ENHANCED FLUX-PINNING VIA CHEMICAL SUBSTITUTION IN BULK SUPERCONDUCTING TL-2212

The flux-pinning characteristics in both pure and doped superconductin g Tl2Ba2CaCu2O8 (referred to as Tl-2212) systems were studied via DC m agnetic hysteresis as well as flux-penetration and flux-expulsion meas urements. At 5 K between 0 and 4.5 T, the 5 at.% Mg-doped Tl-2212 (Tl, Mg-2212) samples displayed enhanced pinning as demonstrated by a fiel d-dependent increase of the magnetic critical current density J(c) by 18-25% as compared with the pure Tl-2212. Excess Mg (10-15 at.%), howe ver, is deleterious. Rietveld refinement of the x-ray diffraction patt ern showed Mg on the Tl sites. AES analysis showed part of the Mg appe aring on grain boundaries. Both Tl-2212 and Tl, Mg-2212 displayed a pe rsisting remanent magnetization M(rem) with a higher value for Tl, Mg- 2212, as well as an asymmetrical behaviour between flux expulsion and flux penetration. The flux-creep activation energies are higher for fl ux expulsion than for flux penetration in both samples; Tl, Mg-2212 di splayed higher individual values. Our results demonstrate an increase in both intrinsic pinning and lock-in transition of flux lines as a re sult of increased density of atomic-size structural defects via Mg dop ing in Tl-2212.