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Pristine nonstoichiometric Mn–Zn ferrites were synthesized using a facile “heating-up” method. A route for achieving very stable single-crystal spinel Mn–Zn ferrites with enhanced magnetic performance and Curie temperature was explored using annealing procedures, where the protective gas flow velocities, heating rates, and annealing temperatures were critical in determining the phase structures and performance. The annealed Mn–Zn ferrites showed ultrahigh saturation magnetizations of ∼120 emu/g and an ultrahigh Curie temperature of ∼750 K. Mössbauer spectra indicated that the valence state of Mn was maintained at Mn2+, and both Mn and Zn were located at A sites in the inverse spinel structure for the highly stable Mn–Zn ferrites. An excellent microwave-absorbing capability in a broad frequency range of 0.1–18 GHz was realized owing to the large magnetic and dielectric losses. The optimal match thickness of Mn–Zn ferrites was found to be 1.5 mm, corresponding to a maximum reflection loss of 22 dB at 16 GHz. These results indicate that the synthesized Mn–Zn ferrites exhibit significant advantages in microwave absorption in the high-frequency range. The demonstrated multicomponent ferrites with high magnetic performance and Curie temperature may find broad applications in various complicated environments, such as those having elevated temperatures.