Two-dimensional flux pinning in multilayered MgB2/Ni thin films prepared by electron beam evaporation

Multilayered MgB₂/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 MgB₂/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 MgB₂/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 MgB₂/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.