The Effect of Cold Working on the Magnetic Properties of Pure Metals

Experiments with aluminum and copper of the highest purity obtainable show that the magnetic susceptibilities of these metals are appreciably altered by cold working. The paramagnetic susceptibility of aluminum is decreased by an amount reaching approximately 15 percent of the original value, and the diamagnetic susceptibility of copper is also, numerically, decreased by nearly the same amount. This strain sensitivity of susceptibility cannot be accounted for by the presence of ferromagnetic impurities, but appears to be related to certain metallurgical changes which occur on cold working. From a consideration of the magnitude of Δχ for different degrees of cold work and the phenomenon of "magnetic self-recovery" (observed in copper but not in aluminum), it is concluded that the combined effects of lattice distortion and fragmentation probably give rise to the observed changes in magnetic susceptibility.An explanation of the results is suggested in terms of trapping of the "free" electrons in the cold-worked metal at points of considerable internal strain, and a simple application of Stoner's theory of the susceptibility of free electrons appears capable of giving a qualitative indication of why cold working should cause a change of χ towards increased paramagnetism in copper but towards decreased paramagnetism in aluminum.