The transition temperature of coupled magnetic planes including fluctuations in the spacer thickness

Abstract

The introduction of a weak coupling between isolated planes, each of which would order ferromagnetically, always raises the transition temperature. The coupled system will have either ferromagnetic or staggered order depending on the sign of the interplanar coupling. If there are fluctuations in the interplanar coupling caused by variations in the spacer thickness, then each plane will experience a random magnetic field caused by the combination of strong correlations in the neighbouring planes and the random exchange. In this paper, the effect of the random field is treated using mean field theory and the Bethe–Peierls approximation. It is shown that the results may be used to fit the data on some real multilayers in which the transition temperature falls below the result for uncoupled films at some values of the average spacer thickness.