Transient response of the spin Peltier effect (SPE) in a Pt/yttrium iron garnet junction system has been investigated by means of a lock-in thermoreflectance method. We applied an alternating charge current to the Pt layer to drive SPE through the spin Hall effect, and measured the AC response of the resultant SPE-induced temperature modulation at frequencies ranging from 10 Hz to 1 MHz. We found that the SPE-induced temperature modulation decreases with increasing the frequency when the frequency is >1 kHz. This is a characteristic feature of SPE revealed by the high frequency measurements based on the lock-in thermoreflectance, while previous low frequency measurements showed that the SPE signal is independent of the frequency. We attribute the decrease of the temperature modulation to the length scale of the SPE-induced heat current; by comparing the experimental results with one-dimensional heat conduction calculations, the length scale of SPE is estimated to be 0.94 {mu}m.
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