Exchange interactions and Curie temperatures of the tetrametal nitrides   Cr4N, Mn4N, Fe4N, Co4N, and Ni4N
  The exchange interactions of Cr4N, Mn4N, Fe4N, Co4N, and Ni4N compounds with perovskite structure were calculated to obtain the Curie temperatures for these compounds from Monte Carlo calculations. Contrary to naive expectation, the exchange interactions vary markedly among these five compounds. In Fe4N, the intra-sublattice interaction of the Fe 3c atoms is strongly negative, leading to a significant reduction of the Curie temperature. The calculated Curie temperatures are 291K (Cr4N), 710K (Mn4N), 668K (Fe4N), 827K (Co4N), and 121K (Ni4N), in good agreement with experimental observations where available. The much lower Curie temperature of Ni4N compared to fcc Ni is explained on the basis of the exchange interactions. 
Markus Meinert
cond-mat.mtrl-sci