Nanomagnetism and Spintronics
Studniarek1,3, U. Halisdemir1, F. Schleicher1, B. Taudul1, E. Urbain1, S. Boukari1, M. Hervé2, C-H Lambert4, A. Hamadeh4, S. Petit-Watelot4, O. Zill1, D. Lacour3, L. Joly1, F. Scheurer1, G. Schmerber1, V. Da Costa1, A. Dixit1, P Guitard5, M. Acosta1, F. Leduc3, F. Choueikani3, E. Otero3, W. Wulfhekel2, F. Montaigne4, E. Monteblanco4*, J. Arabski1, P Ohresser3, E. Beaurepaire1, W. Weber1, M. Alouani1, M. Hehn4, and M. Bowen1
1Institut de Physique et Chimie des Matériaux de Strasbourg UMR 7504, F-67000 Strasbourg, France 2Physikalisches Institut Karlsruhe Institute of Technology Wolfgang-Gaede-Str. 1, 76131 Karlsruhe, Germany, 3Synchrotron SOLEIL L’Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, France, 4Jean Lamour UMR 7198 CNRS Université de Lorraine BP 70239, 54506 Vandoeuvre les Nancy Cedex, France, 5Service de Physique de l’Etat Condensé CEA-IRAMIS-SPEC (CNRS-MPPU-URA 2464) CEA-Saclay F-91191 Gif-sur-Yvette Cedex, France
Our understanding of how devices operate has matured considerably in recent years. Separately conducted device and materials science studies (see e.g. this first report of clear-cut memristance in a spintronic device) have been superseded in scientific insight by ‘operando’ studies that probe the device’s materials properties after the device has been placed in various states of operation. Of course, this approach equates all atoms in a device with its operation, though generally, only a subset of these atoms is actively involved. As an example, it is desirable to hone in on nanocristallites that form near the electrodes of batteries upon repeated cycling and alter its performance. We propose as a next step in operando studies to focus the materials characterization technique onto only those atoms that drive the device’s operation. To do so, the materials science technique excitation is implemented, but the readout is performed using a device operational parameter.
To demonstrate the technique’s interdisciplinary potential, we considered an extreme combination of device class and materials science technique. We’ve chosen magnetic tunnel junctions (MTJs), which are a prototypical spintronics device with industrial penetration toward next-generation memories and bio-inspired computing. Indeed, this device class’s operation is driven by the tunneling through the tunnel barrier’s defects (e.g. oxygen vacancies) of the entirety of the junction’s current. As a result, the macroscale device operates using a minute subset of active atoms. To focus on solely these atoms, we deployed synchrotron-grade x-ray absorption spectroscopy, which is capable of resolving the electronic structure of dilute atomic species buried within a heterostructure, with chemical sensitivity.
We’ve studied the MgO MTJ magnetotransport spectra upon sweeping the soft x-ray photon energy across the O K edge. RP exhibits changes associated with Fe oxide bonds, while RAP does not. We find that these bonds are not only perpendicular to the interface, as anticipated for an ideal MTJ, but also parallel to the interface. This latter observation suggests the presence astride the tunnel barrier’s hotpots of nanoscale FeOx regions. Although this appears counterintuitive considering the high TMR measured, this association of FeOx at both interfaces with high TMR was predicted theoretically. Finally, we find that reaching the O K edge maximum impacts RAP much more strongly than RP. This suggests that the device operation is altered by the absorption of soft x-rays by the device’s active O atoms. We will discuss a first theoretical attempt at explaining the MTJ’s operation in this ‘excited’ state.
Our technique has the potential to considerably simplify progress in device studies spanning a wide interdisciplinary range of research fields. It also strengthens scientific causality between a device’s operation and the materials properties that underscore it, and thus touches upon the philosophy of scientific research itself.
Figure 1. a) XAS references at the O K –edge of both the MgO barrier, either as-grown (black) or annealed (red), and of the encapsulating SiO2 (salmon). Annealing reduces the spectral weight of pre-edge features, which are associated with Fe oxides (blue lines) based on their energy position (grey arrows) away from the Mg 3s XAS peak. Photon energy dependence of the annealed MTJ resistance R at 10 mV and T = 20 K in the b) P and c) AP magnetization states, and d) of the TMR ratio.
 Bias-crafted magnetic tunnel junctions with bistable spin-dependent states, M. Bowen et al, Appl. Phys. Lett. 89 103517 (2006).
 Observation and Quantification of Nanoscale Processes in Lithium Batteries by Operando Electrochemical (S)TEM B. L. Mehdi et al Nano Lett. 2015, 15, 2168.
 Tunneling path toward spintronics, Miao et al, Rep. Prog. Phys. 74, 036501 (2011)
 Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO, F Schleicher et al.Nature Communications 5, 4547 (2014)
 Oxygen-Induced Symmetrization and Structural Coherency in Fe/MgO/Fe(001) Magnetic Tunnel Junctions, C. Tusche, et al, Phys. Rev. Lett. 2005, 95, 176101.