Ybco Thin Films in Crossed AC and DC Fields

We report the effects of superposed ac fields on the dc magnetization in YBCO thin films. The ac fields are seen to result in a much more pronounced decrease of the dc moment as compared to the case of bulk samples, with complete collapse of the irreversible moment for ac fields of about 20Oe. The decrease of the moment is seen to be consistent with the predictions of the critical state models for thin films in perpendicular geometry. The magnetic relaxation is seen to increase rapidly with applied ac fields and appears to take place in bursts at short times.