Critical state analysis in bulk MgB2 by means of a quantitative magneto-optical technique

This paper is aimed at investigating the superconducting properties of the MgB2 new compound (bulk) with particular emphasis on the correlation and/or competition between microstructural and magnetic granularity. Standard magnetic characterizations are reported in order to provide background information about the material under study at higher magnetic fields. At low fields, two samples of different sizes are analysed with the magneto-optical (MO) technique. The central investigation method is an extended quantitative MO analysis coupled with microstructural analysis. The main results consist in finding out that, however large the sample, and notwithstanding the structural granularity, at low fields the magnetic flux penetrates in an apparent single-crystal-like fashion, if macroscopic cracks are not present. The local current distribution inside the small sample is calculated as a function of field at different temperatures by means of an inverted two-dimensional Biot-Savart model, From the results it emerges that the main open question in MgB2 consists in understanding the nature of the vortices entering the grain boundaries as well as investigating whether these grain boundaries can also act as pinning centres. Some hints on this subject are preliminarily provided by the reported experimental results.