  Diluted magnetic semiconductor (DMS) colloidal nanocrystals demonstrate
remarkable magneto-optical properties. However, the behavior of circular
polarization of their emission in high magnetic fields remains unclear. We
measure magneto-optical properties of colloidal CdSe/CdMnS nanoplatelets in
high magnetic fields up to 30 T and at cryogenic temperatures. The degree of
circular polarization of photoluminescence demonstrates non-monotonous behavior
in a magnetic field. In low magnetic fields, the polarization degree is
positive, due to an exchange interaction of excitons with localized spins of
magnetic Mn ions. The exchange interaction is strong enough to overcome the
intrinsic Zeeman splitting, which provides negatively polarized emission in
nonmagnetic CdSe/CdS nanoplatelets. After reaching a maximum the polarization
degree starts to decrease and reverses the sign to negative in high magnetic
fields. The critical magnetic field, in which the sign is reversed, increases
when temperature is elevated. We develop a model, which explains this behavior
by an interplay of bright and dark exciton recombination.
