Charge-to-spin current conversion efficiency in a spin-harvesting structure with spin-current loading and finite conductance

Abstract

Efficiency in charge-to-spin current conversion is a key metric for spin-torque switched magnetic random access memories’ readiness for commercial CMOS integration. Higher efficiency enables faster, more reliable spin-torque writing of a nanomagnet bit. For spin–orbit-torque (SOT) devices, a spin-Hall channel together with a non-magnetic (NM) “spin-harvesting” layer has been proposed as an approach to increase the efficiency beyond the limit of magnetic tunnel junctions. Here, we analyze the scaling relationships between an area-leveraged gain of spin-harvesting and the need to include spin-Hall channel spin-current generator’s impedance and loading. The situation is examined in a spin-Hall channel plus NM spin-conductor stacked structure. We show that spin-harvesting is most effective when the spin-resistance-area product (spin-RA) of the NM harvesting structure is low compared to the spin-Hall channel’s characteristic spin-RA. In appropriate conditions, a harvest gain of well over 4× in efficiency can be expected compared to a magnetic tunnel junction’s theoretical limit. Such spin-harvesting structure can also mitigate the inherent reduction of SOT efficiency at small nanomagnet sizes, enabling potential applications in more advanced CMOS-technology nodes.