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タイトル：Recent advances in magnetic tunnel junction technology for giant tunnel magnetoresistance effect
講演者：介川 裕章 氏
　　　　国立研究開発法人物質・材料研究機構（NIMS）,
　　　　磁性・スピントロニクス材料研究センター スピントロニクスグループ グループリーダー
日時：2025/7/4th（Fri）14:30～16:30  (JST)　ハイブリッド開催
場所：東京大学理学部1号館　913号室　
URL：https://u-tokyo-ac-jp.zoom.us/j/85395800582

Magnetic tunnel junctions (MTJs) are used in magnetic heads for hard disk drives (HDDs) and magnetic random access memory (MRAM) cells, serving as a core component in spintronics applications. In this talk, I will present the recent MTJ research activities in NIMS, especially focused on improving room temperature tunnel magnetoresistance (TMR) ratios based on barrier interface engineering and new material developments.
The discovery of an MgO barrier represents a breakthrough in TMR materials. Compared to a conventional amorphous alumina barrier, a significant increase in the room temperature TMR ratio was achieved. The giant TMR is interpreted to be due to the coherent tunneling effect through an MgO(001) barrier. Although the TMR ratio reached 604% in 2008 [1], it remained unchanged for 15 years. Through the development of single-crystal MTJs, we discovered that MgAl2O4 (MAO) barriers exhibit good lattice matching with CoFe-based ferromagnetic materials. This enables the improvement of the TMR ratios due to the coherent tunneling effect, similar to MgO [2]. In developing spinel barrier based MTJs, we developed precise control techniques for the flatness and oxidation state of the barrier interfaces, which led to the observation of the world’s largest TMR ratio of 631% at room temperature in a single-crystal CoFe/MgO/CoFe(001) MTJ [3]. Furthermore, we observed a significant TMR ratio oscillation with the barrier layer thickness, so called TMR oscillation, in the MTJ, suggesting a relationship with the giant TMR ratio [2, 4].
MTJ applications also require a reduction in the area resistance (RA) to improve HDD recording density and MRAM density. MgO barriers have already been thinned to less than 1 nm, and further significant reduction in RA remains challenging. We are developing a new barrier material by focusing on the gallium-based spinel MgGa2O4 (MGO). MGO has a smaller bandgap (~4.7 eV) than MgO and MAO; its lower barrier height leads to a lower RA [5]. Recently, we achieved a high-quality interface by controlling the insertion of ultra-thin MgO layers at the MGO barrier, resulting in a room temperature TMR ratio of 151% [6].
Finally, I will present our recent study on new crystal orientation fcc(111)-based MTJs toward a giant TMR effect. Masuda et al. predicted a giant TMR ratio exceeding 2,000%, such as in Co/MgO/Co(111), due to an interface resonance effect [7]. The status of (111)-based MTJ development will also be presented [8].

References

[1] S. Ikeda et al., Appl. Phys. Lett. 93, 082508 (2008).
[2] T. Scheike et al., Appl. Phys. Lett. 120, 032404 (2022).
[3] T. Scheike et al., Appl. Phys. Lett. 122, 112404 (2023).
[4] K. Masuda et al., Phys. Rev. B 111, L220406 (2025).
[5] H. Sukegawa et al., Appl. Phys. Lett. 110, 122404 (2017).
[6] R. R. Sihombing et al., Appl. Phys. Lett. 126, 022407 (2025).
[7] K. Masuda et al., Phys. Rev. B 101, 144404 (2020); Phys. Rev. B 103, 064427 (2021).
[8] J. Song et al., arXiv 2308.04149; C. He et al., Acta Mater. 261, 119394 (2023).