Microscopic origin of the zero-bias anomaly and defect-mediated tunneling statistics in Al2O3-barrier magnetic tunnel junctions

In single-barrier Co/Al2O3/Py (Py = Permalloy, a nickel-iron soft ferromagnetic alloy) magnetic tunnel junctions (MTJs), the Fano factor of the tunneling shot noise drops to about 0.65-0.75, exceeds its parallel-state value in the antiparallel magnetic configuration only when the zero-bias conductance anomaly (ZBA) is weak, and returns to the Poissonian value $F\approx 1$ when the amorphous Al2O3 barrier is delta-doped with Cr. Guerrero et al. (2006) attributed this phenomenology qualitatively to sequential spin-flip tunneling through paramagnetic impurity levels inside the barrier. What remains open is the full microscopic account: the unique identification of the ZBA mechanism (sequential impurity tunneling versus magnon- or phonon-assisted tunneling versus Kondo-like scattering), the nature, energy distribution, and spatial positioning of the responsible barrier defect states, and a quantitative theory including charging effects and spin-channel correlations that jointly reproduces the conductance anomaly and the noise statistics, including the relation between nominal Cr coverage and the effective left-right asymmetry of the tunneling.

Background

A magnetic tunnel junction (MTJ) is a ferromagnet/insulator/ferromagnet sandwich whose resistance depends on the relative orientation of the electrode magnetizations (tunneling magnetoresistance, TMR); in early devices the insulator was an amorphous Al2O3 film one to two nanometers thick. When the differential conductance of such a junction is measured at low temperature, a dip in conductance centered at zero applied bias is very commonly observed — the zero-bias anomaly (ZBA). Candidate microscopic explanations, dating from before the source work, include the excitation of magnons (spin waves) by hot tunneling electrons and spin-wave scattering at the ferromagnet/insulator interface, phonon-assisted inelastic tunneling, and two-step (sequential) tunneling in which an electron first hops onto a localized defect or impurity level inside the barrier and then into the second electrode. Shot noise — the current fluctuation arising from the discreteness of the electron charge — probes the statistics of tunneling events in a way dc conductance cannot. It is quantified by the Fano factor $F = S/(2eI)$, the ratio of the noise power $S$ to the Poissonian value $2eI$ of fully uncorrelated tunneling; sequential tunneling through localized states generically yields $F\approx 3/4$, while elastic direct tunneling through a good barrier gives $F\approx 1$. The Kondo effect — screening of a localized magnetic moment by the conduction electrons, producing a resistance contribution that grows logarithmically as the temperature is lowered — provides a further spin-flip mechanism that later work showed can also generate a ZBA in related junction families. Guerrero, Aliev, Tserkovnyak, Santos, and Moodera (Phys. Rev. Lett. 97, 266602 (2006)) performed the first systematic shot-noise measurements on single-barrier Co/Al2O3/Py (Py = Permalloy) MTJs (measuring the noise by cross-correlation of two amplifier channels, a technique that suppresses the amplifiers' own noise), with and without a sub-monolayer Cr delta-layer (a dopant sheet inserted at a nominal depth) at the middle of the barrier. They found: (i) in undoped junctions the Fano factor is suppressed to $F\approx 0.65$–$0.75$ and is enhanced in the antiparallel magnetic configuration, this asymmetry being present only in samples with a weak ZBA and vanishing as the ZBA grows; (ii) Cr delta-doping enhances the ZBA non-monotonically, with large sample-to-sample dispersion, and restores nearly Poissonian noise $F\approx 1$. They explained the data qualitatively with sequential tunneling through a normal region or a single spin-polarized impurity level, characterized by a left-right asymmetry parameter $\beta$ encoding the position/couplings of the level inside the barrier, and concluded that an inelastic spin-flip mechanism in the barrier concurrently enhances the ZBA and reduces the noise asymmetry. The models, however, were qualitative: they neglected charging (Coulomb-blockade, i.e., the energy cost of adding an electron to an already occupied localized state) effects on the localized level and correlations between the two spin channels, treated only a single impurity level, and left the magnon- and phonon-assisted alternatives unexcluded, the barrier defect states unidentified, and the restoration of Poissonian noise by Cr doping only conjecturally attributed to a spatial offset or clustering of the nominally centered Cr layer.

Problem Statement

Determine the microscopic origin of the zero-bias conductance anomaly in polycrystalline MTJs with amorphous Al2O3 barriers (Co/Al2O3/Py and its Cr delta-doped variants), and on that identification build a quantitatively predictive microscopic theory of the spin-dependent tunneling statistics. A complete answer must: (1) establish which mechanism produces the ZBA — sequential spin-flip tunneling through paramagnetic impurity levels inside the barrier, magnon- or phonon-assisted inelastic tunneling, Kondo-like scattering, or a combination — using criteria that discriminate among these candidates under the joint constraints of the dc conductance and shot-noise data (Fano factor suppressed to $F\approx 0.65$–$0.75$ in undoped junctions, enhancement of $F$ in the antiparallel configuration that correlates with the ZBA strength, restoration of $F\approx 1$ upon Cr delta-doping); (2) identify the responsible barrier defect or impurity states — their microscopic nature, energy distribution, and spatial positioning within the barrier — together with the statistical distribution over junctions that produces the observed sample-to-sample dispersion and the non-monotonic dependence on nominal Cr coverage; and (3) construct a quantitatively predictive microscopic theory of the spin-dependent tunneling — extending the source's qualitative single-level models, which neglected charging (Coulomb-blockade) effects on the localized states and correlations between the two spin channels — that reproduces the measured bias dependence of the Fano factor and the magnitude of the ZBA, and that accounts for the restoration of nearly Poissonian noise upon Cr delta-doping, which the source attributed only conjecturally to a spatial offset or clustering of the nominally centered Cr layer producing systematically asymmetric hopping. A purely qualitative model, or a phenomenological fit that matches part of the data without mechanism-level discrimination, does not constitute a solution.

References

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