Trapped magnetic field and vortex pinning properties of MgB2 superconducting bulk fabricated by a capsule method

We have studied the trapped magnetic field and the critical current density for an MgB2 superconducting bulk disc fabricated by a capsule method. The maximum trapped magnetic fields at the centre of bulk surfaces 20 and 30 mm in diameter were, respectively, 1.43 T at 13.4 K and 1.50 T at 16.4 K. The critical current densities, Jcs, were estimated to be 5.5 × 104 and 4.2 × 104 A cm−2 at 20 K for the 20 and 30 mm diameter bulks, respectively, from the measured trapped field at the top surface, by using the analytical equation. The estimated Jc was as small as about 30–60% of the measured Jc in the self-field which was determined from the magnetization hysteresis loop of small pieces cut from the bulk. The results suggest that an MgB2 bulk magnet over 3 T can be realized at 20 K by means of the enhancement of both the Jc values and the mass density. (paper)