Bilinear and biquadratic exchange coupling in bcc Fe/Cu/Fe trilayers: Ferromagnetic-resonance and surface magneto-optical Kerr-effect studies.

Abstract

Ferromagnetic-resonance (FMR) and surface magneto-optical Kerr-effect (SMOKE) studies of the exchange coupling in bcc Fe/Cu/Fe(001) structures are presented. It is shown that the interfaces in bcc Fe/Cu/Fe(001) trilayers grown on a Ag(001) single-crystal substrate can be significantly improved by choosing an appropriate growth procedure. Low-energy electron-diffraction data are presented, which show that the bcc Cu(001) overlayer follows a nearly perfect bcc lattice. The exchange coupling in bcc Fe/Cu/Fe trilayers was studied as a function of interlayer thickness ranging from 7 to 14 monolayers. Quantitative data from FMR and SMOKE measurements are compared. The interpretation of magnetization loops for Fe/Cu/Fe trilayers requires the simultaneous presence of bilinear and biquadratic exchange coupling between the magnetic layers. Computer calculations were used to determine the strength of the bilinear and biquadratic exchange couplings. It is shown that the strength of the biquadratic exchange coupling increases with increasing terrace width. The measured values of the bilinear and biquadratic exchange coupling were compared with a model recently proposed by Slonczewski, which treats the exchange coupling in trilayers with imperfect interfaces. Slonczewski's model was used to deconvolute the data to obtain the intrinsic behavior of the bilinear exchange coupling in bcc Fe/Cu/Fe(001) trilayers. It is shown that the exchange coupling unobscured by interface roughness exhibits a strong short-wavelength oscillatory behavior that is in agreement with recent first-principles band calculations. Composite Ag-Cu structures were also studied. The presence of a few atomic layers of Ag(001) added to the Cu layer rapidly decreases the exchange coupling.