Synthese, cristallogenese, proprietes magnetiques et effets magnetostrictifs spontanes de quelques tetraborures de terres rares

Abstract The study of the magnetic properties of some rare-earth tetraboride powder shows that NdB 4 , SmB 4 , GdB 4 , TbB 4 , DyB 4 and HoB 4 are antiferromagnetic at low temperature, whereas PrB 4 is ferromagnetic. Preliminary results concerning single crystals are also given. PrB 4 is strongly anisotropic and its magnetization is observed only along the c→axis. At low temperature ErB 4 is antiferromagnetic in weak fields; as the field strength increases two magnetic transitions are observed successively, leading finally to a purely ferromagnetic arrangement. No magnetic order has been found for YbB 4 down to 4.2 K and a Mossbauer study shows that the isomer shift of ytterbium is intermediate between those of Yb 2+ and Yb 3+ in compounds with metallic behaviour; the variation of the crystal parameters vs temperature (4.2 – 300 K) shows that only a is affected near the ordering temperature.