Effect of carrier confinement on exchange coupling in dilute magnetic semiconductors with self-organized nanocolumns

The "carrier-confinement" model for phase-separated dilute magnetic semiconductors is proposed, in which the carrier scattering by ferromagnetic strings embedded into a semiconductor host leads to the appearance of quasi-one-dimensional spin-polarized states inside the energy gap of the three-dimensional spectrum. Quasiparticle excitations from these states to the band edge mediate an efficient interstring exchange even in the absence of free carriers in the host. The related exchange integral can switch between ferromagnetic and antiferromagnetic with varying both the interstring distance and the filling of the quasi-one-dimensional spin-polarized states. We discuss the applicability of our model for the description of the magnetic behavior of the array of Mn-rich nanocolumns inserted into a Mn-depleted host in phase-separated (Ge,Si):Mn alloys.