  We propose thermally-populated domain walls (DWs) in an easy-plane
ferromagnetic insulator as robust spin carriers between two metals. The
chirality of a DW, which serves as a topological charge, couples to the metal
spin accumulation via spin-transfer torque and results in the
chirality-dependent thermal nucleation rates of DWs at the interface. After
overpopulated DWs of a particular (net) chirality diffuse and leave the
ferromagnet at the other interface, they reemit the spin current by spin
pumping. The conservation of the topological charge supports an algebraic decay
of spin transport as the length of the ferromagnet increases; this is analogous
to the decaying behavior of superfluid spin transport but contrasts with the
exponential decay of magnon spin transport. We envision that similar spin
transport with algebraic decay may be implemented in materials with exotic spin
phases by exploiting topological characteristics and the associated conserved
quantities of their excitations.
