  We report magnetic behaviour of MnCr$_2$O$_4$, which belongs to a special
class of spinel, known as chromite. Bulk MnCr$_2$O$_4$ shows a sequence of
magnetic states, which follows paramagnetic (PM) to collinear ferrimagnetic
(FM) state below T$_C$ $\sim$ 45 K and collinear FM state to non-collinear FM
state below T$_S$ $\sim$ 18 K. The non-collinear spin structure has been
modified on decreasing the particle size, and magnetic transition at T$_S$
decreases in nanoparticle samples. However, ferrimagnetic order is still
dominating in nanoparticles, except the observation of superparamagnetic like
blocking and decrease of spontaneous magnetization for nanoparticle. This may,
according to the core-shell model of ferrimagnetic nanoparticle, be the surface
disorder effect of nanoparticle. The system also show the increase of T$_C$ in
nanoparticle samples, which is not consistent with the core-shell model. The
analysis of the M(T) data, applying spin wave theory, has shown an unusual
Bloch exponent value 3.35 for bulk MnCr$_2$O$_4$, which decreases and
approaches to 1.5, a typical value for any standard ferromagnet, with
decreasing the particle size. MnCr$_2$O$_4$ has shown a few more unusual
behaviour. For example, lattice expansion in nanoparticle samples. The present
work demonstrates the correlation between a systematic increase of lattice
parameter and the gradual decrease of B site non-collinear spin structure in
the light of magnetism of MnCr$_2$O$_4$ nanoparticles.
