Point Defects, Ferromagnetism, and Transport in Calcium Hexaboride

The formation energy and local magnetic moment of a series of point defects in ${\mathrm{CaB}}_6$ are computed using a supercell approach within the generalized gradient approximation to density functional theory. It is found that the substitution of Ca by La does not lead to the formation of a local moment, while a neutral ${\mathrm{B}}_6$ vacancy carries a moment of 2.4 Bohr magnetons. A plausible mechanism 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 ${\mathrm{CaB}}_6$ structure, it is argued that internal surfaces in polycrystals as well as external surfaces in general will make a large contribution to the observed magnetization.