Spin-injection efficiency and magnetoresistance in a ferromagnet-semiconductor-ferromagnet trilayer

摘要

We present a drift-diffusion transport model to evaluate the spin-injection efficiency η and magnetoresistance (MR) ratio in a ferromagnetic (FM) metal-semiconductor (SC)-FM metal trilayer structure. This model takes into account the differential interfacial resistances (IR) for spin-up and spin-down electrons and spin relaxation within the SC layer. The electrochemical potential μ for both spin directions is analytically solved and expressions for η, spin polarization of current P, and the MR ratio are derived. The presence of IR at the FM-SC boundary is crucial for generating a large spin splitting of μ, and consequently a high injection efficiency η. The IR needs to fulfill the requirements of (ⅰ) of a large magnitude comparable to the resistance of the SC layer and (ⅱ) high asymmetry with respect to the two spin directions. To increase η from 1% to 30%, we require a large IR of 10~(-5) Ωcm~2 and a spin asymmetry of 10:1. There are more stringent requirements for achieving a high MR ratio. An IR of 10~(-5) Ω cm~2 and FM contact polarization P_c of 80% will only yield an MR ratio of 10%. We require a much larger IR ≥ 10~(-4) Ω cm~2 or virtually half-metallic contacts, i.e., P_c of ～100% to achieve high MR ratio exceeding 50%.