Various effects of aliphatic amino acids on the critical current of the MgB₂ superconductor

Carbon-doped MgB₂ polycrystalline bulk samples were prepared by reacting mixtures of Mg and B powders, with and without 0.5 wt.% of the amino acids glycine, alanine, valine, leucine or isoleucine. The amount of carbon introduced into the MgB₂ structure depends on the amino acid and is surprisingly higher for glycine, which has the lowest carbon content among the aliphatic amino acids. The critical temperatures of samples doped with all kinds of additives were lower than for the undoped sample. The critical current density was influenced by the additives to different extents, alanine, valine and leucine yielding the best results. The pinning force is also larger for these additives at T < 30 K. An analysis of the normalized flux pinning force behavior leads to the conclusion that point pinning is predominant in all samples.