STRUCTURE AND MAGNETIC-PROPERTIES OF RARE-EARTH IRON NITRIDES, CARBIDES AND CARBONITRIDES

Iron-rich rare-earth (R) compounds, such as R2Fe17 do not show great potential for high-performance magnet materials due primarily to their low Curie temperatures (T(c) approximately 300-400 K). However, relatively large quantities of nitrogen or carbon atoms can be introduced into the structure, resulting in a dramatic enhancement of magnetic properties including T(c) (> 700 K). The N or C atoms cause a volume expansion of a few percent of the unit cell without changing the crystal structure. The large increase in T(c) can be attributed to the volume dependence of the Fe-Fe exchange interactions. A large uniaxial anisotropy field develops for R=Sm upon nitriding/carbiding with an anisotropy field that is almost double the value for Nd2Fe14B at room temperature. Problems including the precipitation of soft magnetic phases (mainly alpha-Fe) and the limited thermal stability of the nitrides have so far restricted the applications of these compounds. Here data are presented on combined carbide/nitride alloys prepared using a novel technique. These alloys exhibit many of the advantages of the pure compounds but with greater thermal stability and less interference from precipitated phases. A typical material, Sm2Fe17(NxC1-x)3-delta has a T(c) of 758 K and an anisotropy field (mu0H(A)) of 15 T at room temperature.