Utilizing Co/Al2O3/Co magnetic tunnel junctions (MTJs) with Co electrodes of different crystalline phases, a clear relationship between electrode structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (poly-phase and poly-directional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated band structures (DOS) of fcc-Co and hcp-Co.

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We have calculated an accurate exchange-correlation energy of a hole gas, including the complexities related to the valence band coupling as occurring in semiconductors like GaAs, but excluding the band warping. A parametrization for the dependence on the density and the ratio between light-and heavy-hole masses is given. We apply our results to a hole gas in an Al x Ga 1Ϫx As/GaAs/Al x Ga 1Ϫx As quantum well and calculate the two-dimensional band structure and the band-gap renormalization. The inclusion of the valence band coupling in the calculation of the exchange-correlation potentials for holes and electrons leads to a much better agreement between theoretical and experimental data than when it is omitted. ͓S0163-1829͑97͒03931-3͔

The applicability of the strain-induced bcc phase of Co in magnetoresistive devices was studied. Ultrathin bcc Co͑001͒ films and the influence of the additional layers needed for magnetoresistive devices were examined by means of 59 Co nuclear magnetic resonance ͑NMR͒. NMR is shown to be a discriminating technique for determining the presence of structurally and magnetically pure bcc Co. The maximum stability for uncovered and Fe-covered layers grown on Fe͑001͒/GaAs͑001͒ and Fe͑001͒/Ge͑001͒ seed layers is found to be about 2 nm. Growth of an Al 2 O 3 top layer preserves the bcc phase, in contrast to a Cu film which causes a transformation of the bcc structure to the fcc or the hcp phase. The bcc-preserving effects of Al 2 O 3 imply the possibility to fabricate magnetic tunnel junctions with bcc Co͑001͒ bottom electrodes. Although bcc Co is a force-induced structure, thin layers are shown to be stable over a few years when Al 2 O 3 has been grown on top. Junction structures using bcc Co͑001͒ bottom electrodes were grown and characterized.