The quaternary system Sm-Fe-Mo-Al and the effect of Al substitution on magnetic and structural properties of its ThMn12 phase

The Fe-rich corner of the quaternary system Sm-Fe-Mo-Al is studied at isothermal conditions. The phase equilibria of ferromagnetic SmAlxFe10−xMo2 with adjacent phases, especially Sm2AlxFe15.7−xMo1.3, are observed for 1000 °C. Existence ranges are studied by SEM/EDX on samples with various compositions. Magnetic and structural properties were obtained by VSM(PPMS) as well as Permagraph measurements while structural relations were tracked by XRD. Al can be substituted for Fe up to x = 6.5 accompanied by a decrease in high field (8 MA/m ≈ 10 T) polarization from 0.66 T (x = 0.0) to 0.44 T (x = 2.0). Respective Curie temperatures of SmAlxFe10−xMo2 and Sm2AlxFe15.7−xMo1.3 initially increase with Al content reaching a maximum of ≈125 °C at x = 0.5 and ≈135 °C at x = 1.0, respectively. Further Al substitution decreases TC. The coercive fields of optimally annealed nanocrystalline powders decrease slightly from 153 kA/m for x = 0.0–139 kA/m for x = 0.8 and drops to 38 kA/m for x = 2.0. XRD data indicates that lattice parameters of SmAlxFe10−xMo2 continuously increase with Al content. The simultaneous refinement of X-ray and neutron diffraction patterns confirms that Mo prefers 8i sites in the ThMn12 structure and that Al occupies equally 8i and 8j in a ratio of 1:2.