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

Single-phase
$\varepsilon$
-(FeCo)
_x
N 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
_x
N, (FeCo)
_x
N 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.