Abstract
The quantum well (QW) realizes new functionalities due to the discrete electronic energy levels formed in the well‐shaped potential. Magnetic tunnel junctions (MTJs) combined with a quasi‐QW structure of Cr/ultrathin‐Fe/MgAl2O4(001)/Fe, in which the Cr quasi‐barrier layer confines Δ 1 up‐spin electrons to the Fe well, are prepared with perfectly lattice‐matched interfaces and atomic layer number control. Resonant peaks are clearly observed in the differential conductance of the MTJs due to the formation of QWs. Furthermore, enhanced tunnel magnetoresistance (TMR) peaks at the resonant bias voltages are realized for the MTJs at room temperature, i.e., it is observed that TMR ratios at specific and even high bias‐voltages (V bias) are larger than zero‐bias TMR ratios for the MTJs with odd Fe atomic layers, in contrast to the earlier experimental studies. In addition, a new finding in this study is unique sign changes in the temperature coefficient of resistance (TCR) depending on the Fe thickness and V bias, which is interpreted as a signature of the QW formation of Δ1 symmetry electronic states. The present study suggests that the spin‐dependent resonant tunneling via the QWs formed in Cr/ultrathin‐Fe/MgAl2O4/Fe structures should open a new pathway to achieve a large TMR at practically high V bias.
Highlights
quantum well (QW) potential arises from the band mismatch between Cr Δ1 and Fe Δ1↑.[13,21,22,23,24] It was theoretically predicted that the introduced QWs can generate resonant states large Tunnel magnetoresistance (TMR) at practically high Vbias
No postannealing was performed for the top Fe layer to keep it a general free layer and to limit the QW occurring only in the bottom ultrathin-Fe layer
The dominant states of the electron transport in epitaxial magnetic tunnel junctions (MTJs) with crystalline barrier consisting of MgO or MgAl2O4 possess the Δ1 symmetry, while tunneling probabilities of other states, i.e., Δ5, Δ2 (d-like), and Δ2′ (d-like) states, rapidly decay with increasing the barrier thickness.[4,30,31]
Summary
The significant enhancement of TMR at resonant Vbias, compared to the previous studies, is attributed to two major factors: improved structural coherency owing to MgAl2O4 and enhanced QW effect owing to the atomically flat ultrathin Fe layer. The former supplies a much-improved high bias performance as previous works suggested.[27] Inelastic electron scattering that causes suppression of TMR may occur hardly at the electrode/barrier interface free from misfit dislocations. The observed behavior of TCR can be well interpreted as a fact that welldefined QW states consisting of the Δ1 symmetry electrons are formed in the present Cr/Fe/MgAl2O4 structure, in short, as a signature of the well-defined QWs
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