Resonant Tunneling through Electronic Trapping States in Thin MgO Magnetic Junctions

Abstract

We report an inelastic electron tunneling spectroscopy study on MgO magnetic junctions with thin barriers (0.85-1.35 nm). Inelastic electron tunneling spectroscopy reveals resonant electronic trapping within the barrier for voltages V>0.15 V. These trapping features are associated with defects in the barrier crystalline structure, as confirmed by high-resolution transmission electron microscopy. Such defects are responsible for resonant tunneling due to energy levels that are formed in the barrier. A model was applied to determine the average location and energy level of the traps, indicating that they are mostly located in the middle of the MgO barrier, in accordance with the high-resolution transmission electron microscopy data and trap-assisted tunneling conductance theory. Evidence of the influence of trapping on the voltage dependence of tunnel magnetoresistance is shown.

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

“…Remember that our MTJs are dominated by F-type tunnelling, while those of Teixeira et al 24 also exhibit M-type tunnelling. Referring to our data and summary (see Table 2), we find that F/F þ /F þ */F*-assisted tunnelling leads to a relative increase in I P and I AP , with increasing temperature at the positions in the (V, T) dependence of magnetotransport expected within a picture of defect-mediated tunnelling (see Fig.…”

“…We emphasize the importance of spectrally identifying defect species within magnetotransport. Indeed, Teixeira et al 24 , who find inelastic electron tunneling spectroscopy (IETS) signatures of defect-assisted tunnelling at precisely the same energy positions as here, remain uncertain as to why, at V ¼ 0.4 eV, there is an IETS signature in the D 1 -dominated P but not the D 5 -dominated AP channel 12,37 . Our data indicate that this defect does not dominate transport across our MTJs for To250 K. Nevertheless, our work suggests that this defect is a M centre that, furthermore, is predicted 25 to promote coherent, that is, symmetry-and spinconserved transport.…”

“…Although no such calculations exist for M centres, we argue that, owing to a similar energy position within the band gap and a rather constant D 1 decay parameter in this energy range 37 , this means that the M centre is dominantly of 25 the carrier's symmetry to D 1 for enhanced tunnelling probability off of the defect site. We thus explain the absence of an IETS signature of defect-assisted tunnelling in the AP state in certain MgO MTJs 24 . In our MTJs, however, M centres appear to dominate conduction in the AP state for T4250 K (see Supplementary Fig.…”

“…The impact of intrinsic effects such as magnon and phonon generation on TMR has been documented 18,19 . On the other hand, the impact of structural defects in MgO on tunnelling is only emerging in this otherwise widely studied, canonical spintronic system [20][21][22][23][24][25][26] . Indeed, defects have been qualitatively suspected of accounting for much lower tunnelling barrier heights than expected 12,27 .…”

“…Spin-polarized solid-state tunnelling spectroscopy on lithographically processed stacks 8 can reveal the junction electrodes' electronic structure 12,55 , the junction's symmetry-resolved barrier heights 37 and inelastic tunnelling phenomena 19 that includes hopping onto defect states 24 . We note that, since the D 1 complex loop, which spans the band gap and defines the wavefunction decay through tunnelling, is rather flat within at least ± 1.1 eV about the band gap centre, both electron and hole tunnelling with respect to states above and below E F are considered 37 (rescaling the local density approximation (LDA)-derived 4.7 eV MgO band gap to its experimental value of 7.8 eV yields a flat D 1 band over 2.2 eV about the charge-neutrality level-pegged chemical potential 37 .…”