Magnetoreflectivity investigations of the Cd1-xCoxTe crystals with x =
0.004 were carried out in the region of Is exciton in a magnetic fiel
d up to 3.5 T and temperature of 2 K. A giant spin splitting (GSS) of
exciton transitions in sigma(+) and sigma(-) polarizations, as well as
two pi transitions, was observed. A strong asymmetry between the aver
age energies of the sigma and pi exciton components has been found. Th
e model we propose is based on the expanded carrier-ion exchange Hamil
tonian which in addition to the usual Heisenberg-like term with the ex
change constant beta includes also a biquadratic term with exchange co
nstant beta(1). Estimates of the GSS of the exciton components agree w
ith the experimental data if we assign (x'/x)N-0 beta = -1.8 eV and be
ta(1)/beta = 0.26; x' is the active content of magnetic ion and N-0 is
the cation concentration. Previous experimental data obtained in diff
erent laboratories can also be described by this model with the same b
eta(1)/beta ratio. The nature of the biquadratic term as well as the r
ole of spin-orbital interaction in its generation are discussed. Copyr
ight (C) 1997 Published by Elsevier Science Ltd