Role of spin mixing conductance in determining thermal spin pumping near the ferromagnetic phase transition in EuO_(1-x) and La2NiMnO6

We present a comprehensive study of the temperature (T) dependence of the longitudinal spin Seebeck effect (LSSE) in Pt/EuO_{1-x} and Pt/La2NiMnO6 (LNMO) hybrid structures across their Curie temperatures (Tc). Both systems host ferromagnetic interaction below Tc, hence present optimal conditions for testing magnon spin current based theories against ferrimagnetic YIG. Notably, we observe an anomalous Nernst effect (ANE) generated voltage in bare EuO_{1-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 Tc could be fitted to a power law of the form (Tc-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 relate the T-dependence of LSSE to the spin mixing conductance (Gmix) at the heavy metal/ferromagnet (HM/FM) interface, which in turn is known to vary in accordance with the square of the spontaneous magnetization (Ms). Additionally, the T-dependence of the real part of Gmix derived from spin Hall magnetoresistance measurements at different temperatures for the Pt/LNMO structure, further establish the interdependence.