Magnetic tunnel junction
The magnetic tunnel junction, which consists of two ferromagnetic
layers (blue) separated by an insulator (red), exploits tunnel
magnetoresistance (TMR) to switch the output spin current between
high and low. As such, the device can be used as memory to store
information even when the power is turned off. When the
magnetization of the two magnetic layers is parallel, spin-up electrons
can tunnel through the barrier because many unoccupied states are
available in the second ferromagnetic layer (top). When the two layers
are antiparallel, however, fewer up-spin states are available, so
tunnelling is suppressed (bottom). The difference in the tunnel current
as the spin alignment of the ferromagnetic layers is switched between
parallel and antiparallel is known as the TMR ratio, and to be useful for
building practical devices a TMR of about 500% is necessary. Since
1995, when room temperature TMR was first demonstrated,
researchers have obtained much higher TMR values by changing the
insulating material and its interface.