Tunnel magnetoresistance (TMR) is a
magnetoresistive effect that occurs in a
magnetic tunnel junction (MTJ), which is a component consisting of two
ferromagnets separated by a thin
insulator. If the insulating layer is thin enough (typically a few
nanometers),
electrons can
tunnel from one ferromagnet into the other. Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly
quantum mechanical phenomenon.