  We demonstrate the low temperature suppression of the platinum (Pt) spin
Nernst angle in bilayers consisting of the antiferromagnetic insulator hematite
($\alpha$-Fe$_2$O$_3$) and Pt upon measuring the transverse spin Nernst
magnetothermopower (TSNM). We show that the observed signal stems from the
interplay between the interfacial spin accumulation in Pt originating from the
spin Nernst effect and the orientation of the N\'eel vector of
$\alpha$-Fe$_2$O$_3$, rather than its net magnetization. Since the latter is
negligible in an antiferromagnet, our device is superior to ferromagnetic
structures, allowing to unambiguously distinguish the TSNM from thermally
excited magnon transport (TMT), which usually dominates in ferri/ferromagnets
due to their non-zero magnetization. Evaluating the temperature dependence of
the effect, we observe a vanishing TSNM below ~100 K. We compare these results
with theoretical calculations of the temperature dependent spin Nernst
conductivity and find excellent agreement. This provides evidence for a
vanishing spin Nernst angle of Pt at low temperatures and the dominance of
extrinsic contributions to the spin Nernst effect.
