Simultaneous spin pumping and spin Seebeck experiments with thermal control of the magnetic damping in bilayers of yttrium iron garnet and heavy metals: YIG/Pt and YIG/IrMn

We report experiments on the generation of spin currents by simultaneous spin pumping effect and spin Seebeck effect (SSE) in bilayers of yttrium iron garnet (YIG)/Pt and YIG/IrMn. The experiments are performed with the ferromagnetic resonance (FMR) in YIG excited by microwave radiation in the range of 2–6 GHz and subject to a temperature gradient as in studies of the longitudinal SSE. Measurements of the FMR linewidth show that the magnetic damping in YIG can be controlled by spin currents generated by the thermal gradient across the thickness of the bilayer. The simultaneous measurements allow the determination of the spin current generated by SSE and a quantitative comparison of the models proposed for thermal control of the magnetic damping with experimental data. We show that the model based on the flow of angular momentum into or out of the YIG film carried by the coherent part of the spin current influenced by the temperature gradient is the one that best accounts for the observed change in the damping.

Figure 1

(a) A sketch of the experimental setup and sample arrangement used for the simultaneous measurements of the SPE and SSE in bilayers of YIG/Pt and YIG/IrMn. The polarity of the measured voltages is indicated by the plus and minus signs. (b) and (c) Sketches showing the YIG/HM structures and the electrodes used to measure the dc voltage due to the charge current in the metallic layer resulting from the conversion by the ISHE of the spin currents generated by spin pumping and by a thermal gradient in the YIG layer.

Figure 2

(a) The FMR absorption of the YIG/Pt bilayer expressed by the detector voltage versus magnetic field $H$ measured at the frequencies indicated in gigahertz and constant input power of 20 mW. (b) The symbols represent the measured microwave frequency versus the field value at the FMR peak. The solid line is a fit of the Kittel equation for the FMR frequency. The inset shows the absortion derivative of one magnetostatic mode measured at 3.0 GHz and $\mathrm{\Delta }T=0$.

Figure 3

(a) Spin pumping voltage in the YIG/Pt bilayer versus magnetic field $H$ measured at the frequencies indicated in gigahertz with input microwave power of 20 mW and $\mathrm{\Delta }T=0$. The inset shows the linear dependence with the power of the peak voltage measured at 3.5 GHz. (b) Field dependence of the spin Seebeck voltage for $\mathrm{\Delta }T=+4$ and $\mathrm{\Delta }T=+10\mathrm{K}$. The inset shows the values of $V_{SSE}$ measured at $H=\pm 1.0$ kOe for several temperature differences. (c) and (d) Field dependencies of $V=V_{SPE}+V_{SSE}$ with microwave power of 20 mW and several frequencies (indicated in gigahertz) measured with $\mathrm{\Delta }T=-4$ and $\mathrm{\Delta }T=+4\mathrm{K}$.

Figure 4

Field dependencies of the voltages in the YIG/IrMn bilayer produced simultaneously by the spin pumping and spin Seebeck effects with microwave power of 20 mW at the frequencies indicated in gigahertz and (a) $\mathrm{\Delta }T=-4\mathrm{K}$ and (b) $\mathrm{\Delta }T=+4\mathrm{K}$.

Figure 5

(a) The FMR absorption of the YIG/Pt bilayer expressed by the detector voltage versus magnetic-field $H$ measured at the frequencies indicated in gigahertz and constant input power of 20 mW for $\mathrm{\Delta }T=0$ [the black curve, the same as in Fig. 2], for $\mathrm{\Delta }T=-4\mathrm{K}$ (blue curve), and for $\mathrm{\Delta }T=+4\mathrm{K}$ (red curve). (b) The voltage measured in YIG/IrMn under spin pumping with 3 GHz and 20 mW for $\mathrm{\Delta }T=0,\mathrm{\Delta }T=-4$, and $\mathrm{\Delta }T=+10\mathrm{K}$. (c) and (d) The FMR linewidths versus temperature difference measured in YIG/Pt and YIG/IrMn with 3 and 5 GHz.