Abstract
$\mathrm{Th}{\mathrm{Mn}}_{12}$ nitrides are good candidates for high performance permanent magnets. However, one of the remaining challenges is to transfer the good properties of the powder into a useful bulk magnet. Thus, understanding the denitrogenation process of this phase is of key importance. In this study, we investigate the magnetic and structural stability of the ${({\mathrm{Nd}}_{0.75},{\mathrm{Pr}}_{0.25})}_{1.2}{\mathrm{Fe}}_{10.5}{\mathrm{Mo}}_{1.5}{\mathrm{N}}_{x}$ compound ($x=0$ and 0.85) as function of temperature by means of neutron powder diffraction. Thermal dependence of the lattice parameters, formation of $\ensuremath{\alpha}$-(Fe,Mo), as well as the nitrogen content in the nitrides are investigated by heating the compounds up to 1010 K. The decomposition takes place mainly via the formation of the $\ensuremath{\alpha}$-(Fe,Mo) phase, which starts at around 900 K, whereas the nitrogen remains stable in the lattice. Additionally, we show that the magnetic properties of the nitrides [$M(4\phantom{\rule{4.pt}{0ex}}\text{T})=90$ ${\mathrm{Am}}^{2}$/kg and ${H}_{c}=0.55$ T] are maintained after the thermal treatments up to 900 K. This study demonstrates that the $\mathrm{Th}{\mathrm{Mn}}_{12}$ nitrides with the Mo stabilizing element offer good prospects for a bulk magnet provided an adequate processing route is found.
Highlights
The critical and strategic character of rare earth (RE) elements as raw materials has motivated a renewed interest in the search for rare-earth-lean and rare-earth-free hard magnetic materials for permanent magnet applications
The saturation magnetization is reached for the parent compound, giving a value Ms = 90 Am2/kg, which is consistent for Nd-Fe-Mo alloys [15]
The structural properties were investigated by means of powder neutron diffraction measurements performed at different temperatures for the parent compounds and their corresponding nitrides
Summary
The critical and strategic character of rare earth (RE) elements as raw materials has motivated a renewed interest in the search for rare-earth-lean and rare-earth-free hard magnetic materials for permanent magnet applications. Adding an interstitial light element (e.g., N or C) modifies the unit cell without changing the symmetry of the parent compound; this changes the Fe-Fe interaction and establishes a strong positive crystal field coefficient A20 at the Nd(2a) site, which is effective to create a large uniaxial magnetic anisotropy [6,7] The inclusion of such light elements is performed by the gas-phase interstitial modification process, which consists of heating the parent compound under the desired gas atmosphere (e.g., nitrogen) for a certain time which allows the gas-solid reaction to occur [8,9,10]. These results offer good prospects for the use of the ThMn12 nitrides as bulk magnets provided an adequate processing route is found
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