Antiferromagnetism in Nanofilms of Mn-Doped GaN

We theoretically study the role of surfaces in the electronic and magnetic properties of nanofilms made of wurtzite and zinc-blende (Ga,Mn)N. The studied doping reactions suggest that Mn impurities replacing Ga cations preferably stay just below the unsaturated surface rather than near the substrate. The hole-mediated ferromagnetism, typical of (Ga,Mn)N bulk, is absent from these films, and Mn moments for the most stable cationic positions become antiferromagnetically coupled. The holes ascribed to dopants in (Ga,Mn)N semiconductors are here occupied by electrons from dangling bonds. In other less-stable sites, Mn atoms are ferromagnetic; the surface moment can be then parallel to them and small, as in the wurtzite and zinc-blende (111) geometries, or antiparallel and large, as in zinc-blende (001). Hence, the magnetic interplay between surfaces and Mn impurities depends on the surface orientation, which could be useful for the design of magnetic nanodevices.