Ferromagnetic resonance and spin momentum exchange in crystalline magnetic ultrathin films in noncollinear configuration

Abstract

Au ∕ Fe ∕ Au ∕ Fe ∕ Ga As ( 001 ) ultrathin film structures were grown by molecular beam epitaxy on 4×6 reconstructed GaAs(001) substrates. Measurements were carried out using a network microwave analyzer in the range of frequencies from 1to12GHz allowing one to determine both the in-phase and out-of-phase components of the rf magnetic susceptibility. The ferromagnetic resonance (FMR) results indicate that the ultrathin Fe films are of high crystalline quality. The bottom Fe film [Au∕Fe∕GaAs(001)] has its magnetic hard axis along the [1−10] direction due to a strong in-plane uniaxial interface anisotropy. This leads to a unique behavior of the FMR in noncollinear configuration. Beside the 2 two FMR peaks expected an additional FMR peak was observed in a narrow microwave frequency range. This peak can be explained as a “ghost resonance” which was driven purely by magnetic losses. The FMR linewidth is mostly given by the bulk intrinsic Gilbert damping and spin pumping, and is only weakly affected by small local variations of the internal anisotropy fields. An accidental crossover of the collinear FMR branch of the top Fe layer (Au∕Fe∕Au) with the noncollinear branch of the bottom (Au∕Fe∕GaAs) layer was used to investigate nonlocal spin pumping and spin sink effects in a noncollinear configuration of magnetic moments.