CROSSOVER FIELDS IN GRAIN-BOUNDARY FLUX PINNING IN MAGNESIUM DIBORIDE FILMS WITH COLUMNAR GRAINS

Abstract

We studied temperature and magnetic-field dependence of grain boundary pinning effect in MgB 2 films with columnar grains by measuring the angular dependence of the resistivity and the critical current density (Jc). MgB 2 films grown by using a hybrid physical chemical vapor deposition method under appropriate conditions exhibit a peculiar columnar growth with their columns oriented along the c axis. The pinning effect by grain boundaries was manifested in the comparable or even higher J c for magnetic fields (H) parallel to the c axis (H‖c) than that for fields parallel to the ab plane (H‖ab) below certain temperature-dependent crossover fields. The crossover field, B cr , was around 1.6 T at 5 K and decreased to around 0.9 T at 30 K depending weakly on the sample characters, and B cr lay well below the upper critical fields in the phase diagram. Above B cr , J c for H‖c decreased very rapidly. The vortex spacing at B cr was closely correlated with the temperature dependence of the penetration depth, indicating that the degree of the vortex-vortex overlap is an important parameter to determine the number of vortices allowed per unit length of grain boundaries. Thus, B cr was thought to demarcate the accommodation capability of flux lines by the grain boundaries, and a rapid decrease of J c above B cr was ascribed to flux motion of the less strongly pinned vortices located away from the grain boundaries.