  The magnetic properties of Mn, Co and Ni substituted Fe4N are calculated from
first principles theory. It is found that the generalized gradient
approximation reproduces with good accuracy the magnetic moment and equilibrium
volume for the parent Fe4N structure, with the atomic moment largest for the Fe
atom furthest away from the N atom (Fe I site), approaching a value of 3
muB/atom, whereas the Fe atom closer to the N atom (Fe II site) has a moment
closer to that of bcc Fe. Substitution of Fe for Mn, Co or Ni, shows an
intricate behavior in which the Mn substitution clearly favors the Fe II site,
Ni favors substitution on the Fe I site and Co shows no strong preference for
either lattice site. Ni and Co substitution results in a ferromagnetic coupling
to the Fe atoms, whereas Mn couples antiferromagnetically on the Fe II site and
ferromagnetically on the Fe I site. For all types of doping, the total magnetic
moment is enhanced compared with Fe4N only in the energetically very
unfavorable case of Mn doping at the Fe I site.
