Crystal Structure and Magnetic Properties of Hexagonal FeCo Nitrides Prepared Using Ammonia Gas Nitrification

Single-phase $\varepsilon$-(FeCo)_xN compound particles with $x$ = 2.25–2.48 were synthesized using ammonia gas nitrification. The mass magnetization $M$ at 10 K under a magnetic field of 9 T was 77 A$\cdot$m$^{2}$/kg, and Curie temperature $T$_C was 100 K for $x$ = 2.48. These values decreased with increasing nitrogen content. Compared with $\varepsilon$-Fe_xN, (FeCo)_xN had significantly lower $M$ and $T$_C values, even at comparable nitrogen content. Mössbauer spectroscopy suggests that the magnetic moment of Co decreases with increasing nitrogen content and disappears at approximately $x$ = 2.35, even at the lowest measurement temperature of $T$ = 3 K. Griffiths phaselike magnetic behavior was observed in the temperature dependence of magnetic susceptibility. The experimental results indicate that the Fe–Fe interaction may change from ferromagnetic to antiferromagnetic at $x$ = 2.25 when the nitrogen content is low.