Neutron diffraction studies on GdB6 and TbB6 powders

We report here the first powder neutron diffraction study of GdB6 and TbB6. GdB6 and TbB6 order antiferromagnetically at 15 and 21K, respectively. In both compounds the transition at TN is of the first order. Moreover GdB6 presents a second spontaneous magnetic transition at T*=8K. The present study shows that, in both compounds, the magnetic propagation vectors belong to the star and that the direction of the magnetic moment is perpendicular to the component of the wave-vector in GdB6, while in TbB6 the moment is parallel to it. The deduced low-temperature values of the magnetic moments agree with those of the respective rare-earth trivalent ions.