Flux Pinning Centers in ${\rm MgB}_{2}$ Thin Films Prepared by an Electron Beam Evaporation Technique

We prepared MgB
₂
thin films by an electron beam evaporation technique and found that either vacancies and/or B substitutions on Mg sites worked as effective pinning centers in MgB
₂
. We also found that the grain boundaries formed by the columnar grains work as effective pinning centers, by measuring transport
J

_c
's by changing the relative angle between grain boundaries and the applied magnetic field. We also found that introduced MgO nano-particles in MgB
₂
work as effective pinning centers.