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

Sm-Fe-N compounds were prepared by mechanical alloying, subsequent annealing and nitrogenation. For crystal structure investigations of the non-equilibrium phases Sm2Fe17+x and Sm2Fe17+xNy, respectively, formed at the various annealing temperatures TA, XRD with following Rietveld analysis was used. For TA between 600 C and 750 C a modified hexagonal TbCu7 structure (space group P6/mmm) was found in which the Fe(2c) site is replaced by the partially (1/3) and randomly occupied Fe(61) site. The approximated composition is SmFe8.8-9.0. The nitrogenated alloys crystallize in the same structure for 600 C < TA < 700 C. In this case the interstitial nitrogen randomly occupies the 3f site partially (1/3). For 800 C ≤ TA ≤ 900 C a disordered modified Th2Zn17 structure (space group R anti 3m) was found that is formed by introducing additional Fe (6c) and Sm(3a) positions. The degree of order of the Sm- and Fe-atoms in c-direction increases with increasing annealing temperature. The completely ordered stoichiometric Sm2Fe17 structure could not be reached by annealing the ball milled elemental powders. The nitrided alloys already form this intermediate structure at TA = 750 C. The interstitial nitrogen occupies the 9e site. The estimated nitrogen content is higher in the hexagonal phases than in the rhombohedral phase. Optimum magnetic properties, in particular a coercitivity μ01HC = 3.7 T and a good squareness of the demagnetization curve, were obtained for TA = 750 C. Here we found a nitrogen content of y = 3 for Sm2Fe17+xNy. (orig.)