  Heusler alloys with Mn and Co have been found to exhibit interesting
electronic and magnetic properties. Mn$_2$CoAl is well known SGS compound while
Mn$_2$CoGa has weak half metallic character. By using plane wave
pseudo-potential method, we studied the effect of Fe and Cr doping on
half-metalicity and magnetism of these compounds. The doping destroys the SGS
nature of Mn$_2$CoAl while the small-scale doping enhance the half-metallicity
of Mn$_2$CoGa making it perfect half-metal. In case of Mn$_2$CoAl, the doping
decrease the band gap while increase in band width is noticed for Mn$_2$CoGa.
The half-metallicity is destroyed in both cases when the doping level is beyond
certain degree. Moreover, we have also computed magnetic behavior of Mn$_2$CoZ
alloys and we found that total magnetic moments of dopped samples have higher
values than that of pristine compounds.
