  The question of the correlation between magnetization, band splittings, and
magnetic circular dichroism (MCD) in the fundamental gap region of dilute
magnetic semiconductors is examined experimentally and theoretically taking the
case of wurtzite Ga(1-x)FexN as an example. Magnetization and
polarization-resolved reflectivity measurements have been performed down to 2K
and up to 7T for x = 0.2%. Optical transitions originating from all three free
excitons A, B and C, specific to the wurtzite structure, have been observed and
their evolution with the magnetic field determined. It is demonstrated that the
magnitude of the exciton splittings evaluated from reflectivity-MCD data can be
overestimated by more than a factor of 2, as compared to the values obtained by
describing the polarization-resolved reflectivity spectra with appropriate
dielectric functions. A series of model calculations shows that the
quantitative inaccuracy of MCD originates from a substantial influence of the
magnetization-dependent exchange interactions not only on the spin splittings
of excitons but also upon their linewidth and oscillator strength. At the same
time, a method is proposed that allows to evaluate the field and temperature
dependencies of the magnetization from MCD spectra. The accurate values of the
excitonic splittings and of the magnetization reported here substantiate the
magnitudes of the apparent $sp-d$ exchange integrals in (Ga,Fe)N previously
determined.
