Spin-dependent tunneling (SDT) is an imbalance in the electric current carried by up- and down-spin electrons tunneling from a ferromagnet through an insulating barrier. This phenomenon was discovered by Tedrow and Meservey in 1970. Using superconducting layers as detectors they measured the spin polarization of the tunneling current originating from various magnetic electrodes across an alumina barrier.
Qualitatively the origin of SDT can be explained by the fact that in ferromagnetic metals electronic bands are exchange split which implies different densities of states at the Fermi energy for the up- and down-spin electrons. Therefore, the number of electrons that can tunnel through the barrier and consequently the tunneling conductance depend on spin. A more accurate description of SDT involves spin-dependent transmission across the ferromagnet/insulator interface that determines the spin polarization of tunneling conductance.