Towards two-dimensional magnon spin transport in ultrathin magnetic insulator films

Magnetic insulators offer the possibility to develop low-power spintronic devices by utilizing magnons as information carriers that can propagate without Joule heating. However, magnons do suffer from scattering by disorder and phonons. In the thesis, we study the magnon transport in ultrathin yttrium iron garnet (YIG) films. As the thickness of YIG film decreases, a counter-intuitive giant magnon spin conductivity (1.6E8 S/m) is observed in record-thin YIG films (below 10 nm), which is 3-4 orders higher than that of the bulk. A transition from three-dimensional to two-dimensional (2D) magnon transport happens when the film thickness approaches a few lattice constants. Based on such thin films, a magnon transistor structure is applied to study the magnon transport under high magnon density, where we use the source-to-drain magnon current to probe the high magnon density created by large DC current underneath the gate. The modulation efficiency of magnon transistors is more than 200% in such ultrathin films. Meanwhile, we reveal the perpendicular magnetic anisotropy in unit-cell thin YIG film via spin-related phenomena, eg. Spin Hall magnetoresistance, spin Nernst magnetoresistance. Finally, we also report how the spin-related torques behave at high current density in ultrathin-YIG|Pt heterostructures. These results imply that ultrathin YIG films can be superior low-power signal and information transducers between conventional electronic and spintronic devices. They are also attractive platforms to explore electrically excited magnon Bose-Einstein condensation and spin superfluidity at room temperature.