  Ni3C nanoparticles of about 40 nm have been studied experimentally to exhibit
weak ferromagnetic (FM), spin-glass (SG) and paramagnetic (PM) properties. The
freezing temperature of the SG phase at zero applied field is determined as,
TF0 ~ 11.0 K. At T > TF0, a very weak ferromagnetism has been observed over a
PM background. The Curie temperature, TC, is shown to exceed 300 K and the
ferromagnetism at 300 K is determined as about 0.02 emu/g (~6.7*10^{-4}mu_B per
Ni3C formula unit) by subtracting the background paramagnetism. An anomalous
dip appears in the temperature dependent coercivity, HC(T), near the freezing
temperature, TF0. It reflects a distortedly reduced coercivity in the M(H)
hysteresis loop measured at T = TF0 with the applied sweeping field around H =
0. This is attributable to the exchange coupling effect between the SG and the
weak FM phases. The possible origin of the magnetic moments that account for
the observed FM, SG and PM properties is discussed.
