Trapped magnetic field and vortex pinning properties of MgB2superconducting bulk fabricated by a capsule method

We have studied the trapped magnetic field and the critical current density for an MgB 2 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, J c s, were estimated to be 5.5 × 10 4 and 4.2 × 10 4 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 J c was as small as about 30–60% of the measured J c in the self-field which was determined from the magnetization hysteresis loop of small pieces cut from the bulk. The results suggest that an MgB 2 bulk magnet over 3 T can be realized at 20 K by means of the enhancement of both the J c values and the mass density.