Carrier-ion exchange interactions in crystals Cd sub(1-x)Co sub(x)Te

Magnetoreflectivity investigations of the Cd sub(1-x)Co sub(x)Te crystals with x identical with 0.004 were carried out in the region of 1 s exciton in a magnetic field up to 3.5 T and temperature of 2 K. A giant spin splitting (GSS) of exciton transitions in sigma super(+) and sigma super(-) polarizations, as well as two pi transitions, was observed. A strong asymmetry between the average energies of the sigma and pi exciton components has been found. The model we propose is based on the expanded carrier-ion exchange Hamiltonian which in addition to the usual Heisenberg-like term with the exchange constant beta includes also a biquadratic term with exchange constant beta sub(1). Estimates of the GSS of the exciton components agree with the experimental data if we assign (x'/x)N sub(0) beta identical with -1.8 eV and beta sub(1)/ beta identical with 0.26; x' is the active content of magnetic ion and N sub(0) is the cation concentration. Previous experimental data obtained in different laboratories can also be described by this model with the same beta sub(1)/ beta ratio. The nature of the biquadratic term as well as the role of spin-orbital interaction in its generation are discussed.