The formation and local magnetic moment of a series of point defects in CaB
6 are computed using a supercell approach within the generalized gradient a
pproximation to density functional theory. It is found that the substitutio
n of Ca by La does not lead to the formation of a local moment, while a neu
tral B-6 vacancy carries a moment of 2.4 Bohr magnetons. A plausible mechan
ism for the ferromagnetic ordering of these moments is suggested. Since the
same broken B-B bonds appear on the preferred (100) cleavage planes of the
CaB6 structure, it is argued that internal surfaces in polycrystals as wel
l as external surfaces in general will make a large contribution to the obs
erved magnetization.