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

Single-phase <inline-formula><tex-math notation="LaTeX">$\varepsilon$</tex-math></inline-formula>-(FeCo)<italic>_x</italic>N compound particles with <inline-formula><tex-math notation="LaTeX">$x$</tex-math></inline-formula> = 2.25–2.48 were synthesized using ammonia gas nitrification. The mass magnetization <inline-formula><tex-math notation="LaTeX">$M$</tex-math></inline-formula> at 10 K under a magnetic field of 9 T was 77 A<inline-formula><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula>m<inline-formula><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula>/kg, and Curie temperature <inline-formula><tex-math notation="LaTeX">$T$</tex-math></inline-formula>_C was 100 K for <inline-formula><tex-math notation="LaTeX">$x$</tex-math></inline-formula> = 2.48. These values decreased with increasing nitrogen content. Compared with <inline-formula><tex-math notation="LaTeX">$\varepsilon$</tex-math></inline-formula>-Fe<italic>_x</italic>N, (FeCo)<italic>_x</italic>N had significantly lower <inline-formula><tex-math notation="LaTeX">$M$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">$T$</tex-math></inline-formula>_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 <inline-formula><tex-math notation="LaTeX">$x$</tex-math></inline-formula> = 2.35, even at the lowest measurement temperature of <inline-formula><tex-math notation="LaTeX">$T$</tex-math></inline-formula> = 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 <inline-formula><tex-math notation="LaTeX">$x$</tex-math></inline-formula> = 2.25 when the nitrogen content is low.