Determination of surface magnetic profiles of thin Permalloy films using ferromagnetic resonance (abstract)

Abstract

The motivation for this study arose from experimental evidence of oxidation phenomena of Permalloy films, which causes large variation in their magnetic properties as a function of film thicknesses.1 We report a numerical determination of surface magnetic profiles for films of different thicknesses and explain the modification of these profiles in terms of their oxidation. Magnetic profiles of Permalloy films can be obtained from ferromagnetic resonance (FMR) experiments. Approximate calculated profiles of saturation magnetization, exchange constant, and induced anisotropies can be determined using perpendicular resonance alone. For maximum information and accuracy, both perpendicular and parallel resonance spectra are necessary, giving the most probable magnetic profile of the films. To quantify the magnetic profiles of 50-1000-Å-thick NiFe films, we calculated the ferromagnetic resonance of a model structure using a graded saturation magnetization and exchange constant at the interface. This resonance calculation involves, in its simplest form, one second-order differential equation for perpendicular and two coupled differential equations for parallel polarization.2 The results of these calculations are matched to experimental spin-wave resonance spectra obtained at 9.5 and 34.1 GHz in perpendicular and parallel polarization, respectively. Finally, this FMR numerical solution allows us to solve the ambiguity in the source of the pinning of the magnetic moment in these films. It is due to the graded saturation magnetization at the interface and not to any significant surface anisotropy.