strongly and continuously influenced by the O-deficiency. The observed abnormal magnetization ("fishtail" or "bowtie" effect) with H || c was weak at low T but became more pronounced as T and δ increased. No abnormal magnetization was detected with H||ab. As oxygen was removed, B_irr(T) and H_c2(T) separated and both lines shifted to lower T and lower H. Moreover, B_irr was strongly correlated with J_c at low temperature. Determination of the thermodynamic critical field H_c yielded condensation energies F_c(δ) that suffered a strong reduction with increasing δ. As predicted by a simple, single-site pinning model, a good correlation was found between J_c(δ) and the product F_cξ_ab (where ξ_ab is the coherence length in the a-b plane). This correlation was corroborated by flux creep measurements. The results indicate that O-defects weaken the energy barrier of the existing pinning sites and have a negative overall effect on the capacity of YBa₂Cu₃O_7-δ to carry loss-free currents.

Recommended Citation

Ossandon, Jorge G., "Magnetization studies of the effects of oxygen deficiency [delta] on the superconductive properties of aligned YBaℓCuℓOℓℓ[delta] materials. " PhD diss., University of Tennessee, 1991.
https://trace.tennessee.edu/utk_graddiss/11193