, x ~ 2)*

The field of spin caloritronics deals with the fundamental interactions, interconversion, and applications of charge, heat, and spin currents. In magnetic insulators, the spin Seebeck effect produces spin currents from thermal gradients that can be detected in adjacent heavy metal layers through spin-to-charge conversion via the inverse spin Hall effect. Here, we build on existing models of the spin Seebeck effect to include variations of the magnon scattering rates to provide a comprehensive picture of predicted functionality. Specifically, our work accurately describes previously unexplained experimental results and identifies a regime of low-Ms, low-loss, and low-bandwidth magnonic excitations to produce efficient thermally-driven spin pumping. We validate this regime by observing a large spin Seebeck response in the low-Ms, low-loss organic-molecule-based ferrimagnet V[TCNE]_x. Accordingly, the excellent agreement between theory and experiment in this material reveal a magnon diffusion length in V[TCNE]_x exceeding 1μm at room temperature and magnon-life time 1-10μs, longer than YIG. These results establish the foundation of next-generation efficient spin-injection devices utilizing organic-based materials and highlight the importance of expanding the materials used in spin caloritronic applications.