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Synopsis

Magnetic tunnel junctions (MTJs), composed of ferromagnetic layer/insulating barrier/ferromagnetic layer structures, are fundamental devices in spintronics, such as magnetic read heads for hard disks and magnetic random-access memory (MRAM) cells. MTJs directly convert magnetic information into electrical signals. The conversion efficiency is determined by the tunnel magnetoresistance (TMR) ratio, which is the relative change in resistance. Consequently, enhancing the TMR ratio at room temperature has been a key research objective. The discovery of new materials, such as the MgO barrier, has significantly contributed to advancements in TMR ratios.

In my presentation, I will introduce developments in materials and thin film technology in NIMS that have led to significant TMR enhancements. The CoFeB/MgO/CoFeB MTJ structure is widely used in applications due to its ability to achieve large TMR ratios and its versatile compatibility with various substrates. The large TMR ratios observed in MgO barriers are primarily due to the coherent tunneling effect through (001)-oriented MgO barriers. As shown in Fig. 1, the introduction of the MgO barrier in 2004 has dramatically increased the room-temperature TMR ratio, which reached a maximum of 604% in 2008 [1]. However, this value remained stagnant for 15 years. By improving the quality of the barrier interfaces using epitaxial MTJs, including as post-annealing of each layer, nano-insertions at the barrier interfaces, and introducing post-oxidation processes [2], we achieved a record of 631% at room temperature in 2023 with a CoFe/MgO/CoFe(001) epitaxial MTJ [3].

We also applied our interface control technologies to CoFeB/MgO/CoFeB polycrystalline MTJs, which are more industrially viable. Using a high-throughput, automated sputtering system with in-situ crystal orientation evaluations via reflection high-energy electron diffraction (RHEED) and in-situ post-annealing, we improved the MgO interface quality. This approach yielded to a record room-temperature TMR ratio of over 400% in top-exchange-bias-type CoFeB/MgO/CoFeB MTJs [4].

Furthermore, I will introduce our efforts in developing barrier materials. Our interface control technologies have greatly benefited from our discovery of the spinel MgAl₂O₄ barrier in 2009 [5], which exhibits excellent lattice matching with ferromagnetic layers such as CoFeB and Fe. Recently, our spinel-based MTJs achieved a giant TMR ratio of 429% at room temperature [6]. We recently reported that substituting Al with Ga in MgAl₂O₄ to form MgGa₂O₄ barriers achieves lower resistance than conventional MgO barriers [7]. Additionally, we successfully developed a stable five-element oxide barrier, LiTiMgAlGaO, which demonstrated high interface magnetic anisotropy with CoFeB and a TMR ratio exceeding 80% at room temperature [8]. This barrier material is considered a “high-entropy oxide (HEO)” and showcases the potential use of complex oxides in spintronic devices. These advancements in materials technology are expected to further enhance TMR ratios and expand the scope of MTJ applications in the future.

References

[1] S. Ikeda et al., Appl. Phys. Lett. 93, 082508 (2008).
[2] T. Scheike et al., Appl. Phys. Lett. 118, 042411 (2021).
[3] T. Scheike et al., Appl. Phys. Lett. 122, 112404 (2023).
[4] H. Sukegawa et al., The 2025 Joint MMM-Intermag Conference, New Orleans, USA, January 13–17, 2025.
[5] R. Shan et al., Phys. Rev. Lett. 102, 246601 (2009).
[6] T. Scheike et al., Appl. Phys. Lett. 120, 032404 (2022); K. Masuda et al., Phys. Rev. B 111, L220406 (2025).
[7] R. R. Sihombing et al., Appl. Phys. Lett. 126, 022407 (2025).
[8] R. R. Sihombing et al., Mater. Today 88, 12 (2025).