Magnetic relaxation of localized and itinerant electron states in metal solutions

The relaxation properties of electron and nuclear spins change dramatically in the region of the metal-nonmetal (M-NM) transition in lithium-methylamine and sodium-ammonia solutions. Electron spin relaxation rates (T1e-1) decrease markedly with the onset of electron localization, while the converse is true for nuclear spin relaxation rates (T1n-1). Recent magnetic relaxation data for fluid solutions of lithium in methylamine are reviewed. An emphasis is placed on the contrasting relaxation properties of electron and nuclear spins in the localized and itinerant electron regimes. Special attention is devoted to the problem of describing the continuous changes in electron and nuclear spin relaxation across the M-NM transition. While precise theories for the transitional region are not available at the present time, we are able to at least identify the probable origin, and onset, of the observed variations in relaxation properties. In the current approach, we start from a description of the magnetic relaxation properties of itinerant electron states. This overall description is maintained throughout the transition region while intraatomic interactions, representing the incipient localization of electrons, are then introduced in the guise of phenomenological enhancement parameters. In the case of nuclear spin relaxation, Warren has previously shown that the enhancement of (T1n-1), over that predicted by the Korringa relation for itinerant electron states, is indicative of incipient electron localization. An analogous treatment of electron spin relaxation is developed here. We suggest that an enhancement in the electron spin relaxation time (T1e) relative to that predicted by the Elliott relation for nearly free electron metals arises quite naturally if one considers the increasing importance of electron-electron and electron-phonon interactions in the approach to the M-NM transition. These marked differences in relaxation properties between the two electronic regimes may permit the identification of coexisting localized and itinerant electron states in a disordered material, as observed recently in rapidly frozen solutions of alkali metals in hexamethylphosphoramide.