Structural and hard magnetic properties of rapidly solidified Sm–Fe–N

Abstract

Sm–Fe alloys have been produced by rapid quenching and the resulting phases have been investigated in the as‐quenched state and after nitriding. Besides the well‐known equilibrium phases of the binary Sm–Fe system (Fe, Sm₂Fe₁₇, SmFe₃, SmFe₂ and Sm), a hexagonal TbCu₇‐type phase shows up in melt spun ribbons (a=4.88 Å, c=4.23 Å). Its stoichiometry is about Sm₁Fe₉ and it is formed only at wheel velocities above 15 m/s. The Curie temperature and the saturation polarization of this new phase is 210 °C and 1.25 T, respectively. At higher Sm concentrations or lower quenching rates the structure changes to the Th₂Zn₁₇‐type. The Th₂Zn₁₇‐type ribbons are magnetically soft whereas the TbCu₇‐type samples show moderate coercivities of up to 1.7 kA/cm. Nitrogenation leads to an expansion of the lattice and to an overall improvement of the hard magnetic properties for both phases. Their Curie temperatures are increased to 470 °C and the saturation polarizations are raised to 1.40 and 1.51 T for the TbCu₇‐ and the Th₂Zn₁₇‐type phases, respectively. The best hard magnetic properties for isotropic TbCu₇‐type material are obtained for quenched, annealed, and nitrided Sm_10.6Fe_89.4 which shows a coercivity, H_ci, of 4.9 kA/cm, a remanence, J_r, of 0.86 T and an energy product, (BH)_max, of 69.6 kJ/m³. For similarly treated Sm₁₂Fe₈₈, which crystallizes in the Th₂Zn₁₇ structure, a coercivity of 16.7 kA/cm, a remanence of 0.73 T and an energy product of 65.6 kJ/m³ are achieved.