The antiferromagnetic states of GdB 6 are investigated both theoretically and experimentally. A mean-field model is introduced which predicts the coexistence of magnetic and displacement waves starting from isotropic exchange and magnetic ions in a harmonic potential. It provides a coherent interpretation of the puzzling features of GdB 6 magnetic order, in particular the first-order magnetic transition at T N and the two successive antiferromagnetic states. X-ray scattering experiments show ͗ 1 2 0 0 ͘ , ͗ 1 2 1 2 0 ͘ , and ͗ 1 4 1 4 1 2 ͘ satellites consistent with displacement waves imposed by the magnetic ͗ 1 4 1 4 1 2 ͘ structures. In the high-temperature phase, the observation of second-order displacement reflections is evidence of a multi-q, ͗ 1 2 0 0 ͘ , displacement structure: a model of magnetic structure, consistent with both the displacement pattern and the mean-field model predictions, is proposed.

Résume.-On a étudié expérimentalement la susceptibilité paramagnétique des composés intermétalliques ACOZ, où A est une terre rare (Pr, Nd, Gd, Tb, Dy, Ho, Er ou Lu). On a également déterminé la variation avec la pression hydrostatique jusqu'a 4,5 kbar de leur température d'ordre magnétique ainsi que celle du composé isomorphe GdNiz. On discute brièvement des résultats obtenus.

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