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 recent papers

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).

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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