Spin echo studies of heavily cobalt-doped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ZnO</mml:mi> <mml:mo>:</mml:mo> <mml:msup> <mml:mi mathvariant="normal">Mn</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mro

The dynamic nuclear polarization (DNP) in a system of ${\mathrm{Mn}}^{2+}$ spins is studied in a weakly ferromagnetic matrix formed in heavily cobalt-doped ZnO single crystals. It is established that the thermal mixing mechanism of DNP leads to cooling of the common reservoir of electron spin-spin interactions of the ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Co}}^{2+}$ spins to a single spin temperature in the rotating frame. We show the variation of the polarization amplification of the common nuclear subsystem as a function of microwave irradiation frequency.