  Spintronic, spin caloritronic, and magnonic phenomena arise from complex
interactions between charge, spin, and structural degrees of freedom that are
challenging to model and even more difficult to predict. This situation is
compounded by the relative scarcity of magnetically-ordered materials with
relevant functionality, leaving the field strongly constrained to work with a
handful of well-studied systems that do not encompass the full phase space of
phenomenology predicted by fundamental theory. Here we present an important
advance in this coupled theory-experiment challenge, wherein we extend existing
theories of the spin Seebeck effect (SSE) to explicitly include the
temperature-dependence of magnon non-conserving processes. This expanded theory
quantitatively describes the low-temperature behavior of SSE signals previously
measured in the mainstay material yttrium iron garnet (YIG) and predicts a new
regime for magnonic and spintronic materials that have low saturation
magnetization, $M_S$, and ultra-low damping. Finally, we validate this
prediction by directly observing the spin Seebeck resistance (SSR) in the
molecule-based ferrimagnetic semiconductor vanadium tetracyanoethylene
(V[TCNE]$_x$, $x \sim 2$). These results validate the expanded theory, yielding
SSR signals comparable in magnitude to YIG and extracted magnon diffusion
length ($\lambda_m>1$ $\mu$ m) and magnon lifetime for V[TCNE]$_x$
($\tau_{th}\approx 1-10$ $\mu$ s) exceeding YIG ($\tau_{th}\sim 10$ ns).
Surprisingly, these properties persist to room temperature despite relatively
low spin wave stiffness (exchange). This identification of a new regime for
highly efficient SSE-active materials opens the door to a new class of magnetic
materials for spintronic and magnonic applications.
