Nitrogenation of Nd(Fe,Mo)12 powders for sintered magnets

Abstract

The intermetallic Rare Earth-Fe12 (REFe12) compounds are considered as the strongest candidates for alternative high-performance permanent magnet material, thanks to the combination of high magnetocrystalline anisotropy, high Curie temperature and moderate saturation magnetization. However, the NdFe12 based compounds suffer from a weak uniaxial magnetocristalline anisotropy at room temperature. The insertion of light elements, like nitrogen, induces a strong uniaxial anisotropy and an increase of the Curie temperature as well. Nevertheless, the nitrogenation of these compounds have proved to be tricky, as the reaction kinetics is very slow at moderate temperatures, whereas the decomposition of the 1–12 phase occurs at higher temperatures. In the present study the aim is to develop nitrogenated powders suitable for the manufacture of Nd(Fe,Mo)12N-based sintered magnets, starting from Nd(Fe,Mo)12 precursors prepared by strip-casting. The nitrogenation of Nd(Fe,Mo)12 powders was undertaken to analyze the mechanisms of reaction and to determine the experimental conditions allowing to avoid both the formation of α-Fe and the nitrogenation of the Nd-rich intergranular phase. Structural and magnetic investigations are performed on the parent alloys and the nitrogenated powders, allowing to identify the mechanism of the solid-gas reaction and to determine, for the first time to our knowledge, the nitrogen thermodynamic equilibrium pressure between the 1–12 phase and its nitride.