LAUSR

Abstract We have studied the thallium fluoride (TlF 3 ) doping and AC field dependence of the weak links behaviour and AC susceptibility of Bi 1.8−x Tl x Pb 0.35 Sr 2 Ca 2 Cu 3 O y F 3x superconductors with range of x = 0, 0.05, 0.10 and 0.15. The estimated value of the peak temperature ( T p ) from the derivative of the resistivity curve ( dρ / dT ) is increased with TlF 3 doping at 0 and 0.5 teslaand attest on both intragranular and weak links transition, while the full width at half maximum ( FWHM ) value is decreased. The susceptibility measurements χ′(χ″) show that as the magnetic field increases the intra and intergranular AC loss peaks shift to lower temperature with broadening. It was found that the onset temperature of diamagnetism T c is increased with TlF 3 doping. However, the intergranular peak temperature T pj decreased with increasing magnetic field and TlF 3 amount. This behaviour was explained by the poor connectivity between the grains caused by the presence of weak links or secondary phases. The calculated volume fraction of the grains ( f g ) was found to improve with increasing TlF 3 doping and AC magnetic field. Furthermore, the estimated values of the maximum of the extracted matrix susceptibility ( χ m , max ″ ) by using a critical state model proved that the intergranular losses are minimized with TlF 3 doping. On the other hand, the obtained results shown that the coupling between the grains is rather weak than the coupling within the grains and attested of the improvement of superconducting parameters in the doped samples.