Spin transport in insulators without exchange stiffness

The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator Gd 3 Ga 5 O 12 (GGG), a standard substrate for growing magnetic films, can be a spin current generator, but has never been considered as a superior conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, spin transport persists up to temperatures of 100 K ≫ T g = 180 mK, the magnetic glass-like transition temperature of GGG. At 5 K and 3.5 T, we find a spin diffusion length λ GGG = 1.8 ± 0.2 μm and a spin conductivity σ GGG = (7.3 ± 0.3) × 10 4 Sm −1 that is larger than that of the record quality magnet Y 3 Fe 5 O 12 (YIG). We conclude that exchange stiffness is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices.