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^1,2,3 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.

电导率是控制由热力学驱动的各种传输类型（电荷、自旋、热等）的关键材料参数。磁振子，磁序的基本激发，在磁振子化学势梯度下流动^1,2,3与磁振子（自旋）电导率成正比。磁性绝缘体钇铁石榴石是高效磁振子自旋传输的首选材料。在这里，我们报告了当占据的二维子带的数量从大量减少到少数时，厚度低至 3.7 nm 的钇铁石榴石薄膜中的巨磁振子电导率，这对应于从三维到二维的转变维磁振子传输。我们在室温下提取了 1 S 左右的二维磁振子自旋电导率，与毫开尔文温度下 GaAs 量子阱中高迁移率二维电子气的（电子）电导率相当⁴。如此高的电导率为开发基于磁振子的低耗散自旋电子器件提供了机会。