Eur. Phys. J. B 26, 297-300 (2002)
https://doi.org/10.1140/epjb/e20020092

MO analysis of macroscopic supercurrent flow in MgB₂

¹ INFM - U.d.R Torino-Politecnico; INFN - Sez. Torino; Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Torino, Italy
² Edison S.p.A, Foro Buonaparte 31, 20121 Milano, Italy

Corresponding author: ^a mezzetti@polito.it

Received: 31 May 2001
Revised: 5 December 2001
Published online: 15 April 2002

Abstract

We present results from an extended magneto-optical (MO) analysis of two samples cut from high-density pellets of MgB₂. The first sample was studied in order to show that no matter how large the sample is and despite the bulk granularity, the material enters into a critical state in a crystal-like fashion. The second sample was chosen for the quantitative analysis. A numerical approach based on an inverted 2D Biot-Savart model was used to calculate the current paths across the homogeneous polycrystalline bulk, as well as in the vicinity and across some morphological defects. Local current densities in the homogeneous part were estimated as a function of the applied magnetic field at different temperatures, in three regimes: below full penetration, at full penetration and above full penetration, respectively. A hypothesis of interpretation of the apparent absence of magnetic granularity inside the polycrystalline microstructure is presented. It is related to a critical state likely reached by a network of strongly coupled Josephson junctions.