Pinning in high performance MgB2 thin films and bulks: Role of Mg-B-O nano-scale inhomogeneities

Abstract The comparison of nano-crystalline MgB 2 oxygen-containing thin film (140 nm) and highly dense bulk materials showed that the critical current density, J c , depends on the distribution of Mg-B-O nano-scale inhomogeneities. It has been shown that MgB 2 bulks with high J c in low (∼10 6 A/cm 2 in 0-1 T at 10 K) and medium magnetic fields contain MgB 0.6-0.8 O 0.8-0.9 nano-inclusions, where δ T c or a combined δ T c (dominant) / δ l pinning mechanism prevails, while in bulk MgB 2 with high J c in high magnetic fields ( B irr (18.5 K) = 15  T, B c2 (0 K) = 42.1 T) MgB 1.2-2.7 O 1.8-2.5 nano-layers are present and δ l pinning prevails. The structure of oxygen-containing films with high J c in low and high magnetic fields ( J c (0 Т) = 1.8 × 10 7 А/сm 2 and J c (5 Т) = 2 × 10 6 А/сm 2 at 10 К) contains very fine oxygen-enriched Mg-B-O inhomogeneities and δ l pinning is realized. The results of DOS calculations in MgB 2-x O x cells for x = 0, 0.125, 0.25, 0.5, 1 demonstrate that all compounds are conductors with metal-like behaviour. In the case of ordered oxygen substitution for boron the binding energy, E b , does not increase sufficiently as compared with that for MgB 2 , while when oxygen atoms form zigzag chains the calculated E b is even lower ( E b  = −1.15712 Ry).