SEM, STM/STS and heavy ion irradiation studies on magnesium diboride superconductor

We have presented here the results of SEM and STM/STS investigations carried out on MgB2 superconductor. The SEM pictures show porous surface and well-linked granular structure in which bigger grains (few micrometer size) seem to be agglomeration of smaller, nearly hexagonal grains (size nearly 100 nm). Hexagonal structure of Mg and B layers have been directly observed in atomically resolved STM images. The lattice constants have been determined to be a(Mg) = 3.1 Angstrom, a(B) = 1.7 Angstrom and c = 3.5 Angstrom. Grain boundaries (GB) of width ranging from 50 to 200 Angstrom have been observed. Absence of weak link effects despite wide GBs has been attributed to the metallic nature of the amorphous region of the GB interior as inferred from STS analysis. Irradiation with 200 MeV Ag-107 ions gives tracks of about 65 Angstrom in the bulk of the sample. This is expected to give higher value of critical current density owing to the flux pinning mechanism, which is related to possibility of wide practical application of this material. (C) 2002 Elsevier Science B.V. All rights reserved.