Effect of La doping on microstructure and critical current density of MgB2

In the present study, La-doped MgB₂ superconductors with different doping levels (Mg_1-xLa_xB₂; x = 0.00, 0.01, 0.03 and 0.05) have been synthesized by the solid-state reaction route at ambient pressure. Effects of La doping have been investigated in relation to microstructural characteristics and superconducting properties, particularly intragrain critical current density (J_c). The microstructural characteristics of the as-synthesized Mg(La)B₂ compounds were studied employing the transmission electron microscopic (TEM) technique. The TEM investigations reveal inclusion of LaB₆ nanoparticles within the MgB₂ grains, which provide effective flux pinning centres. The evaluation of intragrain J_c through magnetic measurements on the fine powdered version of the as-synthesized samples reveal that J_c values of the samples change significantly with the doping level. The optimum result on J_c is obtained for Mg_0.97La_0.03B₂ at 5 K J_c reaches ~1.4 × 10⁷ A cm^-2 in self-field, ~2.1 × 10⁶ A cm^-2 at 1 T, ~2.5 × 10⁵ A cm^-2 at 2.5 T and ~1.8 × 10⁴ A cm^-2 at 4.5 T. The highest value of intragrain J_c in the Mg_0.97La_0.03B₂ superconductor has been attributed to the inclusion of LaB₆ nanoparticles, which are capable of providing effective flux pinning centres.