Detection of Microwave Spin Pumping Using the Inverse Spin Hall Effect

We report on the electrical detection of the dynamical part of the spin-pumping current emitted during ferromagnetic resonance using inverse spin Hall effect methods. The experiment is performed on a $\mathrm{YIG}|\mathrm{Pt}$ bilayer. The choice of yttrium iron garnet (YIG), a magnetic insulator, ensures that no charge current flows between the two layers and only the pure spin current produced by the magnetization dynamics is transferred into the adjacent strong spin-orbit Pt layer via spin pumping. To avoid measuring the parasitic eddy currents induced at the frequency of the microwave source, a resonance at half the frequency is induced using parametric excitation in the parallel geometry. Triggering this nonlinear effect allows us to directly detect on a spectrum analyzer the microwave component of the inverse spin Hall effect voltage. Signals as large as $30\mu \mathrm{V}$ are measured for precession angles of a couple of degrees. This direct detection provides a novel efficient means to study magnetization dynamics on a very wide frequency range with great sensitivity.

Figure 1

Schematic representation showing the direction of the (a) dc and (b) ac charge currents produced when a pure spin current is pumped from the insulating magnetic YIG (green regions) into the strong spin-orbit Pt metal (yellow regions). The instantaneous magnetization $\mathit{M}(t)$ is shown in a bivariate color map: the blue-red colors code the $x$ component, and the gray shades code the $z$ component. The precession of $\mathit{M}$ at $f_p/2$ around $\mathit{z}$ is driven by parametric excitation: it requires the pumping field $h$, oscillating at $f_p$, to be parallel to the bias magnetic field $H$, and the precession of ($M_x$, $M_y$) to be elliptic (thus, $M_x^2+M_y^2$ is not a constant of the motion). The flow charts on top illustrate that $M_z=\sqrt{M_s^2-M_x^2-M_y^2}$ then oscillates 2 times faster than $M_x$ or $M_y$. Spin-pumping currents are flowing from YIG to Pt (i.e., along the $y$ axis). The injected angular momentum in Pt is carried by the spins (arrows attached to the electrons opposite to their angular momentum). The direction of the instantaneous charge current (flat arrow) is given by the right-hand rule; see Eq. (2).

Figure 2

Standard FMR detected using dc ISHE voltage. The pumping field $h$ oscillating at $f_p=1.8\mathrm{GHz}$ is oriented perpendicularly to the static magnetic field $H$. (a) Larmor absorption peak of the uniform mode detected with a diode (the inset shows the dependence of the linewidth on frequency). (b) Corresponding dc ISHE voltage measured perpendicularly to $H$ (the inset shows the geometry of the experiment).

Figure 3

Parametric excitation detected using the dc ISHE voltage. (a) Measured variation of the resonance frequency $f_{\mathrm{res}}$ as a function of the applied field (dots) using the standard FMR geometry (Fig. 2). The solid line is the Kittel law $f_{\mathrm{res}}=(\gamma /2\pi )\sqrt{H(H+4\pi M_s)}$ with $\gamma =1.785\times {10}^7\mathrm{rad}{\mathrm{s}}^{-1}{\mathrm{Oe}}^{-1}$ and $M_s=139\mathrm{emu}{\mathrm{cm}}^{-3}$. (b) Spin-wave mode spectra detected using the dc ISHE voltage for two different power levels. The bias field $H$ is oriented at 10° from the pumping field $h$ oscillating at $f_p=1.092\mathrm{GHz}$ (the inset shows the geometry of the experiment). At lower power ($+19.8\mathrm{dBm}$), the only peak detected in the spectrum occurs when the Larmor condition is met at $f_p$. At higher power ($+27.8\mathrm{dBm}$), a new peak appears in the spectrum at $f_p/2$, corresponding to the parametrically excited resonance.

Figure 4

Parametric excitation detected using the ac ISHE voltage. The large power ($+24\mathrm{dBm}$) pumping field $h$ at $f_p=3.6\mathrm{GHz}$ is oriented parallel to the static magnetic field $H$. (a) The ac ISHE voltage generated by the parametric excitation at $H=205\mathrm{Oe}$ is monitored on a spectrum analyzer (1 kHz resolution bandwidth): an oscillation voltage is detected at $f_p/2=1.8\mathrm{GHz}$. (b) The amplitude of this oscillation is measured as the bias field is swept from 195 to 225 Oe. The envelope of the curve should be compared to that in Fig. 2. The maximum parametric signal occurs at $H_{\mathrm{res}}=205\mathrm{Oe}$. The inset shows the threshold behavior ($P_c\simeq +20.7\mathrm{dBm}$) of the power dependence of the spectrum analyzer signal at 1.8 GHz to demonstrate the parametric excitation.

Figure 5

Voltage at half the pumping frequency. Red dots correspond to the bias field required to observe the maximum parametric signal detected on the spectrum analyzer at $f_p/2$. The solid line is the Kittel law of the $\mathrm{YIG}|\mathrm{Pt}$ bilayer [see Fig. 3a].