  We present a spin density functional theory (DFT) study for semiconducting
ScN and Mn-substituted ScN.
  Their structural and magnetic properties have been investigated using the all
electrons augmented spherical wave method (ASW) with a generalized gradient GGA
functional for treating the effects of exchange and correlation.
  Band structure calculations show that ScN is semiconductor with a narrow
indirect band gap $\Gamma$-X of 0.54 eV.
  The total-energy versus volume calculations show that ternary
Sc$_{0.75}$Mn$_{0.25}$N nitride is more stable in face centered
tetragonal-rocksalt (fct-rocksalt) structure, found experimentally, than the
cubic rocksalt one. Spin polarized results, at theoretical equilibrium,
indicate that the ground state of Sc$_{0.75}$Mn$_{0.25}$N is ferromagnetic with
a high moment at Mn-atom (3.44$\mu_B$), and zero moment on Sc and N ones. The
magnetovolume effects of Mn-substitution in ScN lattice are discussed. The
electronic structures analyzed from site/spin projected density of states and
chemical bonding, for both the mononitride and the ternary alloy, are reported.
A discussion of the structural and magnetic properties of
Sc$_{0.75}$Mn$_{0.25}$N is given with a comparison to ScN, in order to get
insights of the Mn-substitution effects.
