Spin-based devices
The goal of spintronic devices is to exploit the spin as well as the charge of the electrons that
pass through them. The spin light-emitting diode (spin-LED, top left), for example, in which spinpolarized
electrons injected from a ferromagnetic layer (blue) into a semiconductor structure
(orange) recombine with holes in the active region (yellow) to produce circularly polarized light
(pink, where the arrow indicates the direction of polarization), has already been demonstrated in
the lab and could be useful for encrypted communication. However, it could be several years
before the most immediately useful device — a spin-based transistor — is built. In a lateral spin
transistor (top right) spin-polarized electrons are injected from a ferromagnetic source into a
narrow semiconductor channel (yellow) in which the electron spins can only move in 2D. Here,
the spin can be switched between up and down by an applied magnetic field or the gate voltage,
which thus determines the output spin current in the ferromagnetic "drain" material. An
alternative approach is the single-electron spin transistor (bottom left), in which a ferromagnetic
source injects a polarized electron into a semiconductor nanostructure called a quantum dot,
where its spin state — and thus the output current in the ferromagnetic drain — is controlled by an
applied gate voltage. A third design for a spintronic transistor is the magnetic tunnel transistor
(bottom right), in which the injected electrons are filtered depending on their spin as they tunnel
through a thin insulating layer (red), as happens in a magnetic tunnel junction, before passing
through a Schottky barrier. The output current in the "collector" semiconductor can therefore be
controlled by changing the spin alignment of the "emitter" and "base" ferromagnetic layers.