Daichi Chiba

We demonstrate layer-by-layer programming of built-in strain in sputter-deposited Co bilayers on flexible substrates. Conventional strain engineering, whether epitaxial or based on post-deposition actuation, does not readily allow the strain state to be set independently for each layer of a multilayer. Here, we use the substrate tension during each deposition step as a layer-specific writing parameter, sputter-depositing two Co layers separated by a 3 nm-thick Pt spacer under independently chosen uniaxial tensions. This yields an asymmetrically strained bilayer, in which only one Co layer carries built-in strain, and a cross-strained bilayer, in which the two Co layers carry mutually orthogonal built-in strains. Magnetization measurements show that each Co layer exhibits the magnetic anisotropy expected from its programmed strain state. This layer-specific response is preserved even though the two Co layers are separated by only a few nanometers, providing magnetic evidence for an abrupt change in the effective built-in strain state across the spacer, which we interpret as a nanoscale elastic discontinuity that would be difficult to realize in a coherent epitaxial strain field. These results establish deposition-defined built-in strain as a depth-selective design variable for thin-film heterostructures.

The magnetic properties of Pd/ferromagnet multilayers can be controlled by hydrogen absorption in Pd, which exhibits an excellent hydrogen absorption capability. Although various magnetic properties have been modulated using this method, its effect on magnetic domain wall (DW) dynamics remains incompletely understood. In this study, we investigated the hydrogen-induced modulation of thermally activated DW creep velocity in a perpendicularly magnetized Pd/Co/Pd structure. Hydrogen absorption in Pd was conducted by exposing the sample to hydrogen gas. We found that hydrogen absorption enhanced effective DW pinning strength, resulting in a decrease in velocity. The velocity recovered to its initial value after hydrogen desorption. Analysis of the hydrogen effect on DW energy suggests that the velocity reduction cannot be explained solely by the modulation of magnetic anisotropy; instead, changes in the DW pinning landscape and/or interfacial Dzyaloshinskii–Moriya interaction are likely involved. Our findings provide crucial insights for the development of hydrogen-based spintronics devices.

We have investigated the interlayer chiral coupling (ICC) phenomenon, which occurs via the interlayer Dzyaloshinskii–Moriya interaction, in a ferromagnet/non-magnet/ferromagnet tri-layer structure using sub-micron-sized magnetic tunnel junctions. The tri-layer films were fabricated into sub-micron lateral size tunnel junctions, which exhibited an order of magnitude enhancement of the ICC-induced effective field compared to larger films. Moreover, the magnitude and even the sign of the effective field was observed to vary between devices, indicating that polycrystallinity plays an important role in ICC. We also demonstrate ICC-mediated in-plane magnetization switching induced by out-of-plane magnetic fields.

We report a flexible mechanical sensor based on a magnetic tunnel junction (MTJ) with a CoFeB/MgO/CoFeB structure integrated on a polyimide substrate. The device exhibits a clear magnetoresistance response to uniaxial tensile strain and maintains its sensing performance under high-cycle mechanical loading. To demonstrate this durability, we utilized a custom-developed system to apply over 100 000 repetitive tensile strain cycles with up to 1.25% strain. Throughout the test, the MTJ maintained its zero-strain resistance and strain-resistance characteristics without significant degradation or delamination. Structural analysis revealed partial oxidation within the bottom Ta layer and suggested that the relatively thick bottom electrode stack (Ta/Ru/Ta) functions as a strain buffer layer. These features are considered contributing factors to the observed high durability, although the robustness is likely rooted in the intrinsic properties of the nanoscale thin film. This study provides direct experimental evidence that MTJ-based mechanical sensors can offer both high sensitivity and exceptional endurance, demonstrating their potential as practical strain sensors in flexible and deformable systems for next-generation, high-performance physical sensing.

This paper presents a carbon-based organic field-effect transistor (OFET) developed using screen printing. Carbon materials are electrically conductive, chemically stable, and durable in outdoor settings, rendering carbon-based OFETs highly promising for environmental, Internet of Things, and biosensing applications. However, their temperature-dependent characteristics remain unexplored. We demonstrated carbon-based OFETs operating at 3 V and evaluated their electrical performance across a temperature range relevant to practical environments. By incorporating the observed temperature characteristics into a device simulation model, we achieved excellent reproducibility in circuit simulations. This study advances the understanding and future development of environmentally friendly, printed electronic devices.

Thermal conductivity, a fundamental parameter characterizing thermal transport in solids, is typically determined by electron and phonon transport. Although electrical transport properties are material‐specific, recent advance in understanding phonon transport has led to new insights on controlling the thermal conductivity via phonon engineering techniques. Here, an unconventional way of artificially modulating the heat conduction in solids is demonstrated via engineering of magnon transport. Time‐domain thermoreflectance measurements in ferromagnetic metal/insulator junction systems reveal that the thermal conductivity of the ferromagnetic metals and interfacial thermal conductance vary substantially depending on the spatial distribution of non‐equilibrium spin currents. Systematic measurements of the thermal transport properties with changing the boundary conditions for spin currents unveil that magnons significantly modulate the heat conduction by ≈10 W m⁻¹ K⁻¹ even in ferromagnetic metals at room temperature, upsetting the conventional wisdom that the magnon thermal conductivity is very small in metals except at low temperatures. This magnon‐engineered thermal transport offers a new principle and manner for active thermal management.

Although engineering built-in strain in materials is strategically important for tuning their functional properties, conventional approaches based on epitaxial lattice mismatch are constrained by the specific combinations of materials and underlayers, thereby restricting material selection. In this study, we propose a method to introduce built-in strain into nano-thin films deposited on flexible substrates. By mechanically stretching the substrate during film deposition, a uniaxial compressive built-in strain is induced into the film upon release of the strain. We applied this technique to tailor the magnetic anisotropy of Co and Ni thin films. Systematic modulation of in-plane magnetic anisotropy was observed as a function of the magnitude of the built-in strain. Furthermore, we fabricated a giant magnetoresistive (GMR) device with orthogonally aligned magnetizations in the pinned and free layers. The device exhibited a linear resistance response to external magnetic fields—a characteristic feature of GMR devices with orthogonal magnetization configurations. The proposed method is simple yet versatile for strain engineering, offering a promising route not only for enhancing spintronic device performance but also for exploring strain-induced physical phenomena in functional materials.

The modulation of magnetic properties via chemical state control, particularly using hydrogen (H2), has attracted attention in spintronics. In this study, we investigate the underlayer catalytic effect on the H2-induced modulation of the ferromagnetic (FM) state in a FM thin film/oxide layered system. In a Co/MgO structure with Pt and Au underlayers, H2-annealing restores the magnetization of deeply oxidized Co layers only in Pt underlayer samples, while no restoration occurs in Au underlayer samples. XPS measurements clarify that the chemical state of the Co layer is significantly altered in the Pt-sample after H2-annealing, indicating that the reduction of oxidized Co is caused by the strong catalytic action of Pt. Furthermore, we found that the H2-induced restoration of the FM state occurs even at room temperature, and it is more pronounced when the sample size is reduced. These findings not only highlight the significant role of the catalytic effects in the hydrogen-induced modulation of magnetic properties but also provide the possibility of exploring the mechanism of the catalytic effect using spintronics devices.

A large portion of the ongoing research in spintronics is focused on advancing magnetic memory designs to not only provide improved capabilities but also operat