Effect of flux creep on I-c measurement of electric transport

The influence of flux creep on I, measurement was studied by numerically so lving the nonlinear flux creep equation at different dI/dt, V-c and n*, whe re dI/dt is the sweeping rate of the applied current (I), I-c, V-c. and n* are the critical current, the criterion and the material parameter, respect ively. It is shown that the V-I curve consists of two parts converging at I -p, V-p and (dI/dt)(p) at which the current fully penetrates the sample. In the segment where I < I-p, the V-I curve is parabolic, V <proportional to> I-2, and independent of n*, whereas in the segment where I > I-p, the curv e is a power law, V proportional to I-n*, reflecting the material equation. It is suggested that the appropriate region in the V-I curve to determine n* is where I > I,. Based on the V-I curve, it is concluded that if dI/dt > (dI/dt)(p), I-c decreases with increasing dI/dt. On the other hand, if dI/ dt < (dI/dt)(p), I-c is independent of dI/dt and is therefore suitable. The three critical parameters ((dI/dt)(p), V-p and I-p) are dependent on each other. The parabolic V-I relation can be observed in the giant flux creep s tate (small n*), whereas in the critical state (large n*), V in the parabol ic V-I relation is too small to be detected by a standard voltmeter. This a lso indicates that the pulsed method may underestimate I-c in the case of h igh temperatures or in strong applied fields, but it will not affect I-c in the case of low temperatures, weak applied fields and strong pinning.