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- 저자 : Katsuyuki Nakada (검색결과 1 건)
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Design of MgAl₂O₄ Spinel-Oxide-Based Tunnel Barriers for Advanced Spintronics Devices
Kenji Nawa,Keisuke Masuda,Shinto Ichikawa,Hiroaki Sukegawa,Tsuyoshi Suzuki,Katsuyuki Nakada,Seiji Mitani,Yoshio Miura 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Tunnel magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) is one of the central properties to develop high-performance spintronics devices. MgO(001) with bcc-Fe or CoFe electrodes has been mostly studied as a tunnel barrier of MTJs for the last decades because of its giant TMR ratio originating from the spin-filtering effect of Δ₁-symmetric evanescent states (s, pz, d3z²-r² orbitals), as predicted in 2001. However, the TMR ratio of MgO-MTJs reduces significantly by bias voltage applications, indicating a limitation of the output voltage of the MTJ. Recent experiments show that the use of a (001)-oriented spinel-type oxide, MgAl₂O₄, as a tunnel barrier improves the robustness of the TMR ratio under bias applications, but a theoretical TMR limit in Fe/MgAl₂O₄/Fe(001) MTJs is very small compared to the MgO-MTJs. This is because the in-plane lattice periodicity of Fe electrode is half of that of MgAl₂O₄ and a band-folding effect is induced in the two-dimensional Brillouin zone of the in-plane wave vector in the Fe electrode. This effect provides additional conductive states at the Δ line in MgAl₂O₄-MTJ, contributing to the reduction of the TMR ratio. In this talk, we propose a combined trilayer tunnel barrier, MgO/MgAl₂O₄/MgO, to overcome the above issue of the small TMR limit of MgAl₂O₄-MTJs on the basis of the first-principles calculations. We performed ballistic-conductance calculations in an Fe/MgO(n)/MgAl₂O₄/MgO(n)/Fe(001) MTJ using the non-equilibrium Green’s functions method to clarify the TMR ratio under bias voltage application. Here, number of MgO layers (n-ML) is changed as n = 1, 2, 3. In the case of n = 1, a large TMR ratio of 1184% is obtained at a zero-bias voltage and this large value is almost maintained up to V = 1.2 V (see blue in Fig. 1), leading to a large voltage output. In contrast, a single barrier MgAl₂O₄ shows only a small TMR ratio (~125%), which is constant below V = 1.6 V (see orange in Fig. 1). These results indicate that both the models have a similar tendency in bias voltage dependence of TMR, except for the magnitude of a TMR ratio. Moreover, we clarified that the presence of an MgO interlayer between Fe and MgAl₂O₄ plays an important role in retaining (blocking) the Δ1 evanescent state for majority (minority) spin. The former leads to the robustness of the TMR ratio against bias voltage as observed in single MgAl₂O₄ MTJs, while the latter does to the large TMR ratio as in single MgO MTJs. 〈그림 본문참조〉
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