The spin-diffusion effect in the dipolar ordering state

The theory of spin diffusion is extended to the case of nuclear dipolar order in solids containing paramagnetic impurities. We show that at the beginning of the relaxation process the relaxation function of the dipolar order is non-exponential, due to the direct interaction with paramagnetic impurities, and proceeds to become an exponential function of time, when spin diffusion of the dipolar order takes place. Using experimental data on these two relaxation regimes in CaF₂ doped with Mn²⁺, the diffusion coefficient and the radius of the diffusion barrier were estimated.