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Harmonic oscillation of spin structures is a physical phenomenon that offers great potential for applications in nanotechnologies such as nano-oscillators and bio-inspired computing. The effective tuning of oscillations over wide frequency ranges within a single ferromagnetic nanoelement is a prerequisite to realize oscillation-based nanodevices, but it has not been addressed experimentally or theoretically. Here, utilizing a vortex core structure, one of spin structures, we report a drastic change of oscillation modes over the frequency range from MHz to sub-GHz in a 100 nm-thick permalloy circular disk. Oscillation mode was found to considerably depend on the shape and dimension of the vortex core structure and various oscillation modes over a wide range of frequencies appeared with dimensional change in the vortex core structure. This work demonstrates that oscillation modes of the vortex core structure can be effectively tuned and opens a way to apply spin structures to oscillation-based technology.

Author(s): Han, HS; Lee, S; Je, SG; Kang, M; Ok, HJ; Kim, N; Chao, W; Im, MY; Lee, KS | Abstract: Stochasticity in magnetic nanodevices is an essential characteristic for harnessing these devices to computing based on population coding or the building blocks of probabilistic computing, p-bits. A magnetic tunneling junction (MTJ) consisting of a patterned magnetic element is considered a promising computing unit in the concept of artificial neurons and p-bits. A comprehensive understanding of the stochasticity in the switching of patterned magnetic elements is crucial for realizing MTJ-based probabilistic computing technology. In the present work, the stochastic behavior in the switching process of a perpendicularly magnetized Co/Pt disk within an array was directly observed utilizing full-field soft X-ray microscopy. Within 50 repeated hysteretic cycles, the stochastic magnetization switching of individual Co/Pt disks within disk arrays is identified. We found that the stochasticity in the magnetization switching of disks considerably depends on the disk size. The stochasticity initially decreases as the disk radius gets bigger from 125 to 375 nm (region I), then increases with further enlarging the disk size to 625 nm (region II). The variance of thermal fluctuation relevant to the disk size and the multilevel switching within a disk are severely involved in the observed size-dependent stochasticity. This work provides the way for controlling the stochasticity in the switching of nanopatterned elements, which is a key aspect of MTJ-based probabilistic computing.

Magnetic bubbles are circular magnetic domains that may occur in thin magnetic films with perpendicular magnetic anisotropy (PMA). Because they can form with high topological stability and can be manipulated by external driving forces, magnetic bubbles have been considered as prominent information carriers, which are set to 1 or 0, corresponding to the presence or absence. For practical applications, such information carriers must be written and deleted in a specific area of the magnetic thin film. Herein, we report that the magnetic bubbles can be written and deleted using local magnetic fields. By applying a localized magnetic field from the magnetic tip of a magnetic force microscopy to the stripe domain structures of the PMA multilayer, bubbles can be written at room temperature via the transformation from stripe domains to magnetic bubbles. The deleting of the bubbles in the targeted area demonstrated by the local magnetic field accompanied by a uniform external field. Our findings can provide a key for manipulating information carriers in the spintronic device based on topological magnetic structures such as magnetic skyrmions and bubbles.

The efficient generation, manipulation and detection of magnetic skyrmions are important for the development of future spintronic devices. One approach is to use electric-current-induced spin torques. Recently, thermally induced skyrmion motion has also been observed, but wider experimental evidence and its capabilities remain limited. Here we report the thermal generation, manipulation and thermoelectric detection of nanoscale skyrmions in microstructured metallic multilayers integrated with on-chip heaters. The local application of heat can facilitate a domain morphological transition and the formation of skyrmions at the device edge, where a low energy barrier exists. We observe the unidirectional diffusion of skyrmions from hot regions to cold regions, which is due to the interplay among the repulsive forces between skyrmions, thermal spin–orbit torques, entropic forces and magnonic spin torques. The thermally generated skyrmions can also be electrically detected via the Nernst voltage. Nanoscale magnetic skyrmions that are generated in metallic multilayers using on-chip heating diffuse from hot to cold regions and can be thermoelectrically detected via the Nernst voltage.

Controllable writing and deleting of nanoscale magnetic skyrmions are key requirements for their use as information carriers for next-generation memory and computing technologies. While several schemes have been proposed, they require complex fabrication techniques or precisely tailored electrical inputs, which limits their long-term scalability. Here we demonstrate an alternative approach for writing and deleting skyrmions using conventional electrical pulses within a simple, two-terminal wire geometry. X-ray microscopy experiments and micromagnetic simulations establish the observed skyrmion creation and annihilation as arising from Joule heating and Oersted field effects of the current pulses, respectively. The unique characteristics of these writing and deleting schemes, such as spatial and temporal selectivity, together with the simplicity of the 2-terminal device architecture, provide a flexible and scalable route to the viable applications of skyrmions., Comment: Merged Manuscript & Supplementary Information

We realize a magnetic skyrmion diode operated by a unidirectional skyrmion transport that flows in only one direction, which is highly significant for information processing in spintronic and nanoelectronic devices. We easily control the skyrmion transport by engineering asymmetric shapes of geometric structures. Here, we present a simple method to describe the underlying mechanism behind the unidirectional skyrmion transport by characterizing the topography of potential energy surfaces from a purely geometric perspective. Our approach enables a deeper physical insight into skyrmion transport manipulation and efficient design of skyrmion-based devices in geometric structures.

A significant enhancement in corrosion resistance by Y microalloying was achieved for the Mg–8Sn–1Zn–1Al (wt%) alloy in the extruded condition. Average corrosion rates obtained by immersion tests in 0.6 M NaCl solution at 25 °C were 4.2 and 12.7 mm y−1 for the alloys with and without microalloying with 0.04 wt% Y, respectively. Microstructural investigations indicated that the enhanced corrosion resistance is a result of a transition in a type of second-phase particles toward lowering the difference in electrochemical nobility between the α-Mg and these particles, thereby reducing the susceptibility of the alloy to microgalvanic corrosion.

A Bloch point (BP) is a topological defect in a ferromagnet at which the local magnetization vanishes. With the difficulty of generating a stable BP in magnetic nanostructures, the intrinsic nature of a BP and its dynamic behaviour has not been verified experimentally. We report a realization of steady-state BPs embedded in deformed magnetic vortex cores in asymmetrically shaped Ni80Fe20 nanodisks. Time-resolved nanoscale magnetic X-ray imaging combined with micromagnetic simulation shows detailed dynamic character of BPs, revealing rigid and limited lateral movements under magnetic field pulses as well as its crucial role in vortex-core dynamics. Direct visualizations of magnetic structures disclose the unique dynamical feature of a BP as an atomic scale discrete spin texture and allude its influence on the neighbouring spin structures such as magnetic vortices., The intrinsic nature and dynamics of a Bloch point has not been verified so far. Here, Im et al. report the realization and dynamical character of steady-state Bloch points in the magnetic vortex cores in asymmetrically shaped Ni80Fe20 nanodisks.

In pursuit of the elusive mechanism of high-TC superconductors (HTSC), spectroscopic imaging scanning tunneling microscopy (SI-STM) is an indispensable tool for surveying local properties of HTSC. ...

We introduce an active jumping motion platform hardware device that enables a natural jump experience in an immersive virtual reality environment. The motion platform consists of four units: (1) base plate unit, (2) motor and sprung wire unit, (3) hanging seat unit, and (4) column unit. The base plate unit is a flat plate with pressure sensors that supports the user's jumping motion. The motor and the sprung wire unit support the user's body while using the multi-spring recoil to enable jumping. The hanging seat unit allows a person to sit halfway, sitting only slightly on the hips. Finally, the column unit is placed on the base plate unit and supports the motor and wire unit. This hardware provides a safe jumping experience while wearing a head-mounted display. Using the proposed jumping motion platform, the user can enable a jump operation in conjunction with a virtual character. Additionally, it could be used to measure exercise calories or to analyze knee disease.

Let X be a commutative normed algebra with a unit element e (or a normed field of characteristic different from 2), where the associated norm is sub-multiplicative. We prove the generalized Hyers-Ulam stability of a mean value-type functional equation, f(x)&minus, g(y)=(x&minus, y)h(sx+ty), where f,g,h:X&rarr, X are functions. The above mean value-type equation plays an important role in the mean value theorem and has an interesting property that characterizes the polynomials of degree at most one. We also prove the Hyers-Ulam stability of that functional equation under some additional conditions.

Topological protection precludes a continuous deformation between topologically inequivalent configurations in a continuum. Motivated by this concept, magnetic skyrmions, topologically nontrivial spin textures, are expected to exhibit topological stability, thereby offering a prospect as a nanometer-scale nonvolatile information carrier. In real materials, however, atomic spins are configured as not continuous but discrete distributions, which raises a fundamental question if the topological stability is indeed preserved for real magnetic skyrmions. Answering this question necessitates a direct comparison between topologically nontrivial and trivial spin textures, but the direct comparison in one sample under the same magnetic fields has been challenging. Here we report how to selectively achieve either a skyrmion state or a topologically trivial bubble state in a single specimen and thereby experimentally show how robust the skyrmion structure is in comparison with the bubbles. We demonstrate that topologically nontrivial magnetic skyrmions show longer lifetimes than trivial bubble structures, evidencing the topological stability in a real discrete system. Our work corroborates the physical importance of the topology in the magnetic materials, which has hitherto been suggested by mathematical arguments, providing an important step toward ever-dense and more-stable magnetic devices.

The longitudinal spin Seebeck effects with a ferro- or ferrimagnetic insulator provide a new architecture of a thermoelectric device that could significantly improve the energy conversion efficiency. Until now, epitaxial yttrium iron garnet (YIG) films grown on gadolinium gallium garnet (GGG) substrates by a pulsed laser deposition have been most widely used for spin thermoelectric energy conversion studies. In this work, we developed a simple route to obtain a highly uniform solution-processed YIG film and used it for the on-chip microelectronic spin Seebeck characterization. We improved the film roughness down to ∼0.2 nm because the extraction of thermally induced spin voltage relies on the interfacial quality. The on-chip microelectronic device has a dimension of 200 μm long and 20 μm wide. The solution-processed 20 nm thick YIG film with a 10 nm Pt film was used for the spin Seebeck energy converter. For a temperature difference of Δ T ≈ 0.036 K applied on the thin YIG film, the obtained Δ V ≈ 28 μV, which is equivalent to S

Unraveling nanoscale spin structures has long been an important activity addressing various scientific interests, that are also readily adaptable to technological applications. This has invigorated the development of versatile nanoprobes suitable for imaging specimens under native conditions. Here we have demonstrated the resonant coherent diffraction of an artificial quasicrystal magnet with circularly polarized X-rays. The nanoscale magnetic structure was revealed from X-ray speckle patterns by comparing with micromagnetic simulations, as a step toward understanding the intricate relationship between the chemical and spin structures in an aperiodic quasicrystal lattice. Femtosecond X-ray pulses from free electron lasers are expected to immediately extend the current work to nanoscale structure investigations of ultrafast spin dynamics, surpassing the present spatio-temporal resolution.

A commercial Mg–6Zn–0.5Zr alloy was subjected to low-temperature indirect extrusion for grain refinement and the tensile properties of the extruded alloy at 250 °C were investigated. After extrusion, the alloy showed finely recrystallized grains with an average size of 1.6 μm. High-strain-rate superplasticity was observed in the fine-grained alloy, which exhibited a tensile elongation of up to 800% at a strain rate of 0.01 s−1. Experimental results demonstrating the occurrence of grain boundary sliding during the high-strain-rate superplastic deformation are presented.

Converting body heat into electricity using flexible thermoelectric generators can be useful for self-powered wearable electronic devices. However, the temperature difference that can be obtained by body heat is insufficient, which limits its practical applications. In this study, we present a wearable solar thermoelectric generator driven by a significantly high temperature difference created by introducing a local solar absorber and thermoelectric legs on a polyimide substrate. The solar absorber is a five-period Ti/MgF2 superlattice, in which the structure and thickness of each layer was designed for optimal absorption of sunlight. The thermoelectric legs were prepared by dispenser printing with an ink consisting of mechanically alloyed BiTe-based powders and an Sb2Te3-based sintering additive dispersed in glycerol. Thermoelectric p- and n-type legs have electrical conductivities of ~ 25,000 S m−1 with Seebeck coefficients of 166.37 and −116.38 μV K−1, respectively. When exposed to sunlight, a wearable solar thermoelectric generator comprising 10 pairs of p-n legs has an open-circuit voltage of 55.15 mV and an output power of 4.44 μW. The temperature difference is as high as 20.9 °C, which is much higher than the typical temperature differences of 1.5–4.1 °C of wearable thermoelectric generators driven by body heat. The wearable solar thermoelectric generators have been demonstrated on various surfaces exposed to sunlight, such as clothes or windows.

A concentrating photovoltaic (CPV) cell exhibits the highest conversion efficiency among any solar cells. However, the further enhancement of the CPV efficiency is strongly limited by the heat generation at high solar concentrations. Here, we demonstrate a concentrating photovoltaic/thermoelectric hybrid generator using a single-junction, GaAs-based solar cell and a conventional thermoelectric module as a model system. Our hybrid generator gives rise to the conversion efficiency larger than the single CPV cell by ~3% at the solar concentration of 50 suns. Controlling thermal flow in the hybrid generator and the Peltier cooling effect is the key to achieving high efficiency. Our result provides a framework for designing a highly-efficient hybrid generator using both photo-electric and photo-thermal effects for the clean-energy production.

We fabricated polycrystalline yttrium-iron-garnet (Y3Fe5O12, YIG) samples using the sol-gel synthesis method to develop an energy harvesting material based on the spin Seebeck effect. We confirmed that crystallization occurred during calcination at 850 °C and that only the polycrystalline YIG structure was formed. We found that a sintering process at 1400 °C not only increased the size of the YIG particles and the densification of their microstructure but also enhanced their saturation magnetization (Ms) and dramatically reduced their coercivity (Hc). A mechanical pressing process was carried out between the calcination and sintering treatments to prepare a free-standing YIG fillet sample. We found that Ms was enhanced by almost three times without an associated reduction in Hc. We found that mechanical pressing assists the oxidation reaction during the subsequent sintering process by increasing the surface energy and densification before the heat treatment.

A facile method to fabricate a mechanically robust, stretchable solar absorber for stretchable heat generation and an enhanced thermoelectric generator (TEG) is demonstrated. This strategy is very simple: it uses a multilayer film made of titanium and magnesium fluoride optimized by a two-dimensional finite element frequency-domain simulation, followed by the application of mechanical stresses such as bending and stretching to the film. This process produces many microsized sheets with submicron thickness (∼500 nm), showing great adhesion to any substrates such as fabrics and polydimethylsiloxane. It exhibits a quite high light absorption of approximately 85% over a wavelength range of 0.2-4.0 μm. Under 1 sun illumination, the solar absorber on various stretchable substrates increased the substrate temperature to approximately 60 °C, irrespective of various mechanical stresses such as bending, stretching, rubbing, and even washing. The TEG with the absorber on the top surface also showed an enhanced output power of 60%, compared with that without the absorber. With an incident solar radiation flux of 38.3 kW/m

Integrated electronic circuitries with pressure sensors have been extensively researched as a key component for emerging electronics applications such as electronic skins and health-monitoring devices. Although existing pressure sensors display high sensitivities, they can only be used for specific purposes due to the narrow range of detectable pressure (under tens of kPa) and the difficulty of forming highly integrated arrays. However, it is essential to develop tactile pressure sensors with a wide pressure range in order to use them for diverse application areas including medical diagnosis, robotics or automotive electronics. Here we report an unconventional approach for fabricating fully integrated active-matrix arrays of pressure-sensitive graphene transistors with air-dielectric layers simply formed by folding two opposing panels. Furthermore, this realizes a wide tactile pressure sensing range from 250 Pa to ∼3 MPa. Additionally, fabrication of pressure sensor arrays and transparent pressure sensors are demonstrated, suggesting their substantial promise as next-generation electronics., Electronic skins and health monitoring devices rely on integrated tactile sensors, which often require tailored degrees of sensitivity in specific pressure ranges. Here, the authors fabricate a versatile matrix array of pressure-sensitive graphene transistors operating in the wide 250 Pa to 3 MPa pressure range.

Organic–inorganic hybrid perovskites have emerged as a next-generation candidate for light-emitting device applications due to their excellent optical and electrical properties with narrow band emission compared to organic emitters. The morphological control of perovskite films with full surface coverage and few defect sites is essential for achieving highly efficient perovskite light-emitting diodes (PeLEDs). Here, we obtain a highly uniform perovskite film with a remarkably reduced number of defect sites in a perovskite crystal using chlorobenzene dropping. This effort leads to the enhanced performance of PeLEDs with a CH3NH3PbBr3 film using chlorobenzene dropping with a maximum luminance of 14 460 cd m−2 (at 3.8 V) and a maximum external quantum efficiency (EQE) of 0.71% (at 2.8 V). This research confirms that the role of the solvent in the solvent dropping method is to fabricate a dense and uniform perovskite film and to passivate the defect sites of the perovskite crystal films.

At the interface between ferromagnetic and antiferromagnetic phases, various spin configurations with a higher degrees of complexity than in the bulk states can be derived due to the diverse possible interface atomic structures, where coupling interactions among the constituting atoms can form in consistence with altered atomic configurations. The interface magnetic properties then depend on the collective behavior of such spin structures. In the present work, an extended interfacial configuration of a hypo-oxide state was prepared by establishing the gradient of oxygen concentration across the spatially diffuse interface region between ferromagnetic metallic and antiferromagnetic oxide phases at the nanometer scale. With these mixed ferromagnetic and antiferromagnetic couplings among the atoms in the interfacial hypo- or sub-oxide state, novel magnetic behavior can be induced. We report here, for the first time, a significant increase of saturation magnetization with temperature over a broad temperature range, which is against the conventional expectation for any generally known magnetic materials. And the unusual temperature dependent behavior can be understood as the combined effects of competing ferromagnetic and antiferromagnetic couplings acting on atoms in and near the interface region.

Merons which are topologically equivalent to one-half of skyrmions can exist only in pairs or groups in two-dimensional (2D) ferromagnetic (FM) systems. The recent discovery of meron lattice in chiral magnet Co8Zn9Mn3 raises the immediate challenging question that whether a single meron pair, which is the most fundamental topological structure in any 2D meron systems, can be created and stabilized in a continuous FM film? Utilizing winding number conservation, we develop a new method to create and stabilize a single pair of merons in a continuous Py film by local vortex imprinting from a Co disk. By observing the created meron pair directly within a magnetic field, we determine its topological structure unambiguously and explore the topological effect in its creation and annihilation processes. Our work opens a pathway towards developing and controlling topological structures in general magnetic systems without the restriction of perpendicular anisotropy and Dzyaloshinskii–Moriya interaction., A meron is one half of a skyrmion but whether a single meron pair can be created and stabilized remains a challenging question. Here, Gao et al. develop a method to create and stabilize individual pairs of merons in a continuous Py film by local vortex imprinting from Co disks.

3D reconstruction of subject specific bones from X-Ray images is an important issue in a variety of medical applications such as diagnosis. We propose a method to reconstruct 3D leg bones from 2D X-Ray images using feature analysis. First, bounding boxes are detected by Convolutional Neural Network(CNN). Feature points and feature ellipses are extracted from them. Such features are aligned with feature information of 3D bone model. Then, the boundary of X-Ray is detected from aligned boundary of 3D model. The 3D model is fine-tuned by adjusting the Statistical Shape Model parameter. We believe that this method makes 3D modeling easier by improving the automatic detection of feature information compared to the manual landmark input method.

The non-trivial topology of magnetic structures such as vortices and skyrmions is considered as a key concept to explain the stability of those structures. The stability, dictated by non-trivial topology, provides great potential for device applications. Although it is a very critical scientific and technological issue, it is elusive to experimentally study the topology-dependent stability owing to the difficulties in establishing stably formed magnetic structures with different topologies. Here, we establish a platform for vortex-antivortex structures with different topological charges within Ni80Fe20 rectangular elements thick enough to stabilize a unique three-dimensional magnetic structure with non-uniform magnetization along the thickness of the elements. The detailed magnetization configurations of the three-dimensional vortex-antivortex structures and their annihilations during their field-driven motions are investigated by utilizing magnetic transmission soft x-ray microscopy and micromagnetic simulation. We demonstrate that the stability of vortex-antivortex structures significantly depends on their topologies and the topology-dependent stability is associated with their different annihilation mechanisms. We believe that this work provides in-depth insight into the stability of magnetic structures and its topology dependence.

A strategy for a microbolometer operating over a wide range of temperatures up to 100 °C with low nonlinearity and without hysteresis is described, based on a thermally stable VO2(B) film and 5-stack Ti/MgF2 infrared (IR) absorbers. The highly crystalline VO2(B) film formed on an amorphous SrTiO3 layer shows a high temperature coefficient of resistivity and low resistivity without any significant changes up to 100 °C. No significant changes are observed in the two values even after keeping the film at 100 °C for over a month. This outstanding thermostability makes the microbolometers with and without the IR absorber easily capable of operating up to 100 °C. The microbolometers with the 5-stack Ti/MgF2 IR absorber also shows increases in its responsivity by 4 times at 20 °C and by 3 times at 100 °C compared to that without IR absorber due to the increase of IR absorption. These results are achieved without significant increase in the thermal time constants even with the introduction of thermally massive IR absorber.