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Register a new userGate control of the tunneling magnetoresistance in double-barrier junctions
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Jeronimo Peralta Ramos
Publication date
2008
fields
Physics
and research's language is
English
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We calculate the conductances and the tunneling magnetoresistance (TMR) of double magnetic tunnel junctions, taking as a model example junctions composed of Fe/ZnSe/Fe/ZnSe/Fe (001). The calculations are done as a function of the gate voltage applied to the in-between Fe layer slab. We find that the application of a gate voltage to the in-between Fe slab strongly affects the junctions TMR due to the tuning or untuning of conductance resonances mediated by quantum well states. The gate voltage allows a significant enhancement of the TMR, in a more controllable way than by changing the thickness of the in-between Fe slab. This effect may be useful in the design of future spintronic devices based on the TMR effect, requiring large and controllable TMR values.
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We calculate the tunneling magnetoresistance (TMR) of Fe$mid$ZnSe$mid$Fe$mid$ZnSe$mid$Fe (001) double magnetic tunnel junctions as a function of the in-between Fe layers thickness, and compare these results with those of Fe$mid$ZnSe$mid$Fe simple junctions. The electronic band structures are modeled by a parametrized tight-binding Hamiltonian fitted to {it ab initio} calculations, and the conductance is calculated within the Landauer formalism expressed in terms of Greens functions. We find that the conductances for each spin channel and the TMR strongly depend on the in-between Fe layers thickness, and that in some cases they are enhanced with respect to simple junctions, in qualitative agreement with recent experimental studies performed on similar systems. By using a 2D double junction as a simplified system, we show that the conductance enhancement can be explained in terms of the junctions energy spectrum. These results are relevant for spintronics because they demonstrate that the TMR in double junctions can be tuned and enhanced by varying the in-between metallic layers thickness.
In this contribution, we calculate the spin-dependent ballistic and coherent transport through epitaxial Fe/ZnSe (001) simple and double magnetic tunnel junctions with two different interface terminations: Zn-terminated and Se-terminated. The electronic structure of the junctions is modeled by a second-nearest neighbors {it spd} tight-binding Hamiltonian parametrized to {it ab initio} calculated band structures, while the conductances and the tunneling magnetoresistance are calculated within Landauers formalism. The calculations are done at zero bias voltage and as a function of energy. We show and discuss the influence of the interface structure on the spin-dependent transport through simple and double tunnel junctions.
Magnetite (Fe3O4) based tunnel junctions with turret/mesa structure have been investigated for different barrier materials (SrTiO3, NdGaO3, MgO, SiO2, and Al2O(3-x)). Junctions with a Ni counter electrode and an aluminium oxide barrier showed reproducibly a tunneling magnetoresistance (TMR) effect at room temperature of up to 5% with almost ideal switching behavior. This number only partially reflects the intrinsic high spin polarization of Fe3O4. It is considerably decreased due to an additional series resistance within the junction. Only SiO2 and Al2O(3-x) barriers provide magnetically decoupled electrodes as necessary for sharp switching. The observed decrease of the TMR effect as a function of increasing temperature is due to a decrease in spin polarization and an increase in spin-scattering in the barrier. Among the oxide half-metals magnetite has the potential to enhance the performance of TMR based devices.
Using a simple quantum-mechanical model, we explore a tunneling anisotropic magnetoresistance (TAMR) effect in ferroelectric tunnel junctions (FTJs) with a ferromagnetic electrode and a ferroelectric barrier layer, which spontaneous polarization gives rise to the Rashba and Dresselhaus spin-orbit coupling (SOC). For realistic parameters of the model, we predict sizable TAMR measurable experimentally. For asymmetric FTJs, which electrodes have different work functions, the built-in electric field affects the SOC parameters and leads to TAMR dependent on ferroelectric polarization direction. The SOC change with polarization switching affects tunneling conductance, revealing a new mechanism of tunneling electroresistance (TER). These results demonstrate new functionalities of FTJs which can be explored experimentally and used in electronic devices.
Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder--Tinkham--Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the tunneling-magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings -- that fully determine the tunneling magnetoresistance in the normal-conducting state -- and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip (unconventional) Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double-peaks around the singlet-gap energy, reflecting the competition between the singlet and the newly emerging triplet gap. We thoroughly analyze the Andreev reflections role in connection with superconducting tunneling-magnetoresistance phenomena, and eventually unravel huge conductance and tunneling-magnetoresistance magnetoanisotropies -- easily exceeding their normal-state counterparts by several orders of magnitude -- as another experimentally accessible fingerprint of unconventional Andreev reflections. Our results provide an important contribution to establish superconducting magnetic spin valves as an essential ingredient for future superconducting-spintronics concepts.
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