Wenckebach, W.Th. “Dynamic Nuclear Polarization via the Cross Effect and Thermal Mixing: A. The Role of Triple Spin Flips.” Journal of Magnetic Resonance 299 (February 2019): 124–34.
In dynamic nuclear polarization (DNP) via the cross eﬀect (CE) and thermal mixing (TM) a microwave ﬁeld ﬁrst reduces the polarization of some electron spins, so the electron spin system deviates from thermal equilibrium with the lattice. Next, the mutual interactions combine with their interaction with the nuclear spins to transform this deviation into nuclear spin polarization.