Enhanced flux pinning via chemical substitution in bulk superconducting Tl-2212

The flux-pinning characteristics in both pure and doped superconducting Tl₂Ba₂CaCu₂O₈ (referred to as Tl-2212) systems were studied via DC magnetic hysteresis as well as flux-penetration and flux-expulsion measurements. 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 field-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.%), however, is deleterious. Rietveld refinement of the X-ray diffraction pattern showed Mg on the Tl sites. AES analysis showed part of the Mg appearing on grain boundaries. Both Tl-2212 and Tl, Mg-2212 displayed a persisting 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 flux expulsion than for flux penetration in both samples; Tl, Mg-2212 displayed higher individual values. Our results demonstrate an increase in both intrinsic pinning and lock-in transition of flux lines as a result of increased density of atomic-size structural defects via Mg doping in Tl-2212.