Radiation damping in ferromagnetic resonance induced by a conducting spin sink

We have investigated the damping in the ferromagnetic resonance (FMR) of yttrium iron garnet (YIG) caused by spin pumping into adjacent conducting materials, namely, Pt and the conducting polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS). By a systematic study which also includes multilayers in which the conducting layer is separated from YIG by an insulator, we can show that a considerable part of the damping can be attributed to the so-called radiation damping which originates from the interaction of the magnetic fields caused by the precessing magnetization with the conducting layer. Especially, when PEDOT:PSS is used as a spin sink, the observed damping must be attributed completely to radiation damping, and no contribution from spin pumping can be identified. These results demonstrate that the Gilbert damping as a measure of spin pumping can only be used when careful control experiments accompany the investigation.

Figure 1

AFM for the YIG (200-nm) surface at two different wafer areas.

Figure 2

(a) Schematic or the measurement geometry for the FMR and ISHE. (b) The FMR curves and linewidths obtained at 8 GHz for S1–S3. (c) The FMR curves showing the multiple lines in the spectrum for S2:[YIG-PEDOT:PSS]. The spectrum is taken with the in-plane magnetic field at a frequency of 8 GHz. The solid lines are fitted to Lorentzian line shapes. (d) The FMR linewidth as a function of the resonance frequency. The damping values for S1–S3 are determined from the slope of the linear fit. The errors for the damping values are estimated from the standard error resulting from the linear fit. The errors for the linewidth values are estimated considering both the magnetic-field modulation 0.05 Oe and the deviation of the FMR Lorentz fit from the experimental data.

Figure 3

Test of the suppression of the spin pumping by an $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interlayer. (a) An $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ layer alone does not modify the damping of a single YIG film beyond the error bars. (b) In a $\mathrm{YIG}\text{/}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\text{/}\mathrm{Pt}$ trilayer the ISHE is completely suppressed compared to a YIG/Pt bilayer. (c) Linewidth vs frequency plotted for pure YIG, YIG/PEDOT:PSS, and $\mathrm{YIG}\text{/}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\text{/}\mathrm{PEDOT}:\mathrm{PSS}$. The damping enhancement due to PEDOT:PSS is significant, however, it is identical within the error bars with and without the insulating interlayer, respectively. (d) For the $\mathrm{YIG}\text{/}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\text{/}\mathrm{Pt}$ an additional damping also is observed. Nevertheless, it is significantly smaller than without the $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ interlayer.

Figure 4

Schematic for the measurement geometry during the FMR experiment. The rf field induced by the antenna causes an inhomogeneous precession in the YIG film. This precession induces eddy currents in the Pt layer which is located between the antenna and the YIG. Although the eddy currents induced by the antenna only reduce the amplitude of the signal, the eddy currents induced by the precession can cause additional damping and an increase in linewidth.

Figure 5

(a) Linewidth vs frequency plotted for the sample sequence (S1, S6, S7, S8, S9, and S3). (b) The extracted radiation damping as a function of Pt thickness for the samples with 30 nm of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ between YIG and Pt.

Figure 6

The ISHE for YIG/PEDOT:PSS for in-plane geometry and both magnetic-field polarities +$H$ and −$H$. The curves are fitted using a Lorentz fit. The ISHE voltage is as small as $\sim 3\mathrm{nV}$.