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Tunnel magnetoresistance is often explained using the Jullière model which predicts a magnetoresistance ratio function of two quantities, named polarization, characteristic of each electrode of the junction. However, the dispersion of experimental data indicates that the polarization is not only related to the band structure of the electrode material but rather to the combination electrode barrier. The parabolic band model shows indeed that all the characteristics of the barrier have to be taken into account for defining the polarization of an electrode. Following the Jullière model, the magnetoresistance of an asymmetric tunnel junction should be entirely defined by the polarizations of each of its electrode. We show theoretically that this approach can be used for dissimilar materials and asymmetric barriers. This approach is still valid when a voltage is applied to the junction but, the evolution of the polarization with the voltage is mainly ruled by the deformation of the barrier (and not only by band-structure effects). However, the domain of validity of the Jullière model and its picture of separated electrodes is limited to resistive enough junctions. For low resistive junctions, it is necessary to consider the tunnel junction as a single entity as shown by our calculations. [ABSTRACT FROM AUTHOR]

Micromagnetic features appearing during the reversal of an artificial ferrimagnet used as a hard layer of a magnetic tunnel junction are quantitatively analyzed using the high sensitivity of the spin polarized tunnel current to magnetization fluctuations in the electrodes of the magnetic junctions. We propose an analytical model which takes into account different tunneling paths associated with local magnetization configurations. The model allows a quantitative correlation between the spin polarized transport characteristics and the field-dependent domain structure. The results extracted from the tunnel magnetoresistance measurements are found to be in good agreement with the magnetic domain wall density extracted from magnetic force microscopy experiments. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic moment of the AFi to optimize the magnetotransport response of the MTJ devices.© 2001 American Institute of Physics. Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic moment of the AFi to optimize the magnetotransport respons! e of the MTJ devices.© 2001 American Institute of Physics. Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic moment of the AFi to optimize the magnetotransport response of the MTJ devices.© 2001 American Institute of Physics. Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic! moment of the... [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions have been fabricated, that use Co/Ru/Co and Co/Ru/CoFe artificial ferrimagnet (AFi) systems as hard magnetic electrodes. The ∼22% tunnel magnetoresistance signal obtained, at room temperature, for junctions with Co/Ru/Co was increased to ∼30% by taking advantage of the higher spin polarization of CoFe in contact with the barrier. Along with this improvement, a different reversal behavior for the AFi was obtained. Together with the enhanced thermal stability, up to 350 °C, of the junctions involving the Co/Ru/CoFe system potential use as magnetic sensors is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which focused on the magnetoresistance of spin dependent tunneling structures using artificial antiferromagnetic structure. Brinkman's theory of tunneling; Temperature field dependence of the junction resistance; Conclusions.

Crystallographic and spin polarized transport properties of (100) textured and (100) epitaxial Fe/MgO/Fe magnetic tunnel junctions are compared. Strong similarities in the transport properties show that structural coherence and magnetic quality at the 25 nm grain scale in textured junctions are sufficient to issue signatures of the spin polarized transport specific to a single crystal junction. This demonstrates that the lateral coherence of the Bloch tunneling wave function is identically limited in both systems. Our analysis leads to model the textured tunnel junction as a juxtaposition of nanometer sized single crystal junctions, placed in parallel. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions with a barrier of magnesium oxide were prepared by plasma oxidation of sputter-deposited magnesium. They show magnetoresistance ratios up to 4.5% at room temperature and 5.5% at low temperatures for barrier thickness of 1.6 nm. The material exhibits low barrier heights of around 0.7 eV. These junctions follows the predictions of the free electron model which contrast with the predictions of band structure calculations and experimental results on epitaxial MgO based tunnel junctions. [ABSTRACT FROM AUTHOR]

Fully epitaxial Fe(001)/Fe3O4(001)/MgO(001)/Co micron-sized magnetic tunnel junctions have been elaborated on MgO(001) substrates. X-ray reflectivity and high-resolution transmission electron microscopy revealed a good quality and epitaxial growth of the stack with abrupt interfaces. The magnetotransport measurements exhibit a large negative tunneling magnetoresistance (TMR) value for magnetic tunnel junctions including an Fe3O4 layer and a MgO tunnel barrier (-8.5% at 300 K and -22% at 80 K). Moreover, the sign of the TMR changes with the applied bias. We discuss here the structural quality of the samples and the transport measurement results. [ABSTRACT FROM AUTHOR]

Low temperature (10 K) high voltage bias dynamic conductivity (up to 2.7 V) and shot noise (up to 1 V) were studied in epitaxial Fe(100)/Fe–C/MgO(100)/Fe(100) magnetic tunnel junctions as a function of the magnetic state. The junctions show large tunnel magnetoresistance (185% at 300 K and 330% at 4 K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe–C interface. The shot noise shows a Poissonian character. This demonstrates a pure spin-dependent direct tunneling mechanism and validates the high structural quality of the MgO barrier. [ABSTRACT FROM AUTHOR]

The association of an antiferromagnetic material with a ferromagnetic material in an exchange coupled bilayer is used as a detection layer in a tunnel magnetoresistive sensor. The magnetic response is shown to be reversible and linear depending on the field range to be measured. Both tunnel magnetoresistance and exchange field decrease linearly with the temperature and lead to temperature independent sensor sensibility. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (t[SUBMgO]), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼ 67% at room temperature) have been observed with t[SUBMgO] = 2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Jullière's model, can be understood in the framework of ab initio calculations. [ABSTRACT FROM AUTHOR]

Abstract: We report on spin polarization reduction by incoherent tunneling in single crystal Co2FeAl/MgO/Co50Fe50 magnetic tunnel junctions (MTJs). A large density of misfit dislocations in the Heusler based MTJs has been provided by a thick MgO barrier and its 3.8% lattice mismatch with the Co2FeAl electrode. Our analysis implicates a correlated structural-transport approach. The crystallographic coherence in the real space has been investigated by High Resolution Transmission Electron Microscopy phase analysis. The electronic transport experiments in variable temperature, fitted with a theoretical extended-Glazman–Matveev model, address different levels of the tunneling mechanisms from direct to multi-center hopping. We demonstrate a double impact of dislocations, as extended defects, on the tunneling polarization. Firstly, the breaking of the crystal symmetry destroys the longitudinal and lateral coherence of the propagating Bloch functions. This affects the symmetry filtering efficiency of the states across the (001) MgO barriers and reduces the associated effective tunneling polarization. Secondly, dislocations provide localized states within the MgO gap. This determines temperature activated spin-conserving inelastic tunneling through chains of defects which are responsible for the one order of magnitude drop of the tunnel magnetoresistance from low to room temperature. [Copyright &y& Elsevier]

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the relative magnetic configuration of the junction and also by the asymmetry of the barriers. The proposed theoretical model, based on sequential tunneling through the system and including spin relaxation, successfully accounts for the experimental observations for bias voltages below 0.5 V, where the influence of quantum well states is negligible. A weak enhancement of conductance and shot noise, observed at some voltages (especially above 0.5 V), indicates the formation of quantum well states in the middle magnetic layer. The observed results open up new perspectives for a reliable magnetic control of the most fundamental noise in spintronic structures. [ABSTRACT FROM AUTHOR]

Abstract: Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport. [Copyright &y& Elsevier]