Quadrupolar nuclear resonance in electron-irradiated aluminum: Electric field gradients around vacancies and interstitials

Abstract

Nuclear quadrupolar couplings around defects created by electron irradiation in aluminum have been measured by the nuclear-magnetic-resonance field-cycling technique. The wipe-out numbers are roughly estimated to be 300\ifmmode\pm\else\textpm\fi{}100 and 1000\ifmmode\pm\else\textpm\fi{}200 for the monovacancy and self-interstitial, respectively. The electric-field-gradient values on the four first atomic shells around a monovacancy have been determined and are compared with quadrupolar couplings around impurities in aluminum. The asymmetry parameter $\ensuremath{\eta}$ at the first neighbors of the monovacancy is rather large ($\ensuremath{\eta}=0.65$). Using existing theory, it is shown that this value must be associated with a strong relaxation around the monovacancy. The experimental results rule out the possibility of a spherical symmetry around the self-interstitial, in agreement with other experimental observations. The quadrupolar couplings measured after the different stages of annealing of defects after irradiation are well explained with a model in which free interstitials migrate in stage I, small clusters rearrange themselves in stage II, and monovacancies migrate in stage III.