Chiral charge pumping in graphene deposited on a magnetic insulator

We demonstrate experimentally that a sizable chiral charge pumping can be achieved at room temperature in graphene/yttrium iron garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a nonmonotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model.

Figure 1

Chiral charge pumping in YIG/SLG. (a) Schematic of the experiment. Single layer graphene (SLG) is transferred to yttrium-iron-garnet (YIG) film grown on gallium gadolinium garnet (GGG) substrate. Gold contact patches indicated by ($+$) and ($-$) were used for electric measurements. The system is magnetically saturated normally to the sample plane by means of an applied magnetic field ${\mathit{H}}_0$. Spin waves are excited by microwave (MW) current flowing in one of the two narrow antennas, producing a local MW field. The red arrows represent the precession of the exchange field following the magnetization dynamics. (b) Field dependence of the MW absorption at $f=5$ GHz for the $up$ ($H_{+}$) and down ($H_{-}$) orientation of the static field. Different peaks on the dependencies indicate excitation of different nonuniform spin-wave modes. Note the similarity of the two curves. The red curve corresponding to $H_{+}$ is shifted vertically for the sake of clarity. (c) Field dependence of the dc-voltage for the $up$ ($H_{+}$) and down ($H_{-}$) orientation of the applied field for the same frequency as in (b). Note the different signs of the voltages for the two orientations of the field.

Figure 2

Symmetry properties of the observed effect. Both the induced electric field $\mathit{E}$ and the wave vector of the spin-wave ${\mathit{k}}_{\mathrm{sw}}$ defining the direction of spin-waves propagation are in the plane of the sample, while the applied field ${\mathit{H}}_0$ and the YIG-magnetization ${\mathit{M}}_S$ are oriented normally to the sample plane. The $+$ and $-$ signs indicate the polarity of the measured dc-voltage. (a) The reference geometry. (b) Rotation of the system by ${180}^{\circ }$ around the $z$ axis reverses the directions of both $\mathit{E}$ and ${\mathit{k}}_{\mathrm{sw}}$. In agreement with experiment, the sign of the voltage is reversed. (c) Mirroring with respect to the vertical $xz$ plane ($y\to -y$) does not change vectors $\mathit{E}$ and ${\mathit{k}}_{\mathrm{sw}}$, while axial vectors ${\mathit{H}}_0$ and ${\mathit{M}}_S$ are reversed. In contrast to the experimental findings, the sign of the voltage is not reversed.

Figure 3

Ferromagnetic resonance (FMR) in a YIG film. (a) FMR curves measured at a constant $H_0$ for the $up$ ($H_{+}$) and down ($H_{-}$) orientation of the field. Note a difference in the FMR frequencies $\mathrm{\Delta }f$. (b) Field dependence of $\mathrm{\Delta }f$. The dash line is a guide for the eye.

Figure 4

Distorted SLG near a dislocation in a YIG film. (a) Top view of the distortion induced by a screw dislocation (the dislocation line is represented by the black dot). The red arrows illustrate the in-plane component of the effective exchange field. (b) Isometric illustration of the tilt of the effective exchange field (red arrows) along a path encircling the dislocation line (black arrow). The original field points along the normal to the sample plane, and the tilting is generated by the spin-lattice coupling.

Figure 5

Frequency dependence of the effect. (a) The averaged detected dc-voltage as a function of the absorbed MW power for different frequencies as indicated. The dash lines are linear fits of the experimental data used for calculation of the efficiency coefficient $V/P_{\text{abs}}$. (b) The efficiency coefficient $V/P_{\text{abs}}$ as a function of the spin-wave frequency. Note that a sizable effect is observed between 5 and 7 GHz only. The dash line is a guide for the eye.