Successive spin polarizations underlying a new magnetic coupling contribution in diluted magnetic semiconductors.

We propose a new type of magnetic coupling (MC) that is found in diluted magnetic semiconductors (DMSs). The origin of this is found to be the result of charge transfer processes followed by successive spin polarizations (SSPs) along successive cation-anion segments which include the impurities. The basic process underlying the SSP-based MC (SSP-MC) is the sharing of a single spin orbital by two neighboring impurities. As such, it can be considered as a localized double exchange as it is not mediated by free carriers. SSP-MC can be either ferromagnetic (SSP-FMC) or antiferromagnetic (SSP-AFMC) and, as demonstrated here, the SSP-FMC can be significantly enhanced via codoping; it can act in competition with superexchange and/or double and/or p-d exchange interactions. While the SSP-MC is not directly related to the magnitude of the magnetic moments of the impurities, it depends strongly on the energy difference of the host and impurity d-band centers, the difference of their electronegativities and rather weakly on the coupling interactions between them as well as between the cations and their mediating anions. The validity of the proposed SSP-MC as a new type of magnetic coupling is demonstrated by ab initio results for DMSs, namely ZnO, GaN, GaP, TiO2 and MoS2 monodoped (with Co, Cu and Mn) and codoped (with Co-Cu-Co and Mn-Cu-Mn).