Magnetization and susceptibility of coupled ferromagnetic trilayers calculated with a Green’s function type theory

Abstract

The effect of the interlayer exchange interaction Jinter in coupled Co/Cu/Ni trilayers is studied theoretically, as motivated by recent experiments. In these systems, Jinter induces a considerable Ni magnetization for temperatures above the Curie temperature TC,Ni of the Ni film. We will show that this strong effect can only be understood if magnetic fluctuations in these 2D-like systems are taken into account. A mean field estimate, neglecting these fluctuations, yields a ten times smaller increase of the Ni magnetization. The induced magnetization increases for both a ferromagnetic (Jinter>0) and an antiferromagnetic (Jinter<0) interlayer coupling. The Ni susceptibility, exhibiting a resonance-like peak at T≳TC,Ni, looks different for these two cases. The calculations are performed with the help of a Heisenberg Hamiltonian and a Green’s function approach.