Cryogenic spin Peltier effect detected by a RuO$_2$-AlO$_x$ on-chip microthermometer

We report electric detection of the spin Peltier effect (SPE) in a bilayer consisting of a Pt film and a Y$_{3}$Fe$_5$O$_{12}$ (YIG) single crystal at the cryogenic temperature $T$ as low as 2 K based on a RuO$_2$$-$AlO$_x$ on-chip thermometer film. By means of a reactive co-sputtering technique, we successfully fabricated RuO$_2$$-$AlO$_x$ films having a large temperature coefficient of resistance (TCR) of $\sim 100\% ~\textrm{K}^{-1}$ at around $2~\textrm{K}$. By using the RuO$_2$$-$AlO$_x$ film as an on-chip temperature sensor for a Pt/YIG device, we observe a SPE-induced temperature change on the order of sub-$\mu \textrm{K}$, the sign of which is reversed with respect to the external magnetic field $B$ direction. We found that the SPE signal gradually decreases and converges to zero by increasing $B$ up to $10~\textrm{T}$. The result is attributed to the suppression of magnon excitations due to the Zeeman-gap opening in the magnon dispersion of YIG, whose energy much exceeds the thermal energy at 2 K.