Magnetic properties of stoichiometric Gd monopnictides

We report on the systematic magnetization measurements performed on the stoichiometric Gd monopnictides, GdX_p (X_p = 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 T_c = 58 K. The other GdX_p samples order antiferromagnetically at the Néel temperatures T_N = 15.9, 18.7, 23.4, and 25.8 K for X_p = P, As, Sb, and Bi, respectively. A spin-flop transition is observed at low temperature for GdX_p except GdN. Based on molecular field approximation, the nearest-neighbour and next-nearest-neighbour exchange constants J₁ and J₂ of GdX_p are calculated using the experimental data. Comparing with Eu monochalcogenides, the other group of S-state rare-earth compounds, the absolute values of J₁ and J₂ in GdX_p 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.