Spintronics is an emerging field as a new branch of electronics, in which the charges and spins of electrons are utilized. Spintronic devices are expected to contribute to the realization of low power consumption in the future electronics.
 Spintronics Group is working on the development of tunnel magnetoresistance devices (TMR devices, or magnetic tunnel junctions) which exhibit a large tunnel magnetoresistance ratio (TMR ratio), and on the efficient control of magnetization in magnetic thin films using a new phenomenon called spin-orbit torque. These are expected to be applied to new types of non-volatile magnetic memories (MRAMs) and highly sensitive magnetic sensors. We are also developing ultra-thin magnetic multilayer thin film fabrication technology for new materials exploration.

Introduction of our research

"NIMS" Full-Color LED Demo Unit Using MTJ (TMR Devices)

Introduction of our recent papers

Improved TMR ratio using a "rock-salt-type" MgGaO barrier
Appl. Phys. Lett. 126, 022407 (2025).

Demonstration of Altermagnetism in RuO₂ Thin Films
Nat. Commun. 16, 8235 (2025).

Achieving Low Resistance and High Performance in MTJs Using High-Entropy Oxides, LiTiMgAlGaO
Mater. Today 88, 12 (2025).

Perpendicular magnetic anisotropy appearing at the interface between conductive oxide Li-Ti-O and CoFeB magnetic layer
J. Phys. D: Appl. Phys. 58, 16LT01 (2025).

Atomic layer thickness control of Co/Pt superlattice perpendicular magnetization films using sputtering techniques
APL Mater. 12, 101120 (2024).

Achievement of the world's largest room temperature tunnel magnetoresistance ratio of 631%
Appl. Phys. Lett. 122, 112404 (2023).

This study focused on an MgGa₂O₄ (MGO)-based barrier, which is a promising magnetic tunnel junction (MTJ) barrier due to its band gap of ~4.7 eV, which is much smaller than that of typical barriers, such as MgO (~8 eV). Previously, a relatively high tunnel magnetoresistance (TMR) ratio of 121% was obtained in Fe/MGO/Fe(001) MTJs by our group with Toshiba [Sukegawa et al., Appl. Phys. Lett. 110, 122404 (2017).].
This study demonstrates a significant improvement through the insertion of atomic-scale MgO by sputtering both sides of the MGO barrier: i.e., an Fe/MgO/MGO/MgO/Fe(001) structure. Cross-sectional electron microscope images confirm good lattice matching between the barrier layer and the magnetic layer. Furthermore, unlike conventional MgGa₂O₄ with a spinel structure, this barrier has a rock salt structure; during fabrication, the MgO insertion layers and the MGO barrier intermix to form a single-layer "MgGaO(001)" barrier. The right figure displays the enhanced TMR ratios of 151% at room temperature, which were observed using 0.3-nm MgO insertion layers at both the top and bottom barrier interfaces of Fe/MgGa₂O₄/Fe(001) MTJs. Notably, the low-resistance area (RA) product was maintained even after the MgO insertions due to the low barrier-height.


Figure: [Left] Cross-sectional electron microscope image and atomic model (rock-salt structure) of an Fe/MgGaO/Fe(001) MTJ; [Right] TMR curve at room temperature.

Reference
R.R. Sihombing, T. Scheike, Z. Wen, J. Uzuhashi, T. Ohkubo, S. Mitani, and H. Sukegawa,
"Enhanced tunnel magnetoresistance of Fe/MgGa₂O₄/Fe(001) magnetic tunnel junctions by interface-tuning with atomic scale MgO insertion layers",
Appl. Phys. Lett. 126, 022407 (2025).
Selected as APL Editors' pick
Open access

Details（Press release 2025)

Figure: Image of the altermagnet

Reference
Cong He, Zhenchao Wen, Jun Okabayashi, Yoshio Miura, Tianyi Ma, Tadakatsu Ohkubo, Takeshi Seki, Hiroaki Sukegawa and Seiji Mitani,
"Evidence for single variant in altermagnetic RuO₂(101) thin films"
Nat. Commun. 16, 8235 (2025).

Details（Press release 2025)

Figure: (Top) Cross-sectional electron microscope image of a high-entropy oxide (HEO) single-crystal LiTiMgAlGaO thin film. (Bottom) A graph showing the perpendicular magnetization of a CoFeB magnetic layer formed on LiTiMgAlGaO.

Reference
R. R. Sihombing, T. Scheike, J. Uzuhashi, H. Yasufuku, T. Ohkubo, Z. Wen, S. Mitani, and H. Sukegawa,
"High entropy oxide epitaxial films with interface perpendicular magnetic anisotropy and tunnel magnetoresistance effect toward spintronic applications"
Mater. Today 88, 12 (2025).

In this study, we focused on the perpendicular magnetic anisotropy that appears at the interface between ferromagnetic metals and lithium titanium oxide (Li-Ti-O). Previously, this phenomenon had been studied mainly at the interface between ferromagnetic metals and insulating oxides such as magnesium oxide (MgO). We demonstrated that strong perpendicular magnetic anisotropy appears when a conductive cubic oxide, Li-Ti-O, is used as a barrier. The left figure shows the thin film structure prepared in this study. A chromium (Cr) underlayer (buffer) was deposited on a MgO substrate, followed by a MgO interface insertion layer. Li-Ti-O film was then formed by sputtering of Li₄Ti₅O₁₂ composition, and a magnetic layer of cobalt iron boron (CoFeB) ultra-thin film was deposited on top. After post-annealing treatment, a perpendicular magnetization film, strongly magnetized perpendicular to the film surface, was obtained [right figure]. This is due to the strong perpendicular magnetic anisotropy that appeared at the interface between Li-Ti-O and CoFeB. The combination of conductivity and perpendicular magnetic anisotropy is expected to contribute to the reduction of a device resistance required for tunnel magnetoresistance (TMR) devices for magnetic memory (MRAM) in the future.


Figure: [Left] Thin film stack structure prepared in this study. [Right] Magnetization curve of the magnetic layer (CoFeB). A perpendicular magnetization film has been obtained at the Li-Ti-O interface.
Reference
Hiroki Koizumi, Zhenchao Wen, Jun Uzuhashi, Tadakatsu Ohkubo, Hiroaki Sukegawa, and Seiji Mitani,
“Interface perpendicular magnetic anisotropy in heterostructures consisting of CoFeB and conductive rock-salt Li-Ti-O”,
J. Phys. D: Appl. Phys. 58, 16LT01 (2025).

CoPt films with perpendicular magnetization are important for recording media of hard disk drives and memory cells of magnetoresistive random access memories (MRAMs). To obtain strong perpendicular magnetization using CoPt, a highly (111) orientated growth and an ordered L1₁-type crystal structure consisting of alternating monoatomic layers of Co and Pt are required. High quality epitaxial growth of such ordered alloy films has been demonstrated using well-controlled molecular beam epitaxy (MBE). The MBE method allows atomic layer-by-layer growth, resulting in high quality metallic “superlattice” films with sharp interfaces of Co and Pt. Especially, the control of superlattices with “non-integer atomic monolayer” was demonstrated only by MBE. However, the sputtering-based technology is more favorable due to its cost effectiveness for industrial fabrication.
In this study, we successfully formed an atomic-scale superlattice in sputtered CoPt multilayers (Figures). The X-ray diffraction shows clear peak splitting (Middle figure) due to the achievement of superlattices with non-integer atomic monolayer thicknesses, which have only been reported in MBE studies so far. A high-quality single crystal Ru buffer layer and precisely controlled sputter deposition using Co and Pt targets were used to form sharp Co/Pt interfaces within the superlattices (Right figure). This technique is suitable for future spintronic applications, including high-density MRAMs.


Figure: (Left) Schematic illustration of Co/Pt superlattice stack design. (Middle) X-ray diffraction profiles with peak splitting due to staking with non-integer monolayer periods. (Right) Cross-sectional scanning transmission electron microscope image of a [Co 0.2 nm/Pt 0.2 nm] film.

Reference
Jieyuan Song, Thomas Scheike, Cong He, Zhenchao Wen, Tadakatsu Ohkubo, Kwangseok Kim, Hiroaki Sukegawa, and Seiji Mitani,
"Incommensurate superlattice modulation surviving down to an atomic scale in sputter-deposited Co/Pt(111) epitaxial multilayered films"
APL Mater. 12, 101120 (2024).
Open access

Details（Press release 2023)
-By controlling the "interface" of the barrier (insulator layer), a tunnel magnetoresistance (TMR) ratio of 631% was achieved, the largest in the world at room temperature, breaking the previous record after 15 years
-A phenomenon that the TMR ratio oscillates with the barrier thickness appeared, and the peak-to-valley value reached 141%

(Explanation) The TMR effect is used in high-sensitivity magnetic sensors and non-volatile magnetoresistive random access memory (MRAM). The larger the TMR ratio at room temperature, the better the performance of such devices. However, there have been no new records for TMR ratios at room temperature since 2008, and it was thought that performance improvements had reached the limit. In this study, the limit was broken by precisely controlling the interface of the barrier layer, and a TMR ratio of up to 631% at room temperature was observed. All layers were made single-crystalline, and atomic-level improvements were achieved, such as introduction of ultra-thin metallic magnesium at the interface (Figure left). In addition, a phenomenon that the TMR ratio oscillates with the barrier thickness becomes more significant and was increased to 141%. In the future, further improvements are expected by clarifying the mechanism of this oscillation phenomenon.
Reference
Thomas Scheike, Zhenchao Wen, Hiroaki Sukegawa, and Seiji Mitani,
"631% room temperature tunnel magnetoresistance with large oscillation effect in CoFe/MgO/CoFe(001) junctions"
Appl. Phys. Lett. 122, 112404-1~6 (2023).
Selected as Featured Article
Open access

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