Inelastic electron tunneling spectroscopy (IETS) of MgO-based tunnel junctions with reduced magnetic complexity — a fully nonmagnetic W/MgO/W junction and a single-ferromagnet Co40Fe40B20/MgO/W junction — reveals two spectral features whose microscopic origin has not been assigned: a negative zero-bias anomaly (a conductance dip near 9-10 mV, opposite in sign to the positive anomaly of full magnetic tunnel junctions), tentatively attributed to impurity-mediated tunneling via tungsten atoms implanted into the MgO barrier during fabrication, and a broad antisymmetric (odd) feature peaking near 150 mV and extending to several hundred mV, suspected to arise from excitation of high-energy bulk magnons even though these energies exceed the Curie-temperature scale of the electrode and surface-localized exchange coupling established in STM experiments argues against bulk-magnon excitation in tunneling. In the subsequent literature, work on the zero-bias anomaly and on magnon excitation in MgO magnetic tunnel junctions treats two-ferromagnet junctions and low-energy interface magnons, leaving the sign inversion with nonmagnetic electrodes, the tungsten-implantation hypothesis, and the high-energy bulk-magnon coupling question unresolved. The problem — mechanism assignment for these inelastic spectral features, by controlled junction series with barrier characterization and/or microscopic electron-magnon coupling calculations in the tunneling geometry — remains open.

Background

A magnetic tunnel junction (MTJ) consists of two ferromagnetic electrodes separated by a thin insulating barrier; because the tunneling conductance depends on the relative orientation of the electrode magnetizations, MTJs exhibit tunnel magnetoresistance (TMR). Junctions with crystalline MgO barriers, which act as a symmetry spin filter, reach TMR ratios up to roughly 1000% at low temperature (S. Ikeda et al., Applied Physics Letters 93, 082508 (2008)), but the TMR is two to three times smaller at room temperature and falls strongly with applied bias voltage. Inelastic scattering inside the junction — excitation of phonons, of magnons (the quantized spin waves of a magnetically ordered material), and of impurity- or defect-related states — is a main contributor to this degradation. It can be interrogated by inelastic electron tunneling spectroscopy (IETS): the second derivative of the current-voltage curve, d2I/dV2d^2I/dV^2, shows peaks at bias voltages eVeV matching the excitation energies. In MgO-based junctions the barrier phonons appear near 66 and 81 mV and tungsten electrode phonons near 26 mV (J. Klein et al., Physical Review B 7, 2336 (1973), for oxide-phonon energies).

A long-studied feature of MTJ spectra is the zero-bias anomaly (ZBA), a sharp conductance feature at V=0V=0: J. S. Moodera, J. Nowak and R. J. M. van de Veerdonk (Physical Review Letters 80, 2941 (1998)) attributed it to spin-wave scattering at magnetically disturbed interfaces, and L. Sheng, D. Y. Xing and D. N. Sheng (Physical Review B 70, 094416 (2004)) showed theoretically that inelastic tunneling mediated by impurities in the barrier reproduces it. The Bielefeld group around Drewello, Reiss and Thomas had earlier documented strong magnon contributions to the TMR temperature dependence and the IETS of MgO-based MTJs (V. Drewello et al., Physical Review B 77, 014440 (2008); Physical Review B 79, 174417 (2009)).

In a 2010 preprint, V. Drewello, Z. Kugler, G. Reiss and A. Thomas (arXiv:1008.0326) applied IETS at 13 K to MgO junctions with reduced magnetic complexity: the fully nonmagnetic metal-insulator-metal junction W/MgO/W and a junction with a single ferromagnetic electrode, Co40Fe40B20/MgO/W, in which electrons tunnel into W or into Co-Fe-B depending on bias polarity. Even/odd spectra — the average (difference) of the spectra at positive and negative bias — separate bias-symmetric, phonon-like contributions from antisymmetric ones. Both junction types show a negative ZBA (a conductance dip near 9-10 mV), opposite in sign to the positive ZBA of full MTJs. Tunneling into the ferromagnet shows strong magnon excitation, and the odd spectrum contains, besides the low-energy magnon peak, a second, much broader feature peaking near 150 mV and extending to several hundred mV. These energies correspond to effective temperatures above 1000 K, i.e. above the Curie temperature TCT_C (the temperature above which ferromagnetic order is lost) of the electrode, where the total magnon density of states of a ferromagnet is large. Whether the electron-magnon interaction in a tunnel junction can actually excite such high-energy bulk magnons is unclear: in spin-polarized scanning tunneling microscopy the exchange interaction was found to be too surface-localized to excite bulk magnons (T. Balashov et al., Physical Review Letters 97, 187201 (2006); Physical Review B 78, 174404 (2008)), while in incoherent alumina-based junctions the observed magnon peak sits near 10 mV (P. V. Paluskar et al., Applied Physics Letters 91, 222501 (2007)). For the negative ZBA the source proposes impurity-mediated tunneling, specifically implantation of tungsten atoms of the upper electrode into the MgO barrier during sputter deposition — consistent with the long-standing observation that the zero-bias effect depends on the impurity species (J. R. Cooper and A. F. G. Wyatt, Journal of Physics F 3, L120 (1973)) — but offers no direct verification. Finally, as a minor note, the source observes that the parallel-state IETS spectrum of full MgO MTJs is shifted to negative values, read as a possible signature of a large coherent (symmetry-conserving) tunneling contribution, tentatively ascribed to different MgO growth on different lower electrodes.

Problem Statement

Determine the microscopic origin of the inelastic tunneling spectral features observed in MgO-based tunnel junctions with magnetic and nonmagnetic electrodes, as follows. (1) The negative zero-bias anomaly in W/MgO/W and Co40Fe40B20/MgO/W junctions: establish whether it arises from impurity-mediated tunneling, in particular from implantation of (tungsten) electrode atoms into the MgO barrier during fabrication, by direct chemical/structural analysis of the barrier and by controlled variation of impurity species or implantation conditions, or from another mechanism. (2) The broad high-energy odd feature, with maximum near 150 mV and extending to several hundred mV: establish whether it is produced by excitation of high-energy bulk magnon modes - requiring a determination of whether the electron-magnon interaction potential in the tunneling geometry permits efficient excitation of bulk magnons at energies corresponding to effective temperatures above the Curie temperature TCT_C of the ferromagnetic electrode, whose coupling matrix elements have not been determined - or by another mechanism. A mechanism assignment should account for each feature's sign, energy scale, bias symmetry, and material dependence, supported by experiments on junction series in which impurity content or barrier growth is systematically varied (with barrier characterization), and/or by microscopic calculations of the electron-magnon and impurity-assisted coupling matrix elements and the resulting IETS line shapes in the tunneling geometry.

References

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