In situferromagnetic resonance: an ultimate tool to investigate the coupling in ultrathin magnetic films

The benefit of using in situ ultrahigh vacuum ferromagnetic resonance (FMR) to study exchange coupled magnetic films is demonstrated. Structurally well defined trilayer systems consisting of two ultrathin magnetic films (Ni or Co) separated by a non-magnetic Cu spacer layer are examined. The stepwise construction of the sample reveals how the single uniform resonance mode of the bottom magnetic layer is influenced upon depositing the second layer on top. The coupling leads to an acoustical and an optical mode. The positions, intensities and linewidths of these modes are compared to theory which uses a continuum approach in the framework of the Landau–Lifshitz equation of motion. We discuss how to determine the coupling strength in absolute units which for our trilayers ranges from a few to about 100μeV/atom and then systematically investigate the parameters that influence this coupling. The effect of the spacer thickness and its roughness is studied in detail with the help of scanning tunnelling microscopy to obtain a realistic and quantitative picture of the spacer morphology. In a next step we investigate the influence of a non-magnetic overlayer usually used as a protection layer. For small coupling strengths such a capping layer can change the sign of the coupling. The temperature dependence of the coupling in the framework of existing theoretical models is discussed. Moreover, the resonance method is used to address via the linewidth the dynamical response of the magnetic layers upon being coupled. It is demonstrated that 2D spin fluctuations originating from one layer influence the damping properties within the other. To support the results obtained from the FMR, the measurements are complemented by magneto-optic Kerr effect experiments on the same systems.