Optimum contact resistance for two-terminal magnetoresistance in a lateral spin valve

Abstract

The two-terminal magnetoresistance (2T-MR) due to spin accumulation in a lateral spin valve is determined for devices with a Si channel and Fe/MgO tunnel contacts of varying MgO thickness. Established theory predicts that the 2T-MR exhibits a pronounced maximum for contact resistances comparable to the spin resistance rs of the channel. At large contact resistance (≫rs), the 2T-MR is, indeed, very small, despite the large tunnel spin polarization (TSP) of the contacts (90%). When the contact resistance is reduced toward rs, the 2T-MR increases, but much less than expected because for thinner MgO the TSP decays. For devices with the thinnest MgO and contact resistances near the predicted optimum, the 2T-MR is actually lower, owing to the smaller TSP (14%). The optimum and scaling of the 2T-MR are, thus, profoundly affected by the variation of the TSP with contact resistance. This is relevant for the design of practical two-terminal devices, including those with channel materials other than Si.