Giant magnon spin conductivity in ultrathin yttrium iron garnet films

Conductivities are key material parameters that govern various types of transport (electronic charge, spin, heat and so on) driven by thermodynamic forces. Magnons, the elementary excitations of the magnetic order, flow under the gradient of a magnon chemical potential^1-3 in proportion to a magnon (spin) conductivity. The magnetic insulator yttrium iron garnet is the material of choice for efficient magnon spin transport. Here we report a giant magnon conductivity in thin yttrium iron garnet films with thicknesses down to 3.7 nm when the number of occupied two-dimensional subbands is reduced from a large number to a few, which corresponds to a transition from three-dimensional to two-dimensional magnon transport. We extract a two-dimensional magnon spin conductivity around 1 S at room temperature, comparable to the (electronic) conductivity of the high-mobility two-dimensional electron gas in GaAs quantum wells at millikelvin temperatures⁴. Such high conductivities offer opportunities to develop low-dissipation magnon-based spintronic devices.