Inelastic electron tunneling spectra of MgO-based magnetic tunnel junctions with different electrode designs

Abstract

MgO-based magnetic tunnel junctions with up to 230% tunnel magnetoresistance ratio at room temperature and up to 345% at 13 K are prepared. The lower electrode is either exchange-biased or free, while the top electrode is free or an exchanged-biased artificial ferrimagnet, respectively. Additionally, a pseudo-spin-valve ͑hard-soft switching͒ design with two unpinned electrodes is used. Inelastic electron-tunneling spectra for each of these systems show a strong variation in the zero-bias anomaly with a reduced peak for some of the junctions. At voltages around 200 mV additional structures are found, which are not known from junctions with lower magneto resistance, such as alumina-based junctions. We discuss the spectra for the different electrode types and compare our findings with respect to barrier material and magnetoresistance ratio.

Perpendicular magnetic anisotropy in Ta|Co40Fe40B20|MgAl2O4 structures and perpendicular CoFeB|MgAl2O4|CoFeB magnetic tunnel junction

“…Three peaks, including zero-bias anomaly (ZB) due to magnetic impurity scattering, magnon excitation (M) at the interface of ferromagnetic electrode and barrier, and phonon excitation (Ph) in barrier, are usually found in AlO x and MgO based MTJs. 29 In this work, ZB locating at 5 mV and M locating at 25 mV are identified, which is comparable with in-plane MTJs. 29 The peak at voltage of about 115 mV is identified as Ph, originating from phonon excitation in MgAl 2 O 4 barrier.…”

“…29 In this work, ZB locating at 5 mV and M locating at 25 mV are identified, which is comparable with in-plane MTJs. 29 The peak at voltage of about 115 mV is identified as Ph, originating from phonon excitation in MgAl 2 O 4 barrier.…”

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

“…Yet tunnelling magnetoresistance ratios (TMRs) remain one order of magnitude below those predicted 17 . 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] .…”

“…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 .…”

Symmetry-dependent electron-electron interaction in coherent tunnel junctions resolved by measurements of zero-bias anomaly

“…Related to ZBA is the ZB peak in the second derivative of the current-voltage curves, which is also called inelastic electron tunneling spectroscopy (IETS) and is frequently used to investigate inelastic tunneling processes. In IETS of MTJs [29][30][31][32][33], the ZB peak has been ascribed to various mechanisms, including magnon excitation [34], magnetic impurity scattering [35], and a combination of magnon scattering and EEI [27]. Although some earlier works [25,36] did not distinguish between ZBA and magnon-induced reduction of TMR over a much wider bias range, it was later shown theoretically that interface magnon scattering does not yield sharp peaks at all [37], contradicting the interpretation that the ZBA and the sharp peak in the IETS near zero bias are due to magnon excitation.…”