A new series of compounds with the -type structure (, Dy, Ho, Er and Lu) has been synthesized and studied by x-ray diffraction, ac susceptibility and magnetization versus temperature and field measurements. The maximum Curie temperature is for the Tb compound . Spin reorientation transitions have been observed for the compounds and . A first order magnetization process has been observed at low temperature in and . The importance of the metallic radius of M in the magnetic properties of (for the same rate of M substitution) is discussed.

We report a systematic study of the crystallographic and magnetic structures of the RFe11.5 Ta0.5 (R Lu, Er, Ho, Dy and Tb) compounds carried out by means of neutron powder diffraction. Thermal dependencies of lattice parameters, magnetic moments and magnetization directions have been determined. The hierarchy of the Fe magnetic moments at the 8i, 8j and 8f sites was found to be µ8i >µ8j µ8f for all compounds at all temperatures. The influence of the atomic environments on the strength of the Fe local moments at each of the crystallographic sites is discussed. The results of the magnetic refinement are compared to those previously obtained from magnetic measurements on the same compounds.