Possible ferromagnetism or antiferromagnetism in defective AlN via strain engineering: Hybrid versus GGA calculations

Extensive calculations with hybrid and GGA functionals of the electronic and magnetic properties in cubic AlN for the charged Al and N vacancies are carried out to study the formation energies, the defect equilibrium the induced localized magnetism, the type and range of the magnetic interactions and the possible realization of the magnetic percolation via epitaxial strain engineering during the coherent growth of AlN films. Our calculations are based on the plane wave pseudo-potential spin polarized density functional theory. Our calculations with hybrid and GGA show that only Al vacancies can induce localized magnetic moments. Point defect equilibrium calculation shows that among all the charged Al vacancies, the charge-neutral is the stablest moment carrying state. Study of the collective magnetism between vacancy pairs using the Heisenberg model shows that these defects couple ferromagnetically for the first and third FCC distances and antiferromagnetically for the fourth distance. Thermodynamic considerations show that we cannot achieve magnetic percolation at equilibrium conditions since the minimal vacancy concentration needed for wall to wall percolation is many order of magnitude higher that the equilibrium calculated concentration. Nevertheless, our work shows that we can drastically enhance the vacancy concentration during the coherent growth of AlN films via a tensile epitaxial strain.