Modelling of spin-polarized electron tunnelling from 3d ferromagnets

Spin-polarized electron tunnelling from ferromagnetic Fe and Co Alms is modelled within a quantum-mechanical treatment of the electronic transport and a tight-binding approximation accounting for an accurate band structure of the 3d metals. Calculations have been performed assuming that the band gap of the insulator is much larger than the hopping integrals between the metal and the insulator, the electronic structure of the latter being approximated by two non-coupled s-type tight-binding bands separated by a gap. It is found that within the ballistic regime of conductance the spin polarization of the tunnelling current depends strongly on the type of covalent bonding between the ferromagnet and the insulator. In the case of ssa bonding the tunnelling current is carried only by the s electrons of the ferromagnet and the spin polarization is positive. This is due to the strong s-d hybridization within the ferromagnet which reverses the sign of the spin polarization of the s-electron partial density of states at the Fermi level with respect to the total surface density of states. The absolute values of the spin polarization of the tunnelling current in this case of ss sigma bonding across the metal-insulator interface are in very good agreement with experimental data on tunnelling between 3d ferromagnets and aluminium through an alumina spacer. Including the sd sigma bonding at the interface, however, results in a large contribution of the d-electron tunnelling current, which reduces the spin polarization and leads to a change in its sign for the case of Co.