Spin Seebeck effect at microwave frequencies

We experimentally study the transient voltage response of yttrium iron garnet/platinum bilayer samples subject to periodic heating up to gigahertz frequencies. We observe an intrinsic cutoff frequency of the induced thermopower voltage, which characteristically depends on the thickness of the yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz in a 50-nm-thick yttrium iron garnet film, but drops to below 1 MHz for several-micrometer-thick films. These data corroborate the notion that the magnon spectrum responsible for the spin current emission in the spin Seebeck effect can be shaped by tuning the thickness of the ferromagnetic layer.

Figure 1

(a) At the WMI the thermal voltages generated by an intensity-modulated laser are recorded by a spectrum analyzer. The inset exemplarily shows the transmission $T=S_{21}/(1-S_{11})$ as obtained from the $S$-parameter measurements on the YIG (50 nm) sample. (b) At the UNI KL samples are placed on a microstrip line which is heated periodically by high-power microwave pulses. After filtering out the original microwave frequency, voltages are detected at the pulse frequency.

Figure 2

Normalized spin Seebeck voltage $V_{\mathrm{SSE}}\left(f\right)/V_{\mathrm{SSE}}\left(0\right)$ as a function of the modulation frequency of the applied temperature gradient for the 50- and 2800-nm-thick YIG films recorded at the WMI and the 270-, 6700-, and 30 000-nm-thick YIG films recorded at the UNI KL. An offset between the data for different samples has been added for clarity. The dashed lines indicate the 3-dB points and the solid line depicts the behavior of a 350-MHz low-pass filter for comparison.

Figure 3

The 3-dB rolloff frequency as a function of the YIG film thickness. $f_{3\mathrm{dB}}$ gradually decreases as approximately $d_{\mathrm{YIG}}^{-1}$ indicated by the dashed line.