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We report the observation of anisotropic domain-wall propagation in ultrathin magnetic films with perpendicular anisotropy. A controlled density of step edges was introduced which allowed us to quantify its influence on the domain-wall pinning. For a sawtooth arrangement of the step edges the corresponding wall movement resulted in triangular shaped domains. All aspects of this anisotropic domain-wall evolution could be reproduced by a simulation based on a modified Ginzburg-Landau-type soft-spin model.

We have investigated the thermally activated behavior of the in-plane electrical resistivity of Bi 2 Sr 2 CaCu 2 O 8ϩ␦ films for magnetic fields Bр10 4 G applied parallel to the c axis. The activation energy in the vortex-liquid state changes suddenly at a crossover field B cr . The anisotropy reduction generated by oxygen annealing leads to the increase of the crossover field. For BϽB cr , the activation energy U is weakly magnetic-field dependent. For BϾB cr , U(B,T)ϳ(1ϪT/T c0 )/B 1/2 , which corresponds to an entangled vortex fluid. The observation of vortex-liquid entanglement in the presence of relevant quenched disorder is discussed in connection with the relation between the theoretically predicted entanglement length for a clean system and the collective pinning length along the field direction. Our results suggests that, in the case of a pronounced anisotropy and significant collective pinning, the entanglement field B E ϭB cr Ϸ⌽ 0 /␥ 2 s 2 , where s is the interlayer spacing.

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Thin films of the three members of the superconducting series Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 , n = 1, 2, 3, were prepared by diode sputtering. X-ray characterization shows that all the films are single phase and c-axis oriented and in addition they are epitaxially grown. The latter is found by x-ray pole-figure measurements taken with a four-circle diffractometer. These are emphasized in this work. AC susceptibility measurements show that, while the 2201 films are not superconducting until 4 K, the transition temperatures of the 2212 films are 82 K-90 K and of the 2223 films 84 K-89 K.