Enhancement of the Spin-Mixing Conductance in Co-Fe-B/W Bilayers by Interface Engineering

In ferromagnetic/heavy-metal bilayer systems, spin-current transmission with high efficiency between the heavy-metal layer and the ferromagnetic layer across the interface is crucial for power-efficient spin-orbit torque magnetic random access memory (SOT MRAM). The spin-mixing conductance is one of the critical parameters that determines such efficiency at the interface. For the Co-Fe-B/W bilayer system, which is a promising candidate for SOT MRAM, improving the spin-current transmission efficiency at the interface by interface engineering, however, is not readily well studied. Here, we report that, by inserting an atomically thin α-W layer at the interface of Co-Fe-B/W to construct the Co-Fe-B/α-W/β-W trilayer system, the spin-mixing conductance can be enhanced by about 45%. A modified theoretical model is proposed that, in accordance with the experimental results, by considering the spin-backflow current as an equivalent spin resistance, the enhancement of spin-mixing conductance is attributed to suppression of the spin-backflow current by introducing α-W. This study demonstrates that interfacial phase engineering between multiphases of the same heavy metal is effective at enhancing the spin-mixing conductance in ferromagnetic/heavy metal bilayer systems, which may further improve spin-current transmission efficiency and help realize power-efficient SOT MRAM based on Co-Fe-B/W systems.