Study of fully epitaxial Fe/Pt bilayers for spin pumping by ferromagnetic resonance spectroscopy

We present a study of fully epitaxial Fe/Pt bilayers by means of ferromagnetic resonance (FMR) techniques. The dependence of the resonance field ${H}_{\mathrm{FMR}}$, the Gilbert damping parameter $\ensuremath{\alpha}$, and the FMR linewidth on the Pt thickness is presented and compared with reference layers. Since spin pumping is extremely sensitive to the interface properties, a corresponding study is of large importance. By studying fully epitaxial systems, the existence of an almost perfectly single-crystalline ordered interface is ensured. The measured effects, such as the dependence of the resonant field ${H}_{\mathrm{FMR}}$ on Pt thickness, are thus related to the physics of spin pumping and not to interface disorder effects. The damping parameter $\ensuremath{\alpha}$ has been extracted from the slope of the linewidth dependence on the FMR frequency. By measuring the enhancement of $\ensuremath{\alpha}$ with Pt a value for the effective spin mixing conductance of ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}=(4.9\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$ is obtained. We observe an opposite behavior in the dependence of ${H}_{\mathrm{FMR}}$ when compared with reports on YIG/Pt. In addition, the critical role of the proper choice of a reference layer for the estimation of this value is discussed.