High-Temperature Stability of ThMn12 Magnet Materials

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Abstract
The nitrogenated alloys (Nd0.7Zr0.3)(Fe0.75Co0.25)11.5Ti0.5N1.3 (A) and Nd(Fe0.8Co0.2)11Mo1.0N1.0 (B) and the non-nitrogenated alloy (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5 (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 α-(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 (PO2 > 10 Pa), but the ThMn12 structure remained up to at least 1373 K in an almost oxygen-free atmosphere (PO2 < 10−15 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 800–1000 K. The decomposition rate was clearly dependent on the PO2 in the heated atmosphere, as high PO2 led to sample oxidation, and on the sample composition.

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