Spin-transfer torque

A simple model of spin-transfer torque for two anti-aligned layers. Current flowing out of the fixed layer is spin-polarized. When it reaches the free layer the majority spins relax into lower-energy states of opposite spin, applying a torque to the free layer in the process.
A schematic diagram of a spin valve/magnetic tunnel junction. In a spin valve the spacer layer (purple) is metallic; in a magnetic tunnel junction it is insulating.

Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.

Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic to the carrier. An electric current is generally unpolarized (consisting of 50% spin-up and 50% spin-down electrons); a spin polarized current is one with more electrons of either spin. By passing a current through a thick magnetic layer (usually called the “fixed layer”), one can produce a spin-polarized current. If this spin-polarized current is directed into a second, thinner magnetic layer (the “free layer”), the angular momentum can be transferred to this layer, changing its orientation. This can be used to excite oscillations or even flip the orientation of the magnet. The effects are usually seen only in nanometer scale devices.