1999, Volume 2, Number 3, pp.52--60
Non-Markovian processes and spin memory effects in electron spin relaxation of paramagnetic ions in aqueous, aqua-acid and aqua-glicerol solutions have been investigated. For these purpose the spin-lattice relaxation time of some paramagnetic (Gd3+, Mn2+, Cr3+) ions have been measured by two independent methods: non-resonant paramagnetic absorption method in parallel magnetic fields and EPR. The microscopic mechanism of spin relaxation is the magnetic interparticle dipole-dipole interactions of paramagnetic ions modulated by the translational movement of the particles. Our experimental data for the viscosity dependence of the spin relaxation times exhibited the smooth transition from the liquid-like (diffusional) to the solid-like (rigid-lattice) relaxation scenario in aqueous-acid and aqueous-glicerol solutions. We elaborated the statistical non-Markov theory of paramagnetic relaxation of spins using well-known Zwanzig--Mori's kinetic equations for the normalized spin time correlation functions. Our approach involves the statistical memory effects in combined (spin + solution) system by means of correlation approximation for the second order spin memory function. The theory developed is in good accordance with our experimental results. From our investigations it is obvious that non-Markov processes and spin memory effects play a crucial role in the determination of the character of paramagnetic relaxation of electron spins in solutions.
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