  Half-metallic ferromagnetism (HMFM) occurs rarely in materials and yet offers
great potential for spintronic devices. Recent experiments suggest a class of
compounds with the `ThCr$_{2}$Si$_{2}$' (122) structure -- isostructural and
containing elements common with Fe pnictide-based superconductors -- can
exhibit HMFM. Here we use $ab$ $initio$ density-functional theory calculations
to understand the onset of half-metallicity in this family of materials and
explain the appearance of ferromagnetism at a quantum critical point. We also
predict new candidate materials with HMFM and high Curie temperatures through
A-site alloying.
