J_c Anisotropy and the Columnar-grain Texture in MgB₂ Thin Films

Abstract

We prepared MgB₂ thin films using an electron-beam evaporation technique. TEM observation of the detailed microstructure for the films with and without substrate-spin during deposition revealed that the MgB₂ columnar grains align in the direction of B flux. J_c of the film is higher in perpendicular fields than in parallel fields, whereas B_c2 and B_irr have the opposite anisotropy. A detailed study on the relationship between the J_c anisotropy and the texture geometry of MgB₂ columnar grains revealed that J_c becomes maximal where the magnetic field direction aligns in the direction of the MgB₂ columnar-grain boundaries. The results indicate conclusively that the columnar-grain boundaries in MgB₂ act as effective pinning centers.