Lack of correlation between the spin-mixing conductance and the inverse spin Hall effect generated voltages in CoFeB/Pt and CoFeB/Ta bilayers

We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and CoFeB/Ta bilayers and the correlation between the effective spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$ and the obtained voltages generated by the spin-to-charge current conversion via the inverse spin Hall effect in the Pt and Ta layers. For this purpose, we measure the in-plane angular dependence of the generated voltages on the external static magnetic field and we apply a model to separate the spin pumping signal from the one generated by the spin rectification effect in the magnetic layer. Our results reveal a dominating role of anomalous Hall effect for the spin rectification effect with CoFeB and a lack of correlation between $g_{\mathrm{eff}}^{\uparrow \downarrow }$ and inverse spin Hall voltages pointing to a strong role of the magnetic proximity effect in Pt in understanding the observed increased damping. This is additionally reflected on the presence of a linear dependency of the Gilbert damping parameter on the Pt thickness.

Figure 1

Dependence of the effective Gilbert damping parameter ${\alpha }_{\mathrm{eff}}$ on the thickness of the NM metal. A large increase in damping is observed for the Pt case while a very small but not vanishing increase is observed for Ta. From the change $\mathrm{\Delta }\alpha$ the effective spin-mixing conductance $g_{\mathrm{eff}}^{\uparrow \downarrow }$ is estimated using Eq. (2).

Figure 2

Voltage spectra measured for (a) CoFeB/Ta and (b) CoFeB/Pt at 13 GHz. The solid line is a fit to Eq. (4). The symmetric voltage $V_{\mathrm{sym}}$ and antisymmetric voltage $V_{\mathrm{antisym}}$ contributions are separated and plotted independently (dashed lines). The voltage signal is dominated by $V_{\mathrm{antisym}}$ in the Pt case and by $V_{\mathrm{sym}}$ in the Ta case. The inset in (a) shows schematically the measurement configuration.

Figure 3

(a) Dependence of the ratio S/A $=V_{\mathrm{sym}}/V_{\mathrm{antisym}}$ on the thickness of the NM layer. (b) Dependence of the total voltage on the applied microwave power, proving the measurements were carried out in the linear regime.

Figure 4

Angular dependence of $V_{\mathrm{sym}}$ (top) and $V_{\mathrm{antisym}}$ (bottom) for CoFeB/Pt,Ta samples with NM thickness of 3 nm. The lines are a fit to the model described in Eq. (5).

Figure 5

NM thickness dependence of the different contributions to the measured voltages extracted from the angular dependence of $V_{\mathrm{sym}}$ and $V_{\mathrm{antisym}}$ for Ta (top) and Pt (bottom).