Abstract
The percentage of substitutional doping of magnetic atoms (Mn) in group-IV-based dilute magnetic
semiconductors (DMS) can be increased by co-doping with another conventional electronic
dopant (e-dopant), as demonstrated from rst-principles calculations recently [Zhu et al., Phys.
Rev. Lett. 100, 027205 (2008)]. Here, we report extensive theoretical investigations of the kinetic
and thermodynamic characteristics of several co-doped systems including bulk Si and Ge as hosts
and various group-III and group-V e-dopants. The main ndings are as follows: The n-p pairing
of n-type e-dopants with p-type substitutional Mn is energetically stable in bulk Ge and Si. Mn
atoms move from interstitial sites to substitutional sites easier (with lower kinetic barriers) in the
presence of a neighboring n-type e-dopant. Magnetic coupling between two Mn atoms in bulk Ge
oscillates between positive (ferromagnetic) and negative (antiferromagnetic) values with increasing
Mn-Mn distance, but in Mn/As co-doped Ge the coupling parameter remains positive at all distances
beyond nearest-neighbors and this qualitative di erence does not change with the doping
level. For Mn doped Si, all coupling values except for the nearest neighbor one are positive and do
not change much upon co-doping. We nd an unconventional magnetic anisotropy in the co-doped
system, that is, the dependence of magnetic coupling on the relative positions of the magnetic ions
and their neighboring e-dopants. We map the calculated magnetic coupling to a classical Heisenberg
model and employ Monte Carlo simulations to estimate the Curie temperature (Tc). We nd that
in Mn doped Ge no ferromagnetic order exists for Mn concentrations ranging from 3.13% to 6%.
Instead, a spin-glass phase transition occurs at 5 K at 5% Mn doping. For Mn/As co-doped Ge,
Tc increases nearly linearly with the Mn concentration and reaches 264 K at 5% Mn doping.