Controlled magnon spin transport in insulating magnets: from linear to nonlinear regimes

Magnons are quasiparticle representations of spin waves, which describe the collective excitations of a magnetic system. They can propagate and carry spin information in electrically insulating and magnetically ordered materials, i.e. magnetic insulators. One of the most famous examples is yttrium iron garnet (YIG) due to its uniquely small magnetic damping and long magnon lifetime. The magnon transport set-up is composed of two heavy metal (HM) strips such as platinum (Pt) or tantalum (Ta) on top of a single crystal YIG film. A small ac current is sent through the first HM strip (the magnon injector), where the spin Hall effect (SHE) transfers the electrical current into a transverse spin current. This leads to an electrical spin accumulation at the HM interface which is in contact with the YIG. Due to the conductivity mismatch at the interface, the mobile electrons themselves cannot go deeply into the insulator. However, the spins can be transferred, which gives rise to magnon generation in YIG. The resulting magnons propagate inside the YIG and they can be picked up by the second HM strip (the magnon detector), where the detected spin currents are transferred back to a charge current via the inverse spin Hall effect (ISHE). Under an open circuit condition, an ISHE voltage is measured. Since current supply and voltage measurement are conducted at two different HM strips, this experiment is referred to as nonlocal measurement. The transport properties of magnons are studied by the distance-dependent behavior of nonlocal signals.