Structural and magnetic properties of SmFeN magnets prepared by hydrogenation and nitrogenation processes

Sm-Fe-N permanent magnets were prepared from the starting Sm2Fe17 alloy by hydrogenation and nitrogenation processes. The phase components, structures and magnetic properties at each reaction stage involved in the processes have been investigated systematically. It has been found that many microcracks are present in powdered particles during the hydrogenation process, which is helpful for the nitrogenation process as well as for the remilling process. When the hydrogenation temperature TH is less than 550°C, the main process is hydrogen decrepitation (HD) and the consequent nitrogenation results in the anisotropic Sm2Fe17Nδ powders whose magnetic properties depend not only on the hydrogenation temperature, but also on the size of the particles after the re-milling process. The best values for the magnetic properties are obtained for a particle size of 2 μm average diameter, hydrogenated at TH = 300°C. When TH ≥ 550°C, the hydrogenation mainly shows a hydrogenation-disproportionation-desorption-recombination (HDDR) process. At first, the disproportionation reaction of hydrogen with Sm2Fe17 results in the hydride SmH2 + x and α-Fe and then the desorption-recombination process leads to the formation of metastable SmFe7 phase with TbCu7 structure. With increasing the hydrogenation temperature, the metastable SmFe7 transforms into Sm2Fe17 and the transformation is thoroughly completed at TH = 850°C. Consequently, the Curie temperature of the metastable SmFe7 phase decreases from 188°C at TH = 650°C to 129°C at TH = 850°C of Sm2Fe17 phase. The optimum hydrogenation temperature is 800°C for preparing the isotropic Sm-Fe-N magnets. The best results for the magnetic properties of the Sm-Fe-N powders achieved respectively in the above two processes are: (i) the HD process followed by nitrogenation: Br = 1.19 T (11.9 kG), iHc = 11.3 kA/cm (14.2 kOe), (BH)max = 199.0 kJ/m3 (25.0 MG Oe); and (ii) the HDDR process followed by nitrogenation: Br = 0.81 T (8.1 kG), iHc = 16.7 kA/cm (21.0 kOe), (BH)max = 103.5 kJ/m3 (13.0 MG Oe).