YBaCuO critical magnetic field H_c2 determination from the flow regime dissipation

The direct measurement of H_c2 is difficult owing to its high value if the temperature is not very close to the critical temperature T_c Moreover, because of the very progressive decrease of the magnetization between the irreversibility field (around 1T at 77K) and H_c2, the accuracy on H_c2 direct measurements is poor. We present here an H_c2 determination in YBaCuO ceramics and films based on the flow regime dissipation. We discuss the meaning of both low field measurements (below and around 1T) and medium field measurements (around 5T) where the flow behaviour is fully developed. We show that it is possible to observe a dissipation depending linearly on both magnetic field and current density in this last case. The value of H_c2 deduced from this dependence is around 50 T at 77K, and H_c2 exhibits a linear behaviour with T with a slope of 4.0± 0.3 T.K ^{- 1}. In the low field range the dissipation is mainly due to microstructural effects, leading to a non linear dependence of the dissipation both versus magnetic field and current density.