Magnetism without magnetic ions:: Percolation, exchange, and formation energies of magnetism-promoting intrinsic defects in CaO

We investigate theoretically the prospects of ferromagnetism being induced by cation vacancies in nonmagnetic oxides. A single Ca vacancy V-Ca(0) has a magnetic moment due to its open-shell structure but the ferromagnetic interaction between two vacancies extends only to four neighbors or less. To achieve magnetic percolation on a fcc lattice with such an interaction range one needs a minimum of 4.9% vacancies, or a concentration 1.8x10(21) cm(-3). Total-energy calculations for CaO show, however, that due to the high vacancy formation energy even under the most favorable growth conditions one can not obtain more than 0.003% or 10(18) cm(-3) vacancies at equilibrium, showing that a nonequilibrium vacancy-enhancement factor of 10(3) is needed to achieve magnetism in such systems.