Effect of flux creep on<i>I</i>_<i>c</i>measurement of electric transport

Abstract

The influence of flux creep on Ic measurement was studied by numerically solving the nonlinear flux creep equation at different dI/dt, Vc and n*, where dI/dt is the sweeping rate of the applied current (I), Ic, Vc and n* are the critical current, the criterion and the material parameter, respectively. It is shown that the V-I curve consists of two parts converging at Ip, Vp and (dI/dt)p at which the current fully penetrates the sample. In the segment where I<Ip, the V-I curve is parabolic, V∝I2, and independent of n*, whereas in the segment where I>Ip, the curve is a power law, V∝In*, reflecting the material equation. It is suggested that the appropriate region in the V-I curve to determine n* is where I>Ip. Based on the V-I curve, it is concluded that if dI/dt>(dI/dt)p, Ic decreases with increasing dI/dt. On the other hand, if dI/dt<(dI/dt)p, Ic is independent of dI/dt and is therefore suitable. The three critical parameters ((dI/dt)p, Vp and Ip) are dependent on each other. The parabolic V-I relation can be observed in the giant flux creep state (small n*), whereas in the critical state (large n*), V in the parabolic V-I relation is too small to be detected by a standard voltmeter. This also indicates that the pulsed method may underestimate Ic in the case of high temperatures or in strong applied fields, but it will not affect Ic in the case of low temperatures, weak applied fields and strong pinning.