Two-dimensional flux pinning in multilayered MgB 2/Ni thin films prepared by electron beam evaporation

Abstract

Multilayered MgB 2/Ni thin films on Si substrates by electron beam evaporation were fabricated to enhance the critical current density J c in magnetic fields parallel to the surface. The magnetic field dependence of J c of the multilayered MgB 2/Ni thin films for B//surface shows a peak at the matching field. In the field angular dependence of J c, a sharp peak appears at B//surface. These results indicate that Ni layers which have ferromagnetism act as two-dimensional pinning centers in multilayered MgB 2/Ni thin films. The vortex glass–liquid transition temperature T g and the pinning parameter m were evaluated from the E– J characteristics. The slope of the magnetic field dependence of m in the multilayered MgB 2/Ni thin film under the matching field is larger than that over the matching field. In the T g/ T c– B phase diagram, the remarkable enhancement of T g for B//surface appears around the matching field. These features are characteristic behaviors for two-dimensional pinning centers.