Tatsuya Yamamoto

Chirality-induced spin selectivity (CISS) is a phenomenon resulting from the interaction between chiral molecules and electron spin. One example is chirality-induced magnetoconductance (MC), in which the electrical conduction through a molecule–ferromagnet junction depends on the molecular chirality and the magnetization direction of the ferromagnet. In this study, we investigate the microscopic mechanism of CISS in an electrochemical cell containing a chiral camphorsulfonic acid electrolyte and a ferromagnetic Ni electrode. We find that the MC effect strongly depends on electrode size, which we attribute to a crossover between one-dimensional planar and three-dimensional radial diffusion. Pronounced MC at large electrodes highlights the importance of the one-dimensional planar diffusion regime. This suggests that magnetization reversal modifies exchange coupling between chiral molecules and the ferromagnet, thereby altering the near-electrode concentration gradient rather than the chemical reaction rate per molecule. These findings identify near-electrode diffusion dynamics as a key factor in CISS-related electrochemical responses.

Cryogenic-temperature sputtering deposition at 100 K was used to fabricate crystalline CoFe/Fe stacked ultrathin films grown on a MgO tunneling barrier. The CoFe/Fe stacked layers exhibited a high perpendicular magnetic anisotropy (PMA) energy density of 0.78 mJ/m2 and a voltage-controlled magnetic anisotropy coefficient of 161 fJ/Vm even after annealing at 673 K. The high PMA can be attributed to the addition of the Fe overlayer, which enhances the interfacial PMA originating from orbital hybridization between Fe and O atoms. Although the CoFe/Fe stacked free layers exhibited a clear PMA, Fe films directly grown on MgO exhibited rounded magnetization curves, indicating that the CoFe layer helped grow ultrathin Fe films. Stacked crystalline ultrathin films are expected to be used for the development of free layers, and the CoFe/Fe stacked layers developed in this study can be applied to various applications based on perpendicular magnetic tunnel junctions, e.g., voltage-controlled magnetoresistive random-access memory, achieving extremely low power consumption.

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.

The realization of spin manipulation using electric fields is anticipated to be a promising route toward future spintronic devices with ultra-low driving power as well as zero standby power, such as voltage-controlled (VC) -magnetoresistive random access memory (MRAM). Among various methods, the voltage-controlled magnetic anisotropy (VCMA) effect in magnetic tunnel junctions (MTJs) is regarded as a particularly effective approach for achieving electric-field-induced magnetization control. One of the most important technical challenges in VC-MRAM development is to achieve a large VCMA coefficient, which is essential for ensuring scalability and reliable information writing. This review discusses the fundamental aspects of the VCMA effect and outlines recent attempts to optimize material and device structures to improve the VCMA coefficient in both epitaxial and polycrystalline MTJs.

Spinodal decomposition is a phenomenon that forms self‐assembled microstructures with extremely uniform sizes and shapes. The phenomenon is exploited in the manufacture of alnico magnets, once considered to be the strongest permanent magnets. However, it has hardly been used in spintronics, one of the most important applications of magnetic materials today, because conventional spintronic devices require uniform magnetic thin‐films to achieve a single‐domain state. Here, the study sheds light on how spinodal magnets can be used to create high‐performance spintronics memristors that have synaptic functionalities. Thin‐film spinodal decomposition is used to fabricate an Fe‐Mn‐based storage layer with a nanostructure consisting of Fe‐rich granules in magnetic tunnel junctions. The devices function as nanoscale memristors that offer both long‐term data retention and analog‐like resistance changes controlled by nanosecond voltage pulses and emulate fundamental synaptic functionalities. The results indicate that spinodal MTJs will be a key to realizing nanoscale memristors for energy‐efficient neuromorphic computing.

High-sensitivity force sensors enable the precise and efficient control of actuators to handle and characterize fragile objects. However, strain gauges and piezoelectric elements combined with flexible polymers or microelectromechanical cantilevers are incompatible with monolithic integration of semiconductor circuitry, including sense amplifiers that can maximize sensing performance while minimizing production costs. Here, we develop force sensors made of magnetic tunnel junctions (MTJs) in contact with a magnetic fluid (MF), where the MTJs are fabricated using a semiconductor-compatible mass-production process, and the MF-MTJ sensors are formed simply by adding a droplet of MF on the MTJ devices. While the ordinary strain-induced resistance change is absent in the MTJs, the interplay of mechanics and magnetics in the MF allows the sensor to detect mechanical loads smaller than 100 μN via the tunneling magnetoresistance effect. No degradation in magneto-mechanical properties was detected over the course of 10⁴ cyclic loading tests on these MF-MTJs.

Chirality-induced spin selectivity (CISS) encompasses phenomena such as magnetoresistance and enantiomer separation using ferromagnets. It has recently received notable attention. Despite this growing interest, the microscopic mechanisms driving CISS remain a subject of intense debate. This paper complements and extends the prevailing interpretation that attributes CISS primarily to the electric current in chiral molecules. We propose that molecular vibrations drive spin polarization in chiral molecules and play a critical role in CISS. Our results suggest that magnetic interactions between chiral molecules and ferromagnets, analogous to interlayer exchange coupling, provide a physically consistent mechanism for the observed selectivity. Specifically, we demonstrate that molecular vibrations facilitate the alignment of spin angular momentum dependent on chirality in the presence of an external magnetic field, which is a prerequisite for exchange coupling. These findings necessitate reevaluating spin dynamics, expanding its relevance beyond traditional solid-state physics to fields such as chemical reactions, molecular biology, and drug discovery.

The demonstration of a large voltage-controlled magnetic anisotropy (VCMA) effect in polycrystalline magnetic tunnel junctions (MTJs) is an important technical challenge for realizing low-power spintronic devices, such as voltage-controlled magnetoresistive random access memory. In this study, in order to control the iridium concentration at the ferromagnet/dielectric interface, which is crucial to obtain the large VCMA effect, we have developed a magnetic tunnel junction (MTJ) structure incorporating an ultrathin MgFeO diffusion-blocking layer designed to suppress Ir diffusion. By inserting such an ultrathin MgFeO layer of only 0.15 nm thickness between the voltage-controlled ultrathin CoFeB and Ir doping layer, we observed a clear increase in the VCMA coefficient. We suggest that this acts by suppressing the deep diffusion of Ir atoms into the CoFeB layer during the deposition process. By optimizing the Ir doping concentration and the thickness of the MgFeO blocking layer, we achieved a maximum VCMA coefficient of −167 fJ/Vm in polycrystalline MTJs prepared by sputtering for a mass production process.

Artificial spin ices (ASIs) consisting of 72 stadium-shaped magnetic tunnel junction (MTJ) cells arranged in a honeycomb lattice were fabricated. The magnetization state of each cell was electrically detected using magnetoresistance. Measurements on the MTJ-based ASIs with a lattice constant a ranging from 980 nm to 2 μm confirmed the following: (1) the switching field of the cells depends on the angle of the long axis of the cell with respect to the field direction, reflecting the shape anisotropy of the free layer in the MTJ cells; (2) the switching field distribution was ∼10% of the mean value; and (3) the magnetic coupling increased monotonically with the reduction in a and reached ∼1 mT for the smallest a, which was close to the switching field distribution. Furthermore, during the magnetization of this ASI, reversed cells aligned in a string appeared, corresponding to the dipole strings previously reported for ASIs fabricated with single-layer ferromagnetic films. The appearance of a dipole string connecting emergent monopoles is a feature of the ASI dynamics. The observation of such emergent dynamics in the ASIs microfabricated using MTJ arrays highlights their potential for advancing the development of novel functional devices.

For the realization of high-performance spintronic devices, the development of novel magnetic materials/structures satisfying both large perpendicular magnetic anisotropy (PMA) and ultra-low Gilbert damping is strongly desired. Here, we show that the insertion of an ultrathin GdOx layer as a capping in double-barrier ultrathin magnetic stacks provides such a solution. The inserted GdOx capping layer prevents unintended intermixing among the magnetic layer (CoFeB) and oxide layers (MgO and MgFeO), suppressing the deterioration of both the PMA and the Gilbert damping. Remarkably, the variation in the effective oxygen doping rate of the GdOx layer strongly influences both the PMA and the Gilbert damping, likely affected by a change in the polarization of the orbital magnetic moment. As a result, the GdOx inserted stacks present considerable effective PMA values (a maximum of ∼0.55 erg/cm2) while simultaneously maintaining an ultra-low Gilbert damping (a minimum of ∼1.68 × 10−3) within the range of 1.0–1.5 nm CoFeB thicknesses. These results pave the way to realize practical spintronic devices by providing reliable magnetic structures for efficient spin manipulation.