Geometrical effects on spin injection: 3D spin drift diffusion model

TL;DR

This paper presents a 3D spin drift diffusion model to analyze spin injection in ferromagnet-normal metal/semiconductor interfaces, highlighting the importance of contact area, sample thickness, and tunneling barriers for effective spin injection.

Contribution

It introduces a 3D spin drift diffusion model and investigates the effects of contact area, material thickness, and tunneling barrier quality on spin injection efficiency.

Findings

Small contact area or thick samples enhance spin injection.

Thin Cu films can replace tunneling barriers effectively.

Pinholes reduce spin-injection ratio based on their effective area.

Abstract

We discuss a three-dimensional (3D) spin drift diffusion (SDD) model to inject spin from a ferromagnet (FM) to a normal metal (N) or semiconductor (SC). Using this model we investigate the problem of spin injection into isotropic materials like GaAs and study the effect of FM contact area and SC thickness on spin injection. We find that in order to achieve detectable spin injection a small contact area or thick SC samples are essential for direct contact spin injection devices. We investigate the use of thin metal films (Cu) proposed by S.B. Kumar et al. and show that they are an excellent substitute for tunnelling barriers (TB) in the regime of small contact area. Since most tunnelling barriers are prone to pinhole defects, we study the effect of pinholes in AlO tunnelling barriers and show that the reduction in the spin-injection ratio ($\gamma$) is solely due to the effective area of the pinholes and there is no correlation between the number of pinholes and the spin injection ratio.