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. Spincoherence 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.