Measurement of critical current for bulk superconductors by transport method

The method of critical current measurement in a magnetic field using a DC transport current has been developed for bulk superconductors. The measurement of the critical current, I_c, up to 50 A at a temperature of 77 K became possible by reducing thermal and mechanical stresses caused in the sample due to rapid cooling from the room temperature to 77 K and a electromagnetic force (Lorentz force), etc. The critical current of 50 A is reduced into 14,000 A/cm² of critical current density, J_c, with a cross section of 0.0035 cm². The magnetic field dependence of J_c was investigated from 1 T to 10 T for (Nd_0.33Eu _0.38Gd_0.28)Ba₂Cu₃O_y bulk superconductors. The magnetization measurement was performed for the sample cut out from the same part of the same crystal as that used for the transport measurement of I_c, from which the critical current density was also estimated. In low magnetic fields, the magnitude of critical current density obtained by the transport method, JcT, was larger than that estimated from the magnetization, JcM. This result reflects the difference of criterion to determine the value of JcT and JcM. In higher magnetic fields, however, it was observed that JcT was smaller than JcM and the irreversibility field estimated from JcT-B curve is slightly lower than that given by the magnetic hysteresis. When the external magnetic field was applied, anomalous voltage peaks in the I-V curve were observed below IcT, which may be caused by the sample slightly moving by the Lorentz force.