Determination of self-diffusion mechanisms from high-field nuclear-spin-relaxation experiments

Abstract

The theory of high-field nuclear-spin relaxation due to random-walk diffusion in monoatomic crystals is extended to correlated diffusion mechanisms. It is predicted that the effect of the diffusion mechanism on the relaxation times $T_1$, $T_2$, and $T_{1\rho }$ can be observed in three different ways: (i) In single crystals the relaxation times are shown to depend on the crystallographic orientation of the magnetic field. Whereas on the low-temperature side of the $T_1$ minimum these anisotropies should characterize the diffusion mechanism, they are expected to disappear on the high-temperature side. (ii) Shape and width of the $T_1$ and $T_{1\rho }$ minimum as a function of temperature are found to depend on the diffusion mechanism—in both single crystals and polycrystalline samples. (iii) Assuming that in a single crystal the decay of the transverse magnetization can be described by a single relaxation time $T_2$ (usually true at temperatures above the $T_1$ minimum), deviations from the exponential decay, which should be characteristic for a given diffusion mechanism, are predicted in polycrystalline samples. The numerical results obtained for diffusion via monovacancies in bcc and fcc crystals are compared with those obtained for the limiting case of random-walk diffusion. It is found that for these two mechanisms the orientation dependences of the relaxation times are similar, but that considerable differences in the shapes of the relaxation rates versus temperature should exist.