Superconducting transition of (RE)xBa1-xCuOy (RE: rare earth or Y; 0.25≦x≦0.40) has been studied in magnetic fields up to 6T. Two-step transitions due to higher-Tc and lower-Tc phases have been observed, depending on the RE ion radius. Anomalous hysteresis of the resistivity in the field sweep has been found in the transition region of the lower-Tc phase. This is explained by assuming the exchange interaction between localized Cu spins and conduction electrons to be negative and taking account of the Jaccarino-Peter effect.
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