Quantitative estimation of thermoelectric contributions in spin pumping signals through microwave photoresistance measurements

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion efficiency of heavy metals. Upon microwave excitation, the sample may also be heated, and the parasitic thermoelectric signals could contaminate the spin pumping results. Owing to their identical angular dependences with respect to the magnetic field, it is difficult to isolate one from the other. In this paper, we present a quantitative method to separate thermoelectric contributions from spin pumping signals in both $\mathrm{Py}\left({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\right)/\mathrm{Pt}$ and $\mathrm{YIG}\left({\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}\right)/\mathrm{Pt}$ bilayers through microwave photoresistance measurements. We find that the microwave absorption indeed can raise the temperature of samples, resulting a field-dependent thermoelectric hysteresis loop. However, the additional heat dissipation due to the resonant precession of the magnetization in the ferromagnet is negligibly small compared with the measured spin pumping signal. Thus, we conclude that the spin pumping signal is free of any detectable thermoelectric contributions.

Figure 1

Schematic illustrations of (a) spin pumping (b) longitudinal spin Seebeck effect.

Figure 2

(a) Schematic illustration of the experimental setup for spin pumping measurements of Py/Pt bilayer. (b) Field-dependent voltage for a Py(6 nm)/Pt(3 nm) bilayer stripe with 8.5 GHz microwave irradiation, where the magnetic field is applied along the $y$ direction. The black symbols represent the experimental data, and the red lines are the Lorentz line fittings. The inset presents the zoomed-in feature near zero magnetic field. Here, $2\mathrm{\Delta }{V}_0$ denotes the difference between the voltage backgrounds for the positive and negative fields. (c) Microwave frequency $f$-dependent resonance field $H_{\mathrm{r}}$. Black pentagons are the experimental data, and the red line is the fitting with Kittel equation. (d) Microwave input power-dependent $V_{\mathrm{r}}$ (black hexagon, left scale) and $\mathrm{\Delta }{V}_0$ (red triangle, right scale). The lines are linear fittings.

Figure 3

(a) $R-T$ curve of a Py(6 nm)/Pt(3 nm) bilayer near room temperature, with a slope of 7.12(±0.02) Ω/K. $R_{300\mathrm{K}}=8.86\mathrm{k}\mathrm{\Omega }$. (b) Magnetic field-dependent voltages with direct current $+I_0$ (+0.9 mA, red curve) and $-I_0$ (−0.9 mA, black curve) for the Py/Pt sample. (c) The resistance difference Δ$R$ of the Py/Pt bilayer between microwave on and off states. (d) Linear relation between the thermoelectric ground $\mathrm{\Delta }{V}_0$ and the resistance difference background $\mathrm{\Delta }{R}_{\mathrm{b}}$. (e) Magnetic field-dependent $\mathrm{\Delta }R$ for different $\alpha$. The curves are shifted for clarity. (f) Magnetic field-dependent ${\mathrm{S}}_{21}$ parameter data for a 7 × 7 mm Py(6 nm)/Pt(3 nm) film.

Figure 4

(a) Schematic illustration of the experimental setup for spin pumping measurement of a YIG/Pt bilayer. (b) Field-dependent voltage for a YIG/Pt(5 nm) bilayer stripe with 5-GHz microwave irradiation, where the magnetic field is applied along the $y$ direction. (c) Ferromagnetic resonance field-dependent microwave frequency. (d) $\mathrm{\Delta }{V}_0$ as the function of $\mathrm{\Delta }{R}_{\mathrm{b}}$. The red line is the linear fitting with a slope of $2.8\times {10}^{-3}\mu \mathrm{V}/\mathrm{m}\mathrm{\Omega }$. (e) $H$-dependent $\mathrm{\Delta }R$ for different $\alpha$. The curves are shifted for clarity. (f) The $R-T$ curve of YIG/Pt(5 nm) near room temperature, where the slope is around 3.48(±0.04) Ω/K. $R_{300\mathrm{K}}=3.72\mathrm{k}\mathrm{\Omega }$.

Figure 5

Lock-in modulation frequency-dependent (a) $\mathrm{\Delta }{R}_{\mathrm{b}}$ and (b) $\mathrm{\Delta }{V}_0$ and $V_{\mathrm{r}}$ for Py/Pt deposited on Si and glass substrates. All voltages have been normalized to the value with $f_{\mathrm{mod}}=8.3\mathrm{kHz}$. The applied microwave is 8.5 GHz in frequency and 355 mW in power.