Abstract
Magnetic and calorimetric measurements reveal antiferromagnetic ordering of Gd3+ ions in the tetragonal compounds TlSr2GdCu2O7-δ and (Pb0.5Cu0.5)Sr2GdCu2O7-δ, as well as in the vacuum-annealed orthorhombic compound Pb2Sr2GdCu3O8+y (y0). The Néel temperatures TN(Gd) of 2.3-2.4 K of these semiconducting compounds are almost identical to those for several other high-Tc Gd cuprates, with similar double-CuO2-layer structures. Correlation of TN with the Gd-Gd distance in various Gd cuprate systems indicates a highly anisotropic magnetic coupling. By taking into account another common feature in terms of a broad Schottky-like specific heat below TN, the expected magnetic entropy of ln(2J+1)=R ln8 can be obtained. © 1994 The American Physical Society.