Collapse of ferromagnetism in (Ga, Mn) As at high hole concentrations

Using modulation-doped (Ga,Mn)As/(Al, Ga, Be) As structures, we investigate the properties of (Ga, Mn) As when the hole concentration p increases while the concentration of Mn ions is kept constant. At small values of p the Curie temperature T(C) of (Ga, Mn) As is seen to increase with increasing p, as expected from the p-d Zener model. However, as p continues to increase, this trend is reversed: at some point T(C) begins to decrease, and eventually the ferromagnetism of (Ga, Mn) As disappears altogether, in stark contrast with the p-d Zener model. Mechanisms which can lead to this behavior are discussed.