Giant magnetoresistance arises when a non-magnetic material is sandwiched between thin layers of a magnetic metal. The effect derives from the internal “rotation” of the electron, its spin, which creates a magnetic moment. Each electron behaves like a small magnet, pointing either upwards or downwards. Giant magneto­resistance depends on the difference in electrical resistance for electrons with different spin directions in thin magnetic layers.
A sandwich of nanometre thick magnetic layers with a non-magnetic layer in the middle will allow diffe­rent magnitudes of current to pass through it depending on whether the magnetization in the magnetic layers points in the same direction (the upper figure) or is opposed (the lower figure). In the first case the electrons with the “correct” spin pass straight through the sandwich and an electric current therefore flows through the structure even though electrons with the “wrong” spin are dispersed in the first layer. In the second case, where the magnetization is opposed, all the electrons are “wrong” in one of the two magnetic layers so that the total resistance is large and bars the flow of electric current.