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Quantum-well (QW) states in nonmagnetic metal layers contained in magnetic multilayers are known to be important in spin-dependent transport, but the role of QW states in magnetic layers remains elusive. Here we identify the conditions and mechanisms for resonant tunneling through QW states in magnetic layers and determine candidate structures. We report first-principles calculations of spin-dependent transport in epitaxial Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr tunnel junctions. We demonstrate the formation of sharp QW states in the Fe layer and show discrete conductance jumps as the QW states enter the transport window with increasing bias. At resonance, the current increases by one to two orders of magnitude. The tunneling magnetoresistance ratio is several times larger than in simple spin tunnel junctions and is positive (negative) for majority-(minority-) spin resonances, with a large asymmetry between positive and negative biases.The results can serve as the basis for novel spintronic devices.

Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.

Recent consolidation by Cummings & Hart [Aust. J. Phys. (1988), 41,423-431] of five measured data sets of high-precision Si structure factors and subsequent analysis by Deutsch [Phys. Lett. A (1991), 153, 368-372] produced information on the charge density of Si with precision that is unmatched by any other system. A detailed comparison with newly performed ab initio electronic structure calculation within the local density formalism (LDF) is presented here. The convergence of the calculation is extended to the limit at which the results reflect the predictions of the underlying LDF, unobscured by computational uncertainties. Excellent agreement (e.g. R =0.21% which is three to five times better than previous calculations) is found. This allows the effects of high-index structure factors to be assessed (currently beyond the reach of high-precision measurements) on both static and dynamic deformation charge densities.

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Ultrathin gate-oxide films and alternate dielectrics call for accurate modeling of tunneling currents. Available models, based on the effective-mass approximation, yield good fits to finite-bias data, but fail for infinitesimal biases. Here we report a first-principles theory of tunneling currents. We show that the conductance at infinitesimal bias is a ground-state property and can be calculated accurately using density-functional theory and the local-density approximation for exchange and correlation. At finite biases, a discontinuity in the exchange-correlation potential must be properly included. Challenges for both theory and experiments are identified.

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