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Abstract:
Magnetic resonance studies allow us to distinguish paramagnetic,
ferromagnetic and ferrimagnetic phases in Ga1-xMnxAs. The transition
from ferromagnet to ferrimagnet is correlated with a metal to insulator
transition. The analysis of spin wave resonance spectra, which occur in
the ferrimagnetic phase, allows us to estimate the magnitude and the
distance dependence of exchange coupling. The experimentally evaluated
long range of exchange causes an effective averaging of the fluctuation
of exchange interactions. As a consequence, in the semimetallic phase
both spin subsystems coherently precess forming the ferrimagnetic
structure. In the insulating phase, fluctuations of the local exchange
field lead to a fast decoherence of the carrier spins and only the
localized Mn spins form the ferromagnetic moment.
