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A twisted grain boundary is introduced in the tellurium crystal to effectively block phonon propagation while maintaining high electron mobility for superior thermoelectric properties.

In this study, we examined the dependence of surface morphology and spin Seebeck effect (SSE) voltages on the poly[vinylpyrrolidone] (PVP) concentration in polycrystalline Y3Fe5O12 (YIG) ultrathin films on a silicon substrate synthesized by metal-organic decomposition followed by a crystallization process. During fabrication, PVP concentrations of 0.5–2 g were used while all other conditions remained fixed. Atomic force microscopy and grazing incidence X-ray diffraction (XRD) measurements revealed a strong dependence of crystallinity and sample morphology on PVP concentration. The 1-g PVP sample had the smoothest surface, with a root mean square roughness of 0.2 nm, as well as superior bulk uniformity with respect to the shape and intensity of XRD reflection peaks. This was confirmed by scanning electron microscopy measurements of a cross-section of the sample that revealed a uniform film without pores. SSE measurements were performed to obtain the output SSE voltages (VSSE) of all samples, to which a platinum layer was added as a spin-detection layer. Repeatedly, the 1-g PVP sample had the best performance, demonstrating the importance of film crystallinity and morphology in the spin-to-charge conversion efficiency of YIG films.

A recent study on orbital current, as a result of the orbital Hall effect, has received much attention because it is expected to provide energy-efficient electrical magnetization control in emerging spintronic devices. In this article, we introduce the concept of the orbital-current-induced spin torque, which is called the orbital torque, and discuss the advantages of using the orbital current for magnetization switching. We also summarize the recent theoretical and experimental results for the orbital current and the orbital torque in various material systems.

Spin thermoelectric effect, an electric voltage generation by a thermal spin current in magnetic systems, has been investigated as a new candidate for thermal energy harvesting, but its thermoelect...

Interconversion between charge and spin through spin-orbit coupling lies at the heart of condensed-matter physics. In normal metal/ferromagnet bilayers, a concerted action of the interconversions, the spin Hall effect and its inverse effect of normal metals, results in spin Hall magnetoresistance, whose sign is always positive regardless of the sign of spin Hall conductivity of normal metals. Here we report that the spin Hall magnetoresistance of Ta/NiFe bilayers is negative, necessitating an additional interconversion process. Our theory shows that the interconversion owing to interfacial spin-orbit coupling at normal metal/ferromagnet interfaces can give rise to negative spin Hall magnetoresistance. Given that recent studies found the conversion from charge currents to spin currents at normal metal/ferromagnet interfaces, our work provides a missing proof of its reciprocal spin-current-to-charge-current conversion at same interface. Our result suggests that interfacial spin-orbit coupling effect can dominate over bulk effects, thereby demanding interface engineering for advanced spintronics devices., In normal metal/ferromagnet bilayers, the spin Hall magnetoresistance originating from the spin Hall effect of normal metal possesses positive sign regardless of the sign of spin Hall conductivity of normal metal. Here, the authors report negative spin Hall magnetoresistance of Ta/NiFe bilayers due to interfacial spin orbit coupling at interfaces.

Antiferromagnetic spin waves have been predicted to offer substantial functionalities for magnonic applications due to the existence of two distinct polarizations, the right-handed and left-handed modes, as well as their ultrafast dynamics. However, experimental investigations have been hampered by the field-immunity of antiferromagnets. Ferrimagnets have been shown to be an alternative platform to study antiferromagnetic spin dynamics. Here we investigate thermally excited spin waves in ferrimagnets across the magnetization compensation and angular momentum compensation temperatures using Brillouin light scattering. Our results show that right-handed and left-handed modes intersect at the angular momentum compensation temperature where pure antiferromagnetic spin waves are expected. A field-induced shift of the mode-crossing point from the angular momentum compensation temperature and the gyromagnetic reversal reveal hitherto unrecognized properties of ferrimagnetic dynamics. We also provide a theoretical understanding of our experimental results. Our work demonstrates important aspects of the physics of ferrimagnetic spin waves and opens up the attractive possibility of ferrimagnet-based magnonic devices. Right- and left-handed spin-wave modes are identified in ferrimagnets, and their dynamics are revealed.

Electrical conduction in magnetic materials depends on their magnetization configuration, resulting in various magnetoresistances (MRs). The microscopic mechanisms of MR have so far been attributed to either an intrinsic or extrinsic origin, yet the contribution and temperature dependence of either origin has remained elusive due to experimental limitations. In this study, we independently probed the intrinsic and extrinsic contributions to the anisotropic MR (AMR) of a permalloy film at varying temperatures using temperature-variable terahertz time-domain spectroscopy. The AMR induced by the scattering-independent intrinsic origin was observed to be approximately 1.5% at T = 16 K and is virtually independent of temperature. In contrast, the AMR induced by the scattering-dependent extrinsic contribution was approximately 3% at T = 16 K but decreased to 1.5% at T = 155 K, which is the maximum temperature at which the AMR can be resolved using THz measurements. Our results experimentally quantify the temperature-dependent intrinsic and extrinsic contributions to AMR, which can stimulate further theoretical research to aid the fundamental understanding of AMR.

In this work, we have investigated the bismuth-substituted yttrium iron garnet (Bi-YIG) thin films prepared on glass substrate by modified metal-organic decomposition method. Real-time Faraday rotation spectrometer was used to characterize the magneto-optical properties of Bi-YIG films. The reliability of the measurement system and optimized analyzer setup was confirmed by comparing the Jones matrix calculations and measured results. The effect of annealing condition on the structural, magnetic and magneto-optical properties was investigated. A clear garnet-phase without secondary phase was observed for the Bi-YIG films annealed above 800 °C. Faraday rotation spectra of these films reveal the correlation between their crystallinity and magneto-optical properties. The highest Faraday rotation angle of −3.1 °/μm was achieved for 800 °C annealed sample. Simultaneous evaluation of real-time Faraday rotation spectra and the corresponding hysteresis loops can establish a path for the application of time-resolved magneto-optical spectroscopic analysis of magnetic thin films.

Physical unclonable function (PUFs) utilize inherent random physical variations of solid-state devices and are a core ingredient of hardware security primitives. PUFs promise more robust information security than that provided by the conventional software-based approaches. While silicon- and memristor-based PUFs are advancing, their reliability and scalability require further improvements. These are currently limited by output fluctuations and associated additional peripherals. Here, highly reliable spintronic PUFs that exploit field-free spin-orbit-torque switching in IrMn/CoFeB/Ta/CoFeB structures are demonstrated. It is shown that the stochastic switching polarity of the perpendicular magnetization of the top CoFeB can be achieved by manipulating the exchange bias directions of the bottom IrMn/CoFeB. This serves as an entropy source for the spintronic PUF, which is characterized by high entropy, uniqueness, reconfigurability, and digital output. Furthermore, the device ensures a zero bit-error-rate under repetitive operations and robustness against external magnetic fields, and offers scalable and energy-efficient device implementations.

Spin Hall nano-oscillators (SHNOs) exploiting current-driven magnetization auto-oscillation have recently received much attention because of their potential for oscillator-based neuromorphic computing. Widespread neuromorphic application with SHNOs requires an energy-efficient way to tune oscillation frequency in broad ranges and store trained frequencies in SHNOs without the need for additional memory circuitry. Voltage control of oscillation frequency of SHNOs was experimentally demonstrated, but the voltage-driven frequency tuning was volatile and limited to megahertz ranges. Here, we show that the frequency of SHNO is controlled up to 2.1 GHz by a moderate electric field of 1.25 MV/cm. The large frequency tuning is attributed to the voltage-controlled magnetic anisotropy (VCMA) in a perpendicularly magnetized Ta/Pt/[Co/Ni]n/Co/AlOx structure. Moreover, non-volatile VCMA effect enables control of the cumulative frequency using repetitive voltage pulses, which mimic the potentiation and depression functions of biological synapses. Our results suggest that the voltage-driven frequency tuning of SHNOs facilitates the development of energy-efficient spin-based neuromorphic devices.

Metallic ferrimagnets with rare earth-transition metal alloys can provide novel properties that cannot be obtained using conventional ferromagnets. Recently, the compensation point of ferrimagnets, where the net magnetization or net angular momentum vanishes, has been considered a key aspect for memory device applications. For such applications, the magnetic anisotropy energy and damping constant are crucial. In this study, we investigate the magnetic anisotropy and damping constant of a GdCo alloy, with a Gd concentration of 12–27%. By analyzing the equilibrium tilting of magnetization as a function of the applied magnetic field, we estimate the uniaxial anisotropy to be 1–3 × 104 J m−3. By analyzing the transient dynamics of magnetization as a function of time, we estimate the damping constant to be 0.08–0.22.

In a simple bilayer structure, spin torque induced by a reduced crystal symmetry switches perpendicular magnetization reliably at zero magnetic field.

Electrical control of antiferromagnetic moment is a key technology of antiferromagnet-based spintronics, which promises favourable device characteristics of ultrafast operation and high-density integration compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report electrical manipulation of the antiferromagnetic easy axis in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the antiferromagnetic easy axis and associated exchange bias is gradually modulated between up to ±22 degrees by in-plane current, which is independent of the NiFe thickness, however. This suggests that spin currents arising in the IrMn layer exert SOTs on uncompensated antiferromagnetic moments at the interface and then rotate the antiferromagnetic moments coherently. Furthermore, the memristive features are preserved in sub-micron devices, facilitating nanoscale multi-level antiferromagnetic spintronic devices.

One of the main objectives of spintronics is to provide power-efficient switching of magnetic layers through electrical means, and in order to achieve this goal, alternate material systems with enhanced spin-orbit torque (SOT) must be engineered. In this work we provide evidence of anisotropy in the SOT and spin Hall effect (SHE) in epitaxial $\mathrm{Pt}$(110) grown on $\mathrm{Mg}\mathrm{O}$(110) single-crystal substrates, and find that the spin Hall angle and the dampinglike torque are 20% larger when current is applied along the [001] crystallographic direction as compared to [$1\overline{1}0$], leading to an equivalent reduction in switching current density along [001]. The anisotropy in SOT is attributed to the bulk contributions of the SHE in the $\mathrm{Pt}$ layer through its anisotropic resistance in this specific orientation. Measurements additionally suggest that the Rashba-Edelstein effect at the $\mathrm{Pt}$/$\mathrm{Ti}$ interface due to the $\mathrm{Pt}$(110) surface has a non-negligible effect on the spin diffusion length and SOT. By providing experimental evidence of the crystal orientation dependence of SOT-induced magnetization switching, this work helps to establish a path for energy-efficient magnetization switching through the alignment of devices with crystallographic directions of enhanced SOT generation.

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems., Magneto-optic Kerr effect microscopy is useful for dynamic magnetic studies, but is limited by the weak magneto-optical activity. Here, the authors show that extreme anti-reflection result in a Kerr amplitude as large as 20° and enables real-time detection of sub-wavelength magnetic domain reversals.

Spin-orbit coupling effect in structures with broken inversion symmetry, known as the Rashba effect, facilitates spin-orbit torques (SOTs) in heavy metal/ferromagnet/oxide structures, along with the spin Hall effect. Electric-field control of the Rashba effect is established for semiconductor interfaces, but it is challenging in structures involving metals owing to the screening effect. Here, we report that the Rashba effect in Pt/Co/AlOx structures is laterally modulated by electric voltages, generating out-of-plane SOTs. This enables field-free switching of the perpendicular magnetization and electrical control of the switching polarity. Changing the gate oxide reverses the sign of out-of-plane SOT while maintaining the same sign of voltage-controlled magnetic anisotropy, which confirms the Rashba effect at the Co/oxide interface is a key ingredient of the electric-field modulation. The electrical control of SOT switching polarity in a reversible and non-volatile manner can be utilized for programmable logic operations in spintronic logic-in-memory devices.

Electric-field-driven ion migration can significantly modulate the electric and magnetic properties of solids, creating novel functionalities useful for advanced electromagnetic devices. Earlier works have used vertically stacked structures for this purpose, in which the redox process results from ion migration driven by a vertical electric field through the interfaces. However, the existence of the interfaces between the dissimilar layers causes the oxidation and reduction processes to have high and asymmetric energy barriers, which means that a large electric field is required to control the devices. Here, we show that in a partially oxidized single GdOx wire using a lateral electric field configuration, low and symmetric energy barriers for the oxidation and reduction processes can be achieved. We provide evidence that the redox process is the result of the lateral motion of oxygen ions by directly visualizing the electric-field-driven real-time ionic motion using an optical microscope. An electric field as low as 105 V/m was able to drive oxygen ions at room temperature, allowing controllable modulation of the electrical resistance using a lateral electric field. A large negative magnetoresistance was also observed in the GdOx wire, and its magnitude was significantly enhanced up to 20% at 9 T through oxygen ion control. Our results suggest that the electrical and magnetic properties of single GdOx can be efficiently controlled through oxygen ion motion driven by a lateral electric field, which paves the way for fully functional electromagnetic devices such as artificial synapses. A new approach for creating tiny wires that move oxygen ions instead of electrons can offer researchers efficient control over devices with brain-like conductivity. According to neuroscientists, synapses, the junctions where two neurons meet, store information by activating different types of conductive states. Soogil Lee and Kab-Jin Kim from the KAIST institute in Daejeon, South Korea, and colleagues report similar behavior in nanoscale-thin wires made from gadolinium oxide, a rare-earth material in which oxygen ions can move freely. By pushing ions in the wire laterally between a cathode and an anode using an electric field, the team could establish two different conductivity levels due to slight changes in redox chemistry. The simplicity of this design enables lower-energy ion manipulation than previous studies employing vertically stacked films of gadolinium oxide and other metals. Electric-field-driven ion migration is an effective way to control many physical properties such as electric and magnetic properties of materials. In this research, we demonstrated that remarkably small electric field as low as 105 V/m can effectively modulate resistance of a GdOx microwire due to the lateral motion of oxygen ions. This result enables mimicking a synaptic device with low power consumption like as a brain. Furthermore, huge MR modulation of the GdOx microwire with the electric field provides novel functionality as an electromagnetic device.