Zinc-blende half-metallic ferromagnets are rarely stabilized by coherent epitaxy

The need for spin-injectors having the same zinc-blende-type crystal structure as conventional semiconductor substrates has created significant interests in theoretical predictions of possible metastable half-metallic zinc-blende ferromagnets, which are normally more stable in other structure-types, e.g., NiAs. Such predictions were based in the past on differences Delta(bulk) in the total energies of the respective bulk crystal forms (zinc blende and NiAs). We show here that the appropriate criterion is comparing difference Delta(epi)(a(s)) in epitaxial total energies. This reveals that even if Delta(bulk) is small, still for MnAs, CrSb, CrAs, CrTe, Delta(epi)(a(s))>0 for all substrate lattice constant a(s), so the zinc-blende phase is not stabilized. For CrS we find Delta(epi)(a(s))< 0, but the system is antiferromagnetic, thus not half-metallic. Finally, zinc-blende CrSe is predicted to be epitaxially stable for a(s)>6.2 A and is half metallic.