Satoru Nakatsuji

Developing an ultrafast and energy-efficient nonvolatile switching device may pose a strong impact on emerging computing architectures. However, processing speed has plateaued in the nanosecond regime, as further acceleration demands excessively large write power. We demonstrate ultralow power in picosecond switching using heterostructures of the antiferromagnet Mn ₃ Sn and heavy metal tantalum, which exhibit spin-orbit torque switching by electrical pulses as short as 40 picoseconds. Power consumption in the picosecond regime is several orders of magnitude lower than in ferromagnetic counterparts owing to efficient angular momentum transfer. Compared with previously reported picosecond switching devices, our ultralow-power switching device realizes much less heating, higher endurance, and switching using photocurrent. These results pave the way to ultrafast nonvolatile memory and efficient optical-to-electrical conversion technology.

We present a new approach to high-speed link design based on magnetic tunnel junction devices placed in an array to form a photonic spin register (PSR). After discussing the characteristics and operating principle of the PSR, we investigate the trade-offs involved in designing an electrical interface to read data from it. The variables considered are device noise, input-referred noise of comparators in the readout circuits, power consumption, and data rate. We also depict the relations among multiple variables in a compact way, showing that the comparator power constitutes the dominant portion of the overall power consumption. Also, starting with relatively low data rates (tens of gigabits per second) as a first step, we estimate that post–forward error correction bit error rates of less than 10 ⁻¹⁵ are possible with an energy efficiency in the order of tens of femtojoules per bit.

Interference of electronic wavefunctions may result in the emergence of a flat band, and trigger nontrivial correlated phenomena and phase formations when the flat band crosses the Fermi energy . To date, such an itinerant flat band at has been reported for electronic states without symmetry breaking. If a flat band arises at even with broken symmetry, the ordered state may offer a distinct platform for studying novel phases and spontaneous responses. Here we report experimental and theoretical evidence for itinerant ferromagnetic flat bands formed by spin‐polarized ‐electron orbitals on a stacked honeycomb–kagome lattice, hosting the alternating stack of honeycomb and kagome sublattices. Our theory together with angle‐resolved photoemission spectroscopy and magneto–thermoelectric measurements finds multiple topological flat bands at generating large Berry curvature in the ferrimagnet below its Curie temperature of 940 K. We observe large transverse responses, in particular, gigantic anomalous Nernst effect producing the largest transverse thermoelectric conductivity over 10 A at room temperature. Our study paves a path for developing itinerant magnetic flat bands and to their spintronic and thermoelectric applications.

Topological magnets such as magnetic Weyl and nodal-line semimetals possess topologically non-trivial band structures in magnetically ordered states. In this class of materials, the Berry curvature in momentum space can be significantly enhanced, resulting in thermoelectric responses that exceed empirical scaling laws based on magnetization. Such large transverse thermoelectric effects enable flexible thin-film-based lateral device structures, leading to novel energy-harvesting technologies and sensors beneficial for Internet of Things (IoT) sensors and wearable devices. In this review, we outline recent progress in the study of the large transverse thermoelectric effects in topological magnets.

Antiferromagnetic Weyl semimetals based on Mn₃X(X = Ge, Sn, Ga) kagome compounds exhibit the same ferromagnetic-like responses, including anomalous Hall, Nernst, and magneto-optical effects, as recently discussed for altermagnets. Driven by the Berry curvature due to Weyl fermions, these materials show a disproportionately large magnitude of electromagnetic effects even in the absence of large magnetization. For applications it is crucial to realize these responses in a wide range of temperatures both below and above 300 K. While stoichiometric Mn₃X materials do not offer optimal performance, we show that Mn₃Sn_1−x Ga _x sputtered films with a variable composition offers a tunable Néel temperature, T_N ≈ 425 ± 6–500 ± 15 K, which is crucial for device applications, together with a large tunable anomalous Hall effect. Our thin film growth method enables continuous and precise control over the film composition between x = 0 and x = 1. Through a detailed magnetization and Hall transport, we establish the magnetic phase diagram for the hexagonal Mn₃Sn_1−x Ga _x . Our results reveal an enhanced T_N and antichiral magnetic phase in Ga-doped Mn₃Sn and an enhanced anomalous Hall magnitude in Sn-doped Mn₃Ga compared to their stoichiometric undoped forms. Our work demonstrates a route to optimize the technologically relevant antiferromagnets for various applications.

The exchange bias effect, which causes a shift in the ferromagnetic hysteresis loop due to magnetic coupling with an adjacent antiferromagnetic layer, has been a key area of study for fundamental research and spintronic applications. This effect is known to be static once established through a field cooling procedure and is difficult to manipulate isothermally. Here we present the first room temperature field switching of interlayer magnetic coupling at the metallic heterointerface between a ferromagnet and antiferromaget. Specifically, we demonstrate that the exchange bias can be systematically switched by manipulating the global time-reversal-symmetry-broken antiferromagnetic order of Mn₃Sn via both magnetic field sweeping and field cooling. Furthermore, we confirm that the magnitude and sign of the interlayer coupling can be tuned by selecting the ferromagnet. These findings provide a novel approach for controlling the magnetic state of ferromagnets and functional antiferromagnets, paving the way for advancing spintronic technologies using antiferromagnets.

Magnetically intercalated transition metal dichalcogenides (TMDs) provide a versatile three-dimensional (3D) material platform to explore quantum phenomena and functionalities that emerge from an intricate interplay among magnetism, band structure, and electronic correlations. Here, we report the observation of a nearly magnetization-free anomalous Hall effect (AHE) accompanied by non-Fermi liquid (NFL) behavior and collinear antiferromagnetism (AFM) in V_1/3NbS₂. Our single-crystal neutron diffraction measurements identify a commensurate, collinear AFM order formed by intercalated V moments. In the magnetically ordered state, the spontaneous AHE is tenfold greater than expected from empirical scaling with magnetization, and this strongly enhanced AHE arises in the NFL regime that violates the quasiparticle picture. V_1/3NbS₂ challenges the existing single-particle framework for understanding AHEs based on one-body Berry curvature and highlights the potential of magnetically intercalated TMDs to unveil new electronic functionalities where many-body correlations play a critical role.

Multipolar moments entail a new route to tackle frontier problems in superconductivity (SC). A key progress in the search for multipolar SC is the discovery of Pr$Tr_2$Al$_{20}$ ($Tr =$ Ti, V), which possesses quadrupolar and octupolar but no magnetic dipolar moments. The Kondo entanglement of these multipolar moments with conduction electrons leads to exotic SC within the multipolar ordered phase, though the precise nature of the SC remains unexplored. We experimentally investigate the SC gap structure of SC in PrTi$_{2}$Al$_{20}$ and its La-doping evolution. Our results indicate deviations from a single $s$-wave gap, instead favoring nodal $d$-wave or multiple gaps. While the SC is robust against La dilution, the SC gap structure changes with minimal La doping, coinciding with a sharp change in the ferroquadrupolar (FQ) order. This suggests an intimate link between the quadrupolar order parameter and SC pairing, providing insight into the coexistence of SC with multipolar order.