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.

Results of theoretical studies of spin dependent tunneling in magnetic tunnel junctions at finite bias are presented. A simple model which extends Slonczewski’s ideas is developed. For each spin it assumes tunneling from a single free electron band through a simple barrier. The model predicts a decrease in conductance ratio with bias in good agreement with experimental observations. We find that the decrease of the magnetoconductance ratio, universally seen in experiment, has an intrinsic component resulting from the underlying electronic structure.

Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR.

Theoretical treatment of spin dependent tunneling is used to reveal how asymmetry in the bias dependence of the tunneling magnetoresistance ratio (TMR) arises. The tunneling states in the two ferromagnetic electrodes are taken from the results of ab initio electronic structure calculations. We show that differences between the electronic structure of the electrodes at the two interfaces and/or an asymmetric barrier can cause an asymmetric TMR. Most dramatically, we treat the case of nominally identical electrodes but with different crystalline structures, specifically a Co junction where one electrode is hexagonal-close-packed Co and the other is face-centered-cubic (fcc) Co. In this case we correlate a feature common to both the experiment and the calculations with a specific feature found in the calculated band structure of fcc Co, suggesting the band structure is at the root of the asymmetry.

In this paper we report the results of a theoretical study of the ballistic tunnelling of electrons in magnetic tunnel junctions. We show how first-principles band-structure calculations and published magnetization data can be used as inputs to the model, which is then used to predict the magnetoresistance of a tunnel junction. This approach provides a convenient way to examine effects not readily treated by purely first-principles calculations such as finite bias and finite temperature as well as providing a way to treat the amorphous nature of most tunnelling barriers. Ultimately, the model is used to show how the tunnelling magnetoresistance depends on extrinsic factors such as applied bias, and temperature, as well as on the intrinsic properties of the junction such as the barrier height and thickness. The model generates the asymmetry in the magnetoresistance often seen for forward and reverse bias in asymmetric junctions.