Interpretation and consequences of surface-enhanced itinerant-electron paramagnetism

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Abstract
We reexamine the degree of enhancement of the spin susceptibility near the surface of a nearly magnetic exchange enhanced metal, in the case where a uniform field and a field localized near the surface are applied. We show that it is actually crucial that the precise structure of the noninteracting electron susceptibility be taken into account rigorously. It appears impossible to find any approximate analytic solution which would reasonably reflect the behavior found by the exact numerical computations. However with the help of a variational function we prove that, in agreement with the numerical results, the magnetization near the surface is strongly enhanced compared to the bulk one and can present a ferromagnetic instability for values of the electron-electron interaction for which the bulk is still paramagnetic. We emphasize that this result is obtained within the mean-field theory and we believe that it can be physically related to the oscillating noninteracting electronic density of states near the surface. Such a problem should actually require us to go beyond mean-field theory and in that case we suspect that, as in other problems involving breakdown of translational invariance, fluctuation effects would tend to strongly affect the surface ferromagnetic instability found in the above mean-field theory.