Spin-polarized electron tunneling across a disordered insulator

It is shown that the presence of disorder within an insulator has a dramatic effect on the mechanism and the spin polarization of tunneling in ferromagnet-insulator-metal tunnel junctions. We have calculated the conductance of the tunnel junction within a quantum-mechanical treatment of the electronic transport. The spin-polarized band structure of the ferromagnet was approximated by exchange-split tight-binding bands and the disorder was represented by a randomness in on-site atomic energies of the insulator. We demonstrate that for each realization of the disorder the conductance displays numerous resonances, which are determined by multiple scattering processes. The distribution of the conductance with respect to different random configurations is extremely broad, covering many orders of magnitude. The dominant contribution to the tunneling current comes from a few random configurations of disorder which provide highly conducting resonant electronic channels. We find that the spin polarization P of the tunneling current is determined not only by the intrinsic properties of the ferromagnet alone, but in a regime of relatively high disorder which is typical for experiments, the spin polarization decreases with increasing disorder and the thickness of the insulator. This behavior can be explained qualitatively in terms of quasi-one-dimensional tunnelling through an effective potential barrier, the height of which decreases with increasing disorder and insulator thickness. At high disorder the tunneling magnetoresistance calculated directly by modelling the parallel and antiparallel alignments of the ferromagnets agrees with that predicted by Julliere's formula 2P(2)/(1 + P-2). [S0163-1829(98)03026-4].