  In the present study, La-doped MgB_2 superconductors with different doping
level (Mg1-xLaxB2; x=0.00, 0.01, 0.03 & 0.05) have been synthesized by
solid-state reaction route at ambient pressure. Effect of La doping have been
investigated in relation to microstructural characteristics and superconducting
properties, particularly intragrain critical current density (Jc). The
microstructural characteristics of the as synthesized Mg(La)B2 compounds were
studied employing 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 Jc
through magnetic measurements on the fine powdered version of the as
synthesized samples reveal that Jc of the samples change significantly with the
doping level. The optimum result on Jc is obtained for Mg0.97La0.03B2 at 5K,
the Jc reaches ~1.4x107A/cm2 in self field, ~2.1 x 106A/cm2 at 1T, ~2.5 x
105A/cm2 at 2.5T and ~1.8 x 104 A/cm2 at 4.5T. The highest value of intragrain
Jc in Mg0.97La0.03B2 superconductor has been attributed to the inclusion of
LaB6 nanoparticles which are capable of providing effective flux pinning
centres.
