The magneto-optical properties of a ${\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Se}/{\mathrm{Be}}_{0.93}{\mathrm{Mn}}_{0.07}\mathrm{Te}$ diluted magnetic semiconductor heterostructure with type-II band alignment are investigated at cryogenic temperatures down to 16 mK. The temperature of the Mn spin system, which at the lowest possible laser power reaches 270 mK, is evaluated from the giant Zeeman splitting of the direct exciton in the ${\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Se}$ layers subject to an external magnetic field. The degree of circular polarization of the direct and indirect optical transitions, induced by the magnetic field, is a sensitive indicator for the laser heating of the Mn spin system. Evidence of spin-glass formation in the Mn spin system of the ${\mathrm{Be}}_{0.93}{\mathrm{Mn}}_{0.07}\mathrm{Te}$ layers with the critical temperature of $T_{\mathrm{SG}}=400\mathrm{mK}$ is found.

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• Magneto-optics
• Heterostructures
• Spin glasses