High-temperature stability of ThMn₁₂ magnet materials

The nitrogenated alloys (Nd_0.7Zr_0.3)(Fe_0.75Co_0.25)_11.5Ti_0.5N_1.3 (A) and Nd(Fe_0.8Co_0.2)₁₁Mo_1.0N_1.0 (B) and the non-nitrogenated alloy (Sm_0.8Zr_0.2)(Fe_0.75Co_0.25)_11.5Ti_0.5 (C), having a ThMn₁₂ structure, show interesting magnetic properties and are candidate materials for high-temperature magnets. In this study, the stability of these materials was studied using Curie temperature measurements, differential scanning calorimetry, differential thermal analysis, thermogravimetry from room temperature to 1573 K, and X-ray diffraction of treated samples. The nitrogenated samples (A) and (B) started to decompose into the ¡-(Fe, Co) phase and other X-ray amorphous phases from about 800 and 1000 K, respectively. Sample (C) exists as a metastable phase at room temperature and decomposed above 700 K at a relatively high oxygen partial pressure (P_O2 > 10 Pa), but the ThMn₁₂ structure remained up to at least 1373 K in an almost oxygen-free atmosphere (P_O2 < 10¹¹⁵ Pa). Sample (C) is intrinsically stable at temperatures higher than about 1000 K up to the melting temperature, which was estimated to be 1480 K. The ThMn₁₂ structure in both R = Nd and Sm starting alloys is metastable at room temperature, and becomes unstable under 8001000 K. The decomposition rate was clearly dependent on the P_O2 in the heated atmosphere, as high P_O2 led to sample oxidation, and on the sample composition.