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) Ga As substrates by metal-organic vapour-phase epitaxy. Depending on the growt h parameters, two different magnetic phases of Ga1-xMnxAs can be grown. (i) At low Mn-concentrations, Ga1-xMnxAs alloys are obtained. These alloys exh ibit a paramagnetic behaviour with a strong exchange interaction between th e localised magnetic moments of the Mn(2+)ions and the extended excitonic s tates. (ii) At high Mn-concentrations, Mn(Ga)As clusters are formed within a Ga1-xMnxAs host. The samples are ferromagnetic even above room temperatur e. The ferromagnetism has been investigated by SQUID and ESR measurements. The s-d and p-d exchange integrals have been determined independently by co mbining photoluminescence excitation and spin-flip Raman spectroscopy. A re versal of sign of the valence band exchange integral has been detected alon g with the transition from the paramagnetic to the ferromagnetic phase. (C) 2001 Elsevier Science B.V. All rights reserved.