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 $\alpha$, 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 $\alpha$ has been extracted from the slope of the linewidth dependence on the FMR frequency. By measuring the enhancement of $\alpha$ with Pt a value for the effective spin mixing conductance of $g_{\mathrm{eff}}^{\uparrow \downarrow }=(4.9\pm 0.5)\times {10}^{19}{\mathrm{m}}^{-2}$ 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.

Figure 1

(a) Cross-sectional HRTEM image of the Fe/Pt bilayer deposited on MgO(100), along the [001] MgO zone axis. The (020) MgO, $\left(01\overline{1}\right)$ Fe, and (020) crystal planes are perfectly aligned. (b) Corresponding common SAED pattern, dominated by the [001] zone axis of the substrate. The arrow denotes the growth direction. (c) Magnified part of (b) near the 020 MgO reflection, showing the excellent in-plane epitaxy of the three lattices. (d) Common SAED pattern with the aperture placed more at the deposition side, where the [011] zone axis of Fe is revealed parallel to the [001] zone axis of MgO. Gray indices refer to the Fe lattice reflections, while black indices refer to the MgO lattice reflections.

Figure 2

(a) Example of a FMR spectrum for a Fe(12 nm)/Pt(10 nm) bilayer measured at 14.8 GHz showing the real and imaginary part of the ${\tilde{S}}_{12}$ transmission parameter. (b) Dependence of the resonance frequency on the applied field for a Fe(12 nm)/MgO(10 nm) reference sample. The red line is a fit to Kittel's formula [19] [Eq. (3)] to extract $M_{\mathrm{eff}}$. (c) Dependence of the resonance linewidth on the frequency for the same sample. The red line is a linear fit to Eq. (4) to determine the Gilbert damping parameter.

Figure 3

Dependence of the resonance field $H_{\mathrm{FMR}}$ on the azimuthal in-plane angle $\varphi$ for three fixed resonance frequencies for a Fe(12nm)/MgO(10nm) sample. The lines are fits to Eq. (5). The crystallographic Fe directions and the magnetic easy (e.a.) and hard axis (h.a.) are labeled.

Figure 4

Dependence of the resonance field $H_{\mathrm{FMR}}$ at a fixed frequency (a), the effective damping parameter ${\alpha }_{\mathrm{eff}}$ (b) and the linewidth at a fixed frequency (c) on the Pt thickness for Fe/Pt bilayers. For comparison also the values for the reference samples (Fe/Al, Fe/MgO and Fe/MgO/Pt) are shown. The lines are a guide to the eye.