Enhancing spin injection efficiency through half-metallic miniband conduction in a spin-filter superlattice.

ABSTRACT

We theoretically and numerically studied the band structure and spin transport of electrons subject to a superlattice structure where magnetic semiconductor layers lie between normal semiconductor layers to form periodic spin-filter tunnel barriers. In this alternately deposited superlattice structure, due to the induced periodicity of the envelope wavefunctions, there are additional allowed and forbidden energy regions established, i.e. forming minibands that are far narrower than the conventional conduction bands. The number and thickness of the stacked potential profiles can finely tune these minibands. The spin dependent potential barriers also induce spin splitting at the bottom of each miniband, which generates strongly spin-dependent miniband conduction. Most strikingly, the lowest lying miniband is 100% spin-polarized mimicking a half-metallic behavior on this conduction channel. The total transmission electron current carries thus near-perfectly polarized spin currents when the superlattice falls into suitable miniband conduction regime. This half-metallic miniband enhanced spin-filtering capability paves the way to generate highly polarized spin current without incurring exponentially increased device impedance, as usually happens when only a single spin-filter barrier is applied.