Enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations

Abstract

Motivated by recent experiments, the effect of the interlayer exchange interaction ${J}_{\mathrm{inter}}$ on the magnetic properties of coupled Co/Cu/Ni trilayers is studied theoretically. Here the Ni film has a lower Curie temperature ${T}_{C,\mathrm{Ni}}$ than the Co film in case of decoupled layers. We show that by taking into account magnetic fluctuations the interlayer coupling induces a strong magnetization for $T\ensuremath{\gtrsim}{T}_{C,\mathrm{Ni}}$ in the Ni film. For an increasing ${J}_{\mathrm{inter}}$ the resonancelike peak of the longitudinal Ni susceptibility is shifted to larger temperatures, whereas its maximum value decreases strongly. A decreasing Ni film thickness enhances the induced Ni magnetization for $T\ensuremath{\gtrsim}{T}_{C,\mathrm{Ni}}.$ The measurements cannot be explained properly by a mean-field estimate, which yields a ten times smaller effect. Thus, the observed magnetic properties indicate the strong effect of two-dimensional magnetic fluctuations in these layered magnetic systems. The calculations are performed with the help of a Heisenberg Hamiltonian and a Green's-function approach.