A steady-state population inversion in the ground state of Mn2+ in ZnO was detected by application of continuous microwave and circularly polarized optical pumping in the temperature range of 3-6 K. Multiple spin-flip processes occur in view of a simultaneous saturation in the harmonically related transitions of Mn2+ spins. It is found that an additional relaxation channel arises at 2.7 K due to dynamic polarization of the Mn-55 nuclei through the saturation of the first order electron-nuclear forbidden transitions. The transient populations are created between Mn-55 nuclear sublevels.
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