Are RKKY exchange integrals suitable to study ferromagnetism in diluted ferromagnetic systems ?

Richard Bouzerar

Université de Picardie Jules Verne
Laboratoire de physique de la matière condensée

We use a local-selfconsistent RPA approach to study ferromagnetism in diluted ferromagnetic systems. In our approach, the exchange couplings entering the disordered diluted Heisenberg Hamiltonian are RKKY-type. We also include the effect of the nearest neighbour super-exchange and damping to mimic the finite mean free path of the carrier in the disordered medium. It is shown that a mean field virtual crystal treatment always largely underestimate both the effect of disorder (magnetic impurities randomly distributed) and thermal fluctuations. In the absence of super-exchange we show that the RKKY oscillations has a drastic effect on the ferromagnetism: the region of stability of the ferromagnetism is very narrow. The effect of including damping in the exchange integrals leads to an increase of the stability region of ferromagnetism. In presence of a dominant AF nearest neighbour super-exchange, we find that the ferromagnetism is immediatly suppressed. Our study shows that undamped RKKY exchange integrals are inappropriate to explain the ferromagnetism observed in III-V semiconductors like InMnAs and GaMnAs.