Electrical Detection of Spin Pumping due to the Precessing Magnetization of a Single Ferromagnet

We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.

Figure 1

(a) Schematic diagram of the device. On the lower side, through the shorted end of a coplanar strip, a current $I_{\mathrm{rf}}$ generates an rf magnetic field, denote by the arrows. The Py strip in the center produces a dc voltage $\Delta V=V^{+}-V^{-}$. $H$ denotes the static magnetic field applied along the strip. (b) Scanning electron microscope pictures of the central part of the devices. (c) The $F/N$ structure in which the resonant precession of the magnetization direction $\mathbf{m}$ pumps a spin current ${\mathbf{I}}_s^{\mathrm{pump}}$ into $N$. The spin pumping builds up a spin accumulation ${\mathit{\mu }}_S$ in $N$ that drives a spin current ${\mathbf{I}}_s^{\mathrm{back}}$ back into $F$.

Figure 2

The dc voltage $\Delta V$ generated by a $\mathrm{Pt}/\mathrm{Py}/\mathrm{Al}$ device in response to the rf magnetic field plotted as a function of the static magnetic field. The frequencies of the rf field are as shown. The peaks (dips) correspond to resonance at $f_{\mathrm{high}}$ ($f_{\mathrm{low}}$). The data are offset vertically, for clarity.

Figure 3

(a) Gray scale plot of the dc voltage $\Delta V$, a measured function of static field for different high (low) frequencies of the rf field from the $\mathrm{Pt}/\mathrm{Py}/\mathrm{Al}$ device [21]. The dark (light) curves denote resonance at $f_{\mathrm{low}}$ ($f_{\mathrm{high}}$). (b) The static magnetic field dependence of the resonance frequency of the Py strip (dots). The curve is a fit to Eq. (1). (c) The amplitude of the dc voltage from a $\mathrm{Al}/\mathrm{Py}/\mathrm{Al}$ device as a function of the square of the rf current, at 13 GHz and 139 mT (dots). The line shows a linear fit.

Figure 4

The dc voltage $\Delta V$ generated across the (a) $\mathrm{Al}/\mathrm{Py}/\mathrm{Al}$ and (b) $\mathrm{Pt}/\mathrm{Py}/\mathrm{Pt}$ devices as a function of the static magnetic field. The frequencies of the rf field are as shown.