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

Abstract

The temperature and field dependent dynamical relaxation rate Q(T,H) has been determined for epitaxial c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7/ (Y-123) and HgBa/sub 2/CaCu/sub 2/O/sub x/ (Hg-1212) thin films using an efficient ac susceptibility technique. From a single temperature scan, in which consecutive measurements of /spl chi/' are performed at a number of frequencies in a relatively large ac field H/sub ac/, the frequency dependence of the critical current density J/sub c/(T,H/sub ac/,f) is determined as a function of temperature and ac field and Q(T,H/sub ac/) is subsequently extracted. From measurements of /spl chi/'(H/sub ac/,f) in a slowly ramped dc bias field, both the ac and dc 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 dc bias field fulfils H/sub dc/>2H/sub 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 95 K. Both films exhibit quantum creep of about Q(0)=0.01 in the field range studied.