Current state of the spin exchange theory in dilute solutions of paramagnetic particles. New paradigm of spin exchange and its manifestations in EPR spectroscopy

Kinetic equations are presented for single-particle spin-density matrices with the Heisenberg exchange interaction in bimolecular collisions of particles and the dipole–dipole spin–spin interaction between particles taken into account. It is shown that interactions between individual paramagnetic particles, despite being rather weak compared to the exchange interaction, can result in a nonequivalent spin exchange. Spin-coherence transfer generates collective modes of quantum spin coherence motion. Collective resonance lines have a mixed form (absorption + dispersion). A new interpretation of the exchange narrowing of the spectrum when the spin-coherence transfer rate is sufficiently large is proposed. An advanced theory of paramagnetic relaxation of paramagnetic particles in dilute solutions due to dipole–dipole interaction is discussed. A modern paradigm of spin exchange is also presented. (reviews of topical problems)