ANISOTROPIC GD MAGNETISM IN TLSR2GDCU2O7-DELTA, (PB0.5CU0.5)SR2GDCU2O7-DELTA, AND PB2SR2GDCU3O8+Y

Magnetic and calorimetric measurements reveal antiferromagnetic ordering of Gd3+ ions in the tetragonal compounds TlSr2GdCu2O7-delta and (Pb0.5Cu0.5)Sr2GdCu2O7-delta, as well as in the vacuum annealed orthorhombic compound Pb2Sr,GdCu3O8+y (y congruent to O). The Neel temperatures T-N(Gd) of 2.3-2.4 K of these semiconducting compounds are almost identical to those for several other high-T-c Gd cuprates, with similar double-CuO2-layer structures. Correlation of T-N 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 T-N, the expected magnetic entropy of R 1n(2J + 1) = R 1n8 can be obtained.