  In the present work, we perform a systematic investigation on p-type codoping
in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole
concentration increases, which should theoretically lead to an increase of the
Curie temperature (TC) according to the p-d Zener model. Unexpectedly, although
the film keeps its epitaxial structure, both TC and the magnetization decrease.
The samples present a phase transition from ferromagnetism to paramagnetism
upon increasing hole concentration. In the intermediate regime, we observe a
signature of antiferromagnetism. By using channeling Rutherford backscattering
spectrometry and particle-induced x-ray emission, the substitutional Mn atoms
are observed to shift to interstitial sites, while more Zn atoms occupy Ga
sites, which explains the observed behavior. This is also consistent with
first-principles calculations, showing that the complex of substitutional Zn
and interstitial Mn has the lowest formation energy.
