Spin Pumping with Coherent Elastic Waves

We show that the resonant coupling of phonons and magnons can be exploited to generate spin currents at room temperature. Surface acoustic wave pulses with a frequency of 1.55 GHz and duration of 300 ns provide coherent elastic waves in a ferromagnetic thin-film–normal-metal ($\mathrm{Co}/\mathrm{Pt}$) bilayer. We use the inverse spin Hall voltage in the Pt as a measure for the spin current and record its evolution as a function of time and external magnetic field magnitude and orientation. Our experiments show that a spin current is generated in the exclusive presence of a resonant elastic excitation. This establishes acoustic spin pumping as a resonant analogue to the spin Seebeck effect.

Figure 1

(a) Schematic view of a ${\mathrm{LiNbO}}_3/\mathrm{Co}/\mathrm{Pt}$ hybrid. An external static magnetic field $\mathit{H}$ can be applied within the film plane at an angle $\alpha$ to the SAW propagation direction. (b) The SAW drives resonant magnetization $\mathit{M}$ precession that emits a spin current ${\mathit{J}}_s$ into the Pt. ${\mathit{J}}_s$ is detected via the inverse spin Hall effect in the Pt thin film, i.e., as the voltage $V_{\mathrm{DC}}$. (c) Sample geometry (not to scale). The SAW pulse first traverses the $\mathrm{Co}/\mathrm{Pt}$ bilayer and then is detected at the output IDT.

Figure 2

Time-resolved spectroscopy with $\mathit{H}$ applied at $\alpha =10°$. (a) $P_{\mathrm{IDT}}$ as a function of time with ${\mu }_0{H}_{\mathrm{ref}}=30\mathrm{mT}$, showing the detection of the EMW ($0.2\mu \mathrm{s}$) and the SAW ($0.7\mu \mathrm{s}$) pulses at the output IDT. (b) $\Delta P_{\mathrm{IDT}}(t)=P_{\mathrm{IDT}}(t,{\mu }_{0}H)-P_{\mathrm{IDT}}(t,{\mu }_0{H}_{\mathrm{ref}})$. $\Delta P_{\mathrm{IDT}}(0.7\mu \mathrm{s})<0$ shows that the SAW is damped for ${\mu }_0{H}_{\mathrm{res}}=\pm 4\mathrm{mT}$, indicating acoustically driven FMR. (c) $V_{\mathrm{DC}}$ as a function of time at ${\mu }_0{H}_{\mathrm{ref}}=30\mathrm{mT}$. The EMW is rectified at the bilayer. (d) $\Delta V_{\mathrm{DC}}(t)=V_{\mathrm{DC}}(t,{\mu }_{0}H)-V_{\mathrm{DC}}(t,{\mu }_0{H}_{\mathrm{ref}})$. The change of sign of $\Delta V_{\mathrm{DC}}(0.6\mu \mathrm{s})$ with the reversal of the $\mathit{H}$ direction is a signature of acoustic spin pumping.

Figure 3

(a) $\Delta P_{\mathrm{IDT}}$ for the detection of the SAW (open symbols) and the EMW (solid symbols) pulses at the output IDT as a function of external magnetic field magnitude for $\mathit{H}$ applied at $\alpha =10°$. (b) $\Delta V_{\mathrm{DC}}$ for the detection of the SAW (open symbols) and the EMW (solid symbols) pulses at the bilayer. The characteristic fingerprint for acoustic spin pumping is the antisymmetric behavior of $\Delta V_{\mathrm{DC}}$ with respect to the $\mathit{H}$ orientation.

Figure 4

(a) $\Delta P_{\mathrm{IDT}}$ as a function of $t$ and ${\mu }_{0}H$ for $\mathit{H}$ applied at $\alpha =10°$. (b) $\Delta V_{\mathrm{DC}}(t,{\mu }_{0}H)$ shows EMW rectification (lower part, scaled by 0.1), while the SAW acoustically pumps a spin current which is detected via the inverse spin Hall effect (upper part). (c) We average $\Delta P_{\mathrm{IDT}}$ and $\Delta V_{\mathrm{DC}}$ data for the time span indicated by the dashed lines in (a) and (b), respectively. These data correspond to all elastic excitation of FMR ($\Delta P_{\mathrm{SAW}}$, left scale) and a spin current ($\Delta V_{\mathrm{MSP}}$, right scale).

Figure 5

(a) $\Delta P_{\mathrm{SAW}}$ and (b) $\Delta V_{\mathrm{MSP}}$ as a function of $\mathit{H}$ orientation and magnitude for magnetic field upsweep. The observed angular dependency is characteristic for acoustically driven FMR. The features around 2 mT are due to the $\mathit{M}$ reversal.