Monitoring the sign reversal of the valence band exchange integral in (Ga,Mn)As

We report the successful growth of magnetic Ga1-xMnxAs layers on (1 0 0) GaAs substrates by metal-organic vapour-phase epitaxy. Depending on the growth parameters, two different magnetic phases of Ga1-xMnxAs can be grown. (i) At low Mn-concentrations, Ga1-xMnxAs alloys are obtained. These alloys exhibit a paramagnetic behaviour with a strong exchange interaction between the localised magnetic moments of the Mn(2+)ions and the extended excitonic states. (ii) At high Mn-concentrations, Mn(Ga)As clusters are formed within a Ga1-xMnxAs host. The samples are ferromagnetic even above room temperature. The ferromagnetism has been investigated by SQUID and ESR measurements. The s-d and p-d exchange integrals have been determined independently by combining photoluminescence excitation and spin-flip Raman spectroscopy. A reversal of sign of the valence band exchange integral has been detected along with the transition from the paramagnetic to the ferromagnetic phase. (C) 2001 Elsevier Science B.V. All rights reserved.