Influence of nitrogenation on structure development and magnetic properties of mechanically alloyed and annealed Sm-Fe powders

Sm-Fe-N compounds were prepared by mechanical alloying, subsequent annealin g and nitrogenation. For crystal structure investigations of the non-equili brium phases Sm2Fe17+yNx, formed at various annealing temperatures T-A for 1 h, X-ray diffraction with following Rietveld analysis was used. A volume expansion of 6.2% was observed after nitrogenation, As for the non-nitrided Sm-Fe alloys a modified TbCu7-type structure (space group P6/mmm) and a mo dified Th2Zn17-type structure (space group R (3) over bar m) have been obse rved. However, for nitrogenated Sm-Fe samples the modified Th2Zn17-type str ucture forms already for annealing at T-A=750 degrees C prior to nitrogenat ion. For samples annealed, prior to nitrogenation, between T-A=600 and 700 degrees C the modified TbCu7-type structure was found, in which the nitroge n occupies randomly the 3f position with an occupancy larger than 1/3. The partially ordered, modified Th2Zn17-type structure formed for 750 degrees C <T-A<900 degrees C, is derived from the ideal Th2Zn17-type structure by in troducing additional Fe(6c) and Sm(Sa) positions. The degree of order of th e Sm atoms and Fe-dumbbells increases with increasing T-A. The nitrogen occ upies the octahedral interstitial positions 9e in the latter cases. The nit rogen content is higher in the hexagonal phase than in the rhombohedral pha se. Optimum magnetic properties were obtained for T-A=750 degrees C. Here w e found a coercivity mu(0J)H(c)=3.7 T and a good squareness of the demagnet ization curve. (C) 1999 Published by Elsevier Science S.A. All rights reser ved.