FMR and SMOKE Studies of Bilinear and Biquadratic Exchange Coupling in BCC Fe/Cu,Ag/Fe Trilayers. The Effect of In-Plane Four-Fold Anisotropies on Magnetization Reversal

Abstract

Surface Magneto-Optical Effect (SMOKE) and Ferromagnetic Resonance (FMR) studies of the exchange coupling in bcc Fe/Cu/Fe(001) and Fe/Ag/Fe(001) structures are presented. It is shown that the interfaces in bcc Fe/Cu/Fe(001) and Fe/Ag/Fe(001) trilayers can be significantly improved by choosing an appropriate growth procedure. The exchange coupling in bcc Fe/Cu/Fe and Fe/Ag/Fe trilayers was studied as a function of the interlayer thickness. The interpretation of magnetization loops for Fe/Cu/Fe and Fe/Ag/Fe trilayers requires the simultaneous presence of bilinear and biquadratic exchange coupling between the magnetic layers. The measured values of the bilinear and biquadratic exchange coupling in Fe/Cu/Fe were compared with a model recently proposed by Slonczewski, which treats the exchange coupling in trilayers with imperfect interfaces. It is shown that the exchange coupling unobscured by interface roughness exhibits a strong short wavelength oscillatory behavior which is in agreement with recent first principles band calculations. The improved interfaces in the Fe/Ag/Fe system revealed new features in the exchange coupling which were absent in samples grown entirely at room temperature. FMR and SMOKE measurements showed symmetric long wavelength oscillations (λ~5ML) in the exchange coupling. Magnetization reversal was studied in Fe/Ag/Fe/Ni trilayers. One can grow structures in which the magnetization reversal proceeds in two steps only. A small negative field of ~-15 Oe was needed to reverse the magnetization in the Fe film. A significantly larger field of several hundred Oe was needed to reverse the magnetization in an Fe/Ni film. In these samples minor loops correspond to switching the magnetization of the Fe film from the parallel to the antiparallel configuration with respect to the magnetic moment of the Fe/Ni film.