MAGNETIC AND CALORIMETRIC STUDIES OF ANOMALOUS PR ANTIFERROMAGNETIC ORDERING IN ORTHORHOMBIC PB2SR2PRCU3O8

Magnetic measurements on vacuum-annealed orthorhombic Pb2Sr2PrCu3O8+y (y almost-equal-to 0) reveal an anomalous Pr antiferromagnetic ordering at T(N)(Pr) congruent-to 6 K with transition onset up to 11 K. This Pr-2213 compound shares common structural features with the tetragonal Pr-123 compound PrBa2Cu3O6, which also undergoes a Pr ordering at 10 K. However, oxygenated pseudotetragonal Pb2Sr2PrCu3O8+y (y almost-equal-to 1) exhibits only low transition onset around 3 K, while orthorhombic PrBa2Cu3O7 has a higher T(N)(Pr) value of 17 K. Complementary studies by adiabatic calorimetry on Pb2Sr2PrCu3O8 yield a specific-heat anomaly with a peak near the magnetically derived T(N)(Pr), with onset around 11 K. A large heavy-fermion-like linear term in the specific heat prevails as often observed in other Pr-containing high-T(c) systems. The magnetic entropy DELTAS(m) below 11 K onset is relatively small, only 13% of R ln3 for Pr3+ with a quasitriplet ground state, implying a quasi-2D nature of the magnetic ordering with a strong 2D short-range magnetic correlation which persists to higher temperatures in this extremely anisotropic structure.