Topologically Driven Giant Effective Spin Mixing Conductance in Antiferromagnetic FeSn/Py Heterostructures

The topological semimetal FeSn antiferromagnet, characterized by its kagome lattice, two-dimensional flat bands, and Dirac-like surface states, holds immense promise for spintronic applications. In this work, for the first time, we investigate the spin pumping behavior in epitaxial-FeSn/Py (Ni$_{80}$Fe$_{20}$) heterostructures. We report a giant effective spin mixing conductance (g$^{\uparrow \downarrow}_{\mathrm{eff}}$) of $(116\pm 7)$~nm$^{-2}$, which is nearly one order of magnitude higher than that of standard Pt/Py heterostructures. The insertion of a 3 nm Al spacer layer results in a two-fold reduction in the effective damping, confirming the interfacial origin of the large g$^{\uparrow\downarrow}_{\mathrm{eff}}$. Consistently, we observe an order-of-magnitude higher inverse spin Hall effect voltage in the FeSn/Py system compared to a reference Pt/Py film stack. We attribute the giant g$^{\uparrow\downarrow}_{\mathrm{eff}}$ to the direct interfacing of the Py layer with the topologically active [001]-kagome surface of epitaxial-FeSn. These findings establish the critical role of topologically active interfaces for advanced quantum-material-based spintronic devices.