  The structural and magnetic properties of transition metal-doped Mn$_3$SnC
are studied with an aim to understand the effect of transition metal atom on
magnetostructural properties of the antiperovskite compound. The doped
Mn$_{2.8}$T$_{0.2}$SnC (T = Cr, Fe, Co, Ni and Cu) compounds show a distinctly
different magnetic behavior which can be related to electronic filling of the
$d$ band of the transition metal atom. While Cr and Fe doped Mn$_3$SnC show
properties similar to that of Mn$_3$SnC, the Co, Ni and Cu doped compounds
exhibit nucleation of secondary phases which are devoid of carbon and having
Heusler and DO19 type hexagonal structure. A strong magnetic interaction is
observed between the impurity phases and the major antiperovskite phase leading
to a sharp decrease in magnetostructural transition temperature of the
antiperovskite phase and a cluster glassy ground state.
