EFFECT OF HELIUM ION IRRADIATION AT MEDIUM ENERGIES ON THE ANISOTROPYOF CRITICAL CURRENTS IN YBA2CU3O7-DELTA THIN-FILMS

The effect of 350 keV He+ ion irradiation with fluences Phi (ions cm(- 2)) on the anisotropy of the critical current density j(c)(Theta, H, P hi) of c-axis oriented YBa2Cu3O7-delta thin films is reported. Here, T heta is the angle between the c-axis and the direction of the external magnetic field mu(0) H less than or equal to 5 tesla. An almost isotr opic decrease of critical current density due to the ion bombardment u p to fluences Phi less than or equal to 4.0 x 10(14) cm(-2) is observe d. This result is different from that found for irradiation with heavy ions at high energies, for which an anisotropic change of the critica l current density is obtained with a maximum near the incident directi on of the ion beam due to the formation of amorphous columnar defects. The angular dependence of the normalized current densities j(c)(Theta , H, Phi)/j(c)(0, H, Phi) exhibits a universal relation with a maximum at Theta = 90 degrees. Significant deviations from j(c)(0, H, Phi) to wards larger values, however, occur only for angles above 40 degrees i n contrast to the predictions of standard models. Similarly, the field dependence of the normalized current densities j(c)(Theta, H, Phi)/j( c)(Theta, 0, Phi) is found to be unaffected by the ion bombardment. Co pyright (C) 1996 Elsevier Science Ltd