  We study magnetic, transport and thermal properties of Cobalt carbide
nanocomposite with a mixture of Co2C and Co3C phases in 1:1 ratio, with an
average particle diameter of 40$\pm 15$ nm. We show that the behavior of the
nanocomposite is completely different from that of either Co3C or Co2C. We
observed that with decreasing temperature the saturation magnetization MS(T)
increases, however, below 100 K, there is a steep rise. A detail analysis shows
the increase in MS(T) down to 100 K is explained via the surface spin freezing
model. However, below 100 K the steep increase in MS(T) is explained by a
finite size effect related to a confinement of spin waves within the nano
particles. The measurement of heat capacity shows broad peak at 100 K along
with presence of another anomaly at a lower temperature 43 K(=Tex). Resistance
measurement in the nanocomposite shows metallic behavior at high T with an
unusual anomaly appearing at Tex, which is near the T regime where MS(T) begins
to increase steeply. A measurement of the temperature gradients across the
sample thickness indicates an abrupt change in thermal conductivity at Tex
which suggests a phase transition at Tex. Our results are explained in terms of
a transformation from a magnetically coupled state with a continuous spectrum
of spin waves into a magnetically decoupled state below 100 K with confined
spin waves.
