Efficient spin transport in a paramagnetic insulator

The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator $mathrm{Gd}_3mathrm{Ga}_5mathrm{O}_{12}$ (GGG) is a superior substrate for growing magnetic films, but has never been considered as a conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, the spin transport persists up to temperatures of 100 K $gg$ $T_{mathrm{g}} = 180$ mK, GGGu0027s magnetic glass-like transition temperature. At 5 K we find a spin diffusion length ${lambda_{mathrm{GGG}}} = 1.8 pm 0.2 {mu}$m and a spin conductivity ${sigma}_{mathrm{GGG}} = (7.3 pm 0.3) times10^4$ $mathrm{Sm}^{-1}$ that is larger than that of the record quality magnet $mathrm{Y}_3mathrm{Fe}_5mathrm{O}_{12}$ (YIG). We conclude that exchange coupling is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices.