PHASE-DIAGRAM OF SUPERCONDUCTING UPT3 BY THE MAGNETOCALORIC EFFECT

Measurements of the magnetocaloric effect are reported which elucidate the phase diagram of superconducting UPt3 in a magnetic field B perpendicular to the c-direction. In particular, the transition between low-field (C) and high-field (B) phases is observed as a clear step in DELTAT/DELTAB versus B. The data are complemented by specific-heat measurements on the same sample. Our results confirm the previous assumption based on ultrasonic measurements of a thermodynamic phase transition between C and B. A consistent thermodynamic analysis is possible with four second-order phase transition lines meeting at a tetracritical point (T*, B*) for B perpendicular-to c, as has been suggested before. Based on the analysis of our thermodynamic data for B parallel-to c, the existence of a tetracritical point also for B parallel-to c is possible but cannot be unequivocally proven.