Nanostructural inhomogeneities acting as pinning centers in bulk MgB₂ with low and enhanced grain connectivity

Regularly distributed structural inhomogeneities in the MgB₂ matrix, such as nano-areas with a high concentration of boron (MgB_x) and impurity oxygen (Mg–B–O nano-layers or inclusions), are observed in all materials independently of the preparation method, pressure (0.1 MPa–2 GPa) and temperature (600–1100 ^°C), and in materials with different connectivity (18–98%) and density (55–99%). Such inhomogeneities can act as pinning centers in MgB₂ because the variation of their size and distribution are well correlated with variations of the critical current density, j_c. The decrease in size of MgB_x inclusions, the transformation of 15–20 nm thick Mg–B–O nano-layers into separated inclusions, and the localization of impurity oxygen are accompanied by an increase in critical current density in low and medium magnetic fields. The efficiency of these defects is evidenced by a shift from grain-boundary pinning to point pinning. (paper)