Electronic states of Mn impurities and magnetic coupling between Mn spins in diluted magnetic semiconductors

Electronic states of single Mn impurities and magnetic couplings between Mn spins in diluted magnetic semiconductors have been studied systematically. It has been clearly shown that in the ground state, Mn spin antiferromagnetically (AFM) couples to surrounding As(N) when p-d hybridization V-pd is large and both the hole level E-v and the impurity level E-d are close to the middle of the gap; or very weak ferromagnetically (FM) when V-pd is small and both E-v and E-d are deep in the valence band. The Mn spin couplings are Heisenberg AFM for half-filled hole orbits; on the contrary, the couplings between Mn spins are double-exchange-like FM when the hole occupation in the p orbits is away from half-filling, and this accounts for the FM order in III-V semiconductors. The important role of the antisite As(N) compensation or the hole phase separation for the stability of FM ground state in wide-gap diluted magnetic semiconductors is emphasized.