  We have analyzed the electronic structure of half-metallic magnets based on
first principles electronic structure calculations of a series of semi-Heusler
alloys. The characteristic feature of the electronic structure of semi-Heusler
systems is a d-d gap in the density of states lying at/close to the Fermi level
depending on the number of valence electrons. We have employed various
indicators of chemical bonding to understand the origin of the gap in these
systems, which is crucial for their half-metallic property. The density of
states of other half-metallic magnets also supports a gap and it is a generic
feature of these systems. We have discussed in some details the origin of
magnetism, in particular, how the presence of the gap is crucial to stabilize
half-metallic ferro and ferri magnetism in these systems. Finally, we have
studied the role of magnetic impurities in semiconducting semi-Heusler systems.
We show with the aid of model supercell calculations that these systems are not
only ferromagnetic but also half-metallic with possibly high Curie temperature.
