  The addition of transition metals (TM) to III-V semiconductors radically
changes their electronic, magnetic and structural properties. In contrast to
the conventional semiconductor alloys, the lattice parameter in magnetic
semiconductor alloys, including the ones with diluted concentration (the
diluted magnetic semiconductors - DMS), cannot be determined uniquely from the
composition. By using first-principles calculations, we find a direct
correlation between the magnetic moment and the anion-TM bond lengths. We
derive a simple formula that determines the lattice parameter of a particular
magnetic semiconductor by considering both the composition and magnetic moment.
The formula makes accurate predictions of the lattice parameter behavior of
AlMnN, AlCrN, GaMnN, GaCrN, GaCrAs and GaMnAs alloys. This new dependence can
explain some of the hitherto puzzling experimentally observed anomalies, as
well as, stimulate other kind of theoretical and experimental investigations.
