  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.
