Spin Signal Optimization in Metallic Non-Local Spin Valves via Tuned Interface Resistance*

Magnetic Tunnel Junction (MTJ)-based devices are instrumental in data storage. Due to the drive to smaller devices, however, MTJs face challenges related to high resistance-area products (RA), which limit the signal-to-noise ratio due to impedance mismatch. There thus exists a pressing need to explore all-metal spintronic devices, particularly non-local spin valves (NLSVs), although their applications are spin-signal-limited. The primary approach to enhancing signals in such NLSV devices is incorporation of tuned-RA “tunnel” contacts at the ferromagnetic/nonmagnetic interfaces, thereby mitigating back diffusion of injected spins. Here, we report on a comprehensive study of highly-spin-polarized Al/AlO_x/Co₇₅Fe₂₅-based NLSVs with RA tuned from deep in the tunneling regime (10⁶ Ωμm²) to the transparent limit via controlled Al oxidation. We determine the RA range over which non-local spin signals can be detected, characterize the interfacial barriers via voltage- and temperature-dependent measurements, and comprehensively probe the spin signal enhancement vs. RA. Surprisingly, part of the enhancement is found to derive from increased spin diffusion length in the Al channels when their surfaces are controllably oxidized. We also uncover a systematic suppression of the spin polarization at moderate RA, which appears to be the primary limiting factor in the current spin signal in such devices.