Spin injection into a ballistic semiconductor microstructure

A theory of spin injection across a ballistic semiconductor embedded between two ferromagnetic leads is developed for the Boltzmann regime. Spin injection coefficient y is suppressed by the Sharvin resistance of the semiconductor r* N (=(h/e 2 )(π 2 /S N ), where S N is the Fermi-surface cross section. It competes with the diffusion resistances of the ferromagnets r F , and y is small, γ∼r F /r* N <<1, in the absence of contact barriers. Efficient spin injection can be ensured by contact barriers. Explicit formulas for the junction resistance and the spin-valve effect are presented.