Non-local spin-transport structures and governing length-scales

Non-local spin-current conduction can serve to concentrate spin-current in a nanostructure in spin-torque switchable nanomagnets in solid-state memory. This is sometimes called spin-harvesting or spin-funneling. We use a simplified building block for understanding spin-harvest transport and some associated physical length-scales, identifying key materials and interface conductance parameters. A simple rotationally symmetric non-magnetic spin conductor is used for solving its corresponding drift-diffusion transport equation and for investigating the role finite interface spin-resistance areas (RAs) play in affecting the spin-harvesting length-scale. A finite-element model is used to illustrate quantitative size and contact resistance-dependent spin transport, followed by discussion of a simplified sheet-resistance-limit solution for illustrating a characteristic lateral length-scale that governs the spin-harvesting distance. Two key findings from this study are as follows: (1) The effective spin-harvesting range is bound by spin drift-diffusion length λ but usually shorter due to spin-RA-related loading effects, and (2) there is a strong interplay among spin-flip diffusion length λ and the bulk and interface spin-conductance of the nonmagnetic metal (NM). The more conducting the bulk NM is and the more resistive a bottom interface RA is for spin-conduction, the longer range one can efficiently harvest spin-current. These results provide guidance for device structure designs utilizing spin-harvesting to maximize spin-current coupling into nanomagnets for spin-transfer-torque switching in magnetic random-access memory.