Importance of giant spin fluctuations in two-dimensional magnetic trilayers

Abstract

Two ultrathin ferromagnetic films of Co and Ni separated by a nonmagnetic spacer of Cu are taken to study the spin-spin correlations of weakly coupled ferromagnets. The Ni film thickness ranging between ${d}_{\mathrm{Ni}}=2$ and 6 monolayers (ML) is chosen to study the two-dimensional $2\mathrm{D}\ensuremath{\rightarrow}3\mathrm{D}$ crossover in ferromagnets. The spacer thickness ranges from ${d}_{\mathrm{Cu}}=2\ensuremath{-}8\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ to monitor the oscillatory behavior of the interlayer exchange coupling. The measured temperature-dependent magnetizations and the corresponding Curie temperatures are accompanied by a microscopic many-body Green's function theory. Both experiment and theory give firm evidence that for nanostructured magnets a static mean field description is insufficient, higher order spin-spin correlations are important and explain the observed increase of the Curie temperature by up to $\ensuremath{\sim}200%$ due to the interlayer exchange coupling. The results are visualized in a three-dimensional diagram as a function of both the Ni thickness and the Cu spacer thickness.