Relaxation measurements of flux pinning by a double pulsed field

We have developed a novel method to measure the relaxation time of flux pinning of type-II superconductors by using a double pulsed field. The magnetic fields are applied to the specimen two times successively. The time between two pulsed fields, t, is adjustable in a time range of milliseconds to any longer time by using twin capacitor banks. The magnetization curve by the first field starts from zero and shows a residual magnetization M₁. In the second field, the magnetization starts from M₂, which is smaller than M₁ due to relaxation. But as the field increases and in high fields, the magnetization curve is identical with the first one and comes back again to the same residual magnetization M₁. We can measure M₁ and M₁ - M₂ precisely and obtain a t-dependence of ( M₁ - M₂) M₁. This method is applied to study the relaxation measurement of YBCO samples and the result is consistent with the theory.