Flux pinning in columnar-structured and single crystalline MgB2 films

Flux pinning effect have been investigated in columnar-structured and single crystalline MgB2 films. Columnar grain boundary is acted as a strong pinning source in MgB2 films. An abnormal double-peak behavior in Fp(B) was observed in columnar-structured MgB2 films. We have investigated the flux pinning effect of columnar grain boundary in columnar-structured and single crystalline MgB2 films. The MgB2 films with columnar structure showed much higher Jc than that of single crystalline thin film, and sample having smaller grain size had a higher Jc in high magnetic fields. At 5 K, the MgB2 film with grain size of 460 nm showed an abnormal double-peak behavior in pining force density, Fp(B), caused by competition of different types of pinning sites, such as planar defects and point defects. Field dependences of Fp in columnar-structured films suggest that the columnar grain boundary is a strong pinning source in the MgB2 film and it plays a crucial role in enhancing Jc over a wide range of magnetic fields and temperatures.