  Spin backflow and spin-memory loss have been well established to considerably
lower the interfacial spin transmissivity of metallic magnetic interfaces and
thus the energy efficiency of spin-orbit torque technologies. Here we report
that spin backflow and spin-memory loss at Pt-based heavy metal/ferromagnet
interfaces can be effectively eliminated by inserting an insulating
paramagnetic NiO layer of optimum thickness. The latter enables the thermal
magnon-mediated essentially unity spin-current transmission at room temperature
due to considerably enhanced effective spin-mixing conductance of the
interface. As a result, we obtain dampinglike spin-orbit torque efficiency per
unit current density of up to 0.8 as detected by the standard technology
ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall
ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient,
integration-friendly, and high-endurance spin-current generators that provide
>100 times greater energy efficiency than sputter-deposited topological
insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and
advance spin-torque technologies.
