We report on the systematic magnetization measurements performed on the stoichiometric Gd monopnictides, ( = N, P, As, Sb, and Bi), at temperatures between 1.6 and 350 K in magnetic fields up to 42 T. GdN shows the ferromagnetic behaviour with a Curie temperature . The other samples order antiferromagnetically at the Néel temperatures = 15.9, 18.7, 23.4, and 25.8 K for = P, As, Sb, and Bi, respectively. A spin - flop transition is observed at low temperature for except GdN. Based on molecular field approximation, the nearest-neighbour and next-nearest-neighbour exchange constants and of are calculated using the experimental data. Comparing with Eu monochalcogenides, the other group of S-state rare-earth compounds, the absolute values of and in are anomalously large, and the ferromagnetic RKKY interactions due to semimetallic carriers are very weak. These anomalous properties are discussed with a new theoretical model developed very recently by Kasuya.

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