Role of spin mixing conductance in determining thermal spin pumping near the ferromagnetic phase transition in EuO1−x and La2NiMnO6

We present a comprehensive study of the temperature ($T$) dependence of the longitudinal spin Seebeck effect (LSSE) in $\mathrm{Pt}/{\mathrm{EuO}}_{1\ensuremath{-}x}$ and $\mathrm{Pt}/{\mathrm{La}}₂{\mathrm{NiMnO}}₆$ (LNMO) hybrid structures across their Curie temperatures (${T}_c$). Both systems host ferromagnetic interaction below ${T}_c$, and hence present optimal conditions for testing magnon spin current based theories against ferrimagnetic yttrium iron garnet. Notably, we observe an anomalous Nernst effect generated voltage in bare ${\mathrm{EuO}}_{1\ensuremath{-}x}$, however, we find LSSE predominates the thermal signals in the bilayers with Pt. The $T$ dependence of the LSSE in small $T$ range near ${T}_c$ could be fitted to a power law of the form ${({T}_c\ensuremath{-}T)}^P$. The derived critical exponent $P$ was verified for different methods of LSSE representation and sample crystallinity. The results are explained based on the magnon-driven thermal spin pumping mechanism that relates the $T$ dependence of LSSE to the spin mixing conductance (${G}_{\text{mix}}$) at the heavy metal/ferromagnet interface, which in turn is known to vary in accordance with the square of the spontaneous magnetization (${M}_s$). Additionally, the $T$ dependence of the real part of ${G}_{\text{mix}}$ derived from spin Hall magnetoresistance measurements at different temperatures for the Pt/LNMO structure further establishes the interdependence.