A spin valve is a device which consist of two metallic magnetic layers separated by a non-magnetic spacer whose electric resistance can change depending on the relative alignment of magnetization in the two magnetic layers, electrons moving through a non-magnetic material normally have random spins, so the net effect is cancelled out. when the magnetization of both the magnetic material are in the same direction or parallel, the resistance in the sandwiched layer becomes low so that the electrons in the that layer have spins all aligned in one direction - either all up or all down, allowing a new way to store binary data in the form of one's (all spins up) and zeroes (all spins down).