An efficient AC susceptibility technique to study flux creep in HTS thin films.

The temperature and field dependent dynamical relaxation rate Q(T,H) has been determined for epitaxial c-axis oriented YBa2Cu3O7 (Y-123) and HgBa2CaCu2Ox (Hg-1212) thin films using an efficient ac susceptibility technique. From a single temperature scan, in which consecutive measurements of chi' are performed at a number of frequencies in a relatively large ac field H-ac, the frequency dependence of the critical current density J(c)(T, H-ac,f) is determined as a function of temperature and ac field and Q(T,H-ac) is subsequently extracted. From measurements of chi'(H-ac,f) in a slowly ramped de bias field, both the ac and de field dependence of Q(T,H) can be studied. Q(T,H) for a Hg-1212 thin film is found to be independent of the ac field when the de bias field fulfils H-dc>2H(ac). Compared to conventional magnetic relaxation, M-H loop recording and traditional ac susceptibility techniques, our ac susceptibility method is found to yield a considerably larger amount of information for a given measuring time. The ac susceptibility technique is found to reproduce the characteristic plateau-like temperature dependence of the relaxation rate for the Y-123 system. The relaxation rate for the Hg-1212 thin film shows a monotonic increase with temperature up to a maximum at about 95K. Both films exhibit quantum creep of about Q(0)=0.01 in the field range studied.