A magnetic tunnel junction (MTJ) consists of two layers of magnetic metal, such as cobalt-iron, separated by an ultrathin layer of insulator, typically aluminum oxide with a thickness of about 1 nm. The insulating layer is so thin that electrons can tunnel through the barrier if a bias voltage is applied between the two metal electrodes. In MTJs the tunneling current depends on the relative orientation of magnetizations of the two ferromagnetic layers, which can be changed by an applied magnetic field. This phenomenon is called tunneling magnetoresistance (TMR) which is a consequence of spin-dependent tunneling.
Nowadays MTJs that are based on transition-metal ferromagnets and Al2O3 barriers can be fabricated with reproducible characteristics and with TMR values up to 50% at room temperature. Recently large values of TMR observed in crystalline MTJs with MgO barriers further boosted interest in spin dependent tunneling. MTJs are nowadays used in magnetic random access memories.