Current state of the spin exchange theory in dilute solutions of paramagnetic particles. New paradigm of spin exchange and its manifestations in EPR spectroscopy
K.M. Salikhov E.K. Zavoiskii Kazan Physicotechnical Institute, Kazan Scientific Centre of the Russian Academy of Sciences, Sibirskii trakt 10/7, Kazan, 420029, Russian Federation
Kinetic equations are presented for single-particle spin-density matrices with consideration for Heisenberg exchange interaction in bimolecular collisions of particles and dipole—dipole spin—spin interaction between particles. It is shown that interactions between individual paramagnetic particles albeit rather weak compared to exchange interaction may result in a nonequivalent spin exchange. Spin-coherence transfer generates collective modes of quantum spin coherence motion. Collective-resonance lines feature a mixed form (absorption + dispersion). A new interpretation of the exchange narrowing of spectrum in case the rate of spin-coherence transfer 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.