Pinning and trapped field in MgB₂- and MT-YBaCuO bulk superconductors manufactured under pressure

The relevant pinning centers of Abrikosov vortices in MgB₂-based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgB_x (x>4) phases. The high critical current densities, j _c, of 10⁶ and 10³A/cm² at 1 and 8.5 T, respectively, at 20 K can be achieved in polycrystalline materials (prepared at 2 GPa) containing a large amount of admixed oxygen. Besides, oxygen can be incorporated into the MgB₂ structure in small amounts (MgB_1.5O_0.5), which is supported by Auger studies and calculations of the DOS and the binding energy. The j _c of melt textured YBa₂Cu₃O_7-δ (or Y123)-based superconductors (MT-YBaCuO) depends not only on the perfectness of texture and the amount of oxygen in the Y123 structure, but also on the density of twins and micro-cracks formed during the oxygenation (due to shrinking of the c-lattice parameter). The density of twins and microcracks increases with the reduction of the distance between Y2BaCuO5 (Y211) inclusions in Y123. At 77 K j_c=8•10⁴ A/cm² in self-field and jc=10³ A/cm2 at 10 T were found in materials oxygenated at 16 MPa for 3 days with a density of twins of 2235 per μm (thickness of the lamellae: 45-30 nm) and a density of micro-cracks of 200280 per mm. Pinning can occur at the points of intersection between the Y123 twin planes and the Y211 inclusions. MTYBaCuO at 77 K can trap 1.4 T (38×38×17 mm, oxygenated at 0.1 MPa for 20 days) and 0.8 T (16 mm in diameter and 10 mm thick with 0.45 mm holes oxygenated at 10 MPa for 53 h). The sensitivity of MgB₂ to magnetic field variations (flux jumps) complicates estimates of the trapped field. At 20 K 1.8 T was found for a block of 30 mm in diameter and a thickness of 7.5 mm and 1.5 T (if the magnetic field was increased at a rate of 0.1 T) for a ring with dimensions 24×18 mm and a thickness of 8 mm.