Spin-Polarized Density of States and Electron Tunnelling from the CO/Al 2 O 3 Interface
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ABSTRACT In order to elucidate the mechanism of spin-polarized electron tunneling in thin-film ferromagnet-insulator junctions, self-consistent band structure calculations of the Co/A120 3 interface have been performed using a new LMTO technique. Since the results of the calculations are very sensitive to the distance between the Co and Al planes, we have minimised the total energy with respect to this distance. Our calculations show that at the Fermi energy a strong bonding between the 3d-electrons of Co with the sp-electrons of Al at the interface can have an important influence on the spin polarization of the layer-projected density of states (LPDOS) of inner Al and 0 layers. Since the Fermi energy lies within the minority-spin d-band of Co but above the majority-spin d-band, the sp-d bonding results in a smaller LPDOS of the minority-spin electrons of the interfacial Al layers in comparison to that of the majority-spin electrons. This asymmetry in the LPDOS extends to the inner A12 0 3 layers implying a positive spin polarization of the tunneling density of states. The result is consistent with experimental observations on tunnelling from cobalt through alumina where positive values of the spin polarization of the tunnelling current were measured. INTRODUCTION Values of the spin polarization of the tunnelling current from a ferromagnetic metal through an insulator film are known experimentally for numerous ferromagnetic metals and alumina spacers [1]. It was found that for all ferromagnetic 3d-metals these values are positive. Various theoretical explanations were proposed in order to explain these positive values of the spin polarization [2-3]. Although quantitative agreement with experimental observations was shown in [2-3], no accurate calculations of the tunnelling current were performed and results were based mainly on semi-quantitative estimates. Recently, spin-polarized electron tunnelling from ferromagnetic Fe and Co films was modelled within a rigorous quantum-mechanical treatment of the electronic transport and a tight-binding approximation accounting for an accurate band structure of the ferromagnetic metals [4]. It was 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 ssu bonding the tunnelling current is carried only by the s electrons of the ferromagnet and the spin polarization was positive. Including the sda bonding at the interface, however, resulted in a large contribution of the d electron tunnelling current, which reduced the spin polarization and lead to a change in its sign for the case of Co. The importance of the interfacial contribution to the spin polarization of tunnelling was earlier emphasized in [5]. In general, the bonding between the electronic orbitals of the metal atoms and the electronic states of the oxide ions depends strongly on 319
Mat. Res. Soc. Symp. Proc. Vol. 492 01998 Materials Research Society
the band struc
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