ON THE THEORY OF DIFFUSION IN A MAGNETIC FIELD

Abstract

Diffusion in the weakly paramagnetic aluminum (rich) – copper system has been investigated in a magnetic field. Aluminum – aluminum/3% copper solid diffusion couples were diffusion annealed in a magnetic field of 30 and 20 kilo-oersteds, and then analyzed for diffusion using a microhardness technique. A statistically significant 25% decrease in the diffusivity was found for the couples annealed in a 30-kilo-oersted field applied perpendicular to the diffusion direction. However, from the accuracy of the results it is not possible to determine whether the field affects only the frequency factor D 0 , the activation energy E, or both factors. A theory based on the plasma–agnetohydrodynamic properties of the alloy system is developed to explain the results. According to the theory, the magnetic field decreases the diffusivity by the factor 1/(1 + ω ce 2 /ν e 2 ), where ω ce and ν c are the cyclotron and collision frequencies respectively of the diffusion-transported electrons. The plasma-oscillation screening effect is discussed, and it is shown that the field does not significantly affect charge screening. It is thus proposed that the diffusivity is decreased through the frequency factor D 0 , and not through the activation energy of the atom-vacancy jump process.