Isospin of magnons and birefringence-like spin transport in antiferromagnetic insulators

Collinear antiferromagnets with uniaxial anisotropy afford spin wave excitations (dubbed magnons) of both right and left handed chirality, forming an internal degree of freedom - magnonic isospin - capable of encoding information and transporting spin angular momenta over long distances without incurring Joule heating. However, it is commonly conceived that biaxial antiferromagnets (with coexisting easy-axis and easy-plane anisotropy) are not able to function as their uniaxial cousins in spin transport because magnons are linearly polarized and do not carry angular momenta due to rotational symmetry breaking. Here we challenge this seemingly established property by reporting experimental evidence that hematite thin film, in which magnons are linearly polarized, can efficiently transmit spins over micrometer distances. Using proper boundary conditions, we extract the spin decay length showing an unconventional temperature dependence which cannot be explained solely by thermal magnon scattering. We interpret our observations as an interference of the two linearly-polarized propagating magnons in analogy to the birefringence effect in optics, which can further be characterized by an effective precession of the isospin. Furthermore, our devices can realize a bi-stable spin-current switch with a 100% on/off ratio under zero remnant magnetic field. These findings open new intriguing arenas for non-volatile and low-field manipulation of spin transport in antiferromagnetic materials.