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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.

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.

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.

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.

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.

We explore the optimal condition for cylindrical core/shell hard-soft exchange-coupled magnetic nanostructures by obtaining full hysteresis loops for various geometries by obtaining full hysteresis loops for various geometrical variables, including the dimensional scale and soft/hard-magnetic phase volume ratio through micromagnetic simulations. For achieving maximum energy product (BH), it is essential to increase the demagnetizing field by increasing the volume fraction of the soft magnet while maintaining a positive nucleation field and, which can be possible by the scaling-down. To scale up the nanostructure to a bulk magnet having high BH can be achieved by forming an array of needle-shaped exchange-coupled cylinders. These findings could lead to the flexible design and scalable fabrication of exchange-coupled permanent magnets.

The corrosion resistance of conventional and experimental Mg–3Al–1Zn-based alloys with different Mn and Y content was comparatively investigated. After immersion for 72 h in a 0.6 M NaCl solution at 25 °C, an experimental Mg–3Al–1Zn–0.05Mn–0.03Y alloy exhibited a corrosion rate of 0.91 mm y−1, which is about four times slower than that of a conventional Mg–3Al–1Zn–0.3Mn alloy. Microstructural factors underlying the improved corrosion resistance achieved by the trace additions of Mn and Y are discussed.

We report a replaceable and multifunctional triboelectric nanogenerator (TENG) with a triangular prism shaped supporter to enhance the uniformity of contact and separation during walking. The supporter was ergonomically designed by considering the walking style, therby enhancing the uniformity of the contact between the Al and PDMS film inside the TENG. The TENG with the supporter generated a high output performance of 64 V and 55 μA when walking, showing an enhancement of approximately 600% when compared with the flat TENG. We also demonstrate a self-powered pressure distribution sensor to monitor the human gait patterns by pressing the TENG arrays when walking, a useful technology in reliable health monitoring systems.

Chirality of the magnetic vortex plays an essential role in dynamic excitations of the magnetic vortex structure. In a harmonic excitation of the vortex gyrotropic motion, it has been known that the chirality determines its phase to the driving force. From our micromagnetic simulations, we find an additional role of chirality in the harmonic excitation of the vortex gyration. The shear deformation of the three-dimensional structure of the vortex core is determined by the chirality of the vortex. We confirm that this is due to the gyrotropic field. For the same vortex core motion with the same polarization but with opposite chirality, it turns out that the opposite gyrotropic field is formed at the spiral magnetization in the vicinity of the vortex core structure.

The87Rb nuclear magnetic resonance (NMR) in a RbTiOAsO4 single crystal was investigated by employing a Bruker FT NMR spectrometer. Instead of one central line, four central lines were obtained. There were two sets of crystallographically inequivalent Rb+ ions: Rb(1) and Rb(2). Two resonance lines in the Rb(1) nucleus and two resonance lines in the Rb(2) nucleus were caused by magnetically inequivalent sites. The angular dependences led to different values for the quadrupole coupling constant and the asymmetry parameter: e2 qQ/h = 19.26 ± 0.03 MHz and η = 0.59 ± 0.02 for the Rb(1) ion, and e2 qQ/h = 23.58 ± 0.07 MHz and η = 0.44 ± 0.05 for the Rb(2) ion. The EFG tensors of Rb(1) and Rb(2) were non-axially symmetric, and the orientations of their principal axes did not coincide. The Rb(1) ions, which are surrounded by nine oxygen atoms, are low in symmetry while the Rb(2) ions, which are surrounded by nine oxygen atoms, show high symmetry.

The susceptibility spectra originating from both relaxation processes of reversible domain wall motion and magnetization rotation are decomposed from the susceptibility spectrum for 3% SiFe samples implanted by N-ion in the fluence range of n=5×10−5–5×1018 ions/cm2. The static susceptibilities due to domain wall motion and magnetization rotation are maximum at n=5×1017 ions/cm2. The relaxation frequencies of domain wall motion and magnetization rotation are not changed significantly up to n=5×1017 ions/cm2, and then rapidly increased with the further increase in the ion fluence. The variation of static susceptibilities and relaxation frequencies with n indicate that increase in number of pinning sites for n⩽5×1017 ions/cm2 to facilitate magnetization, and then increase in internal stress for n>5×1017 ions/cm2 to hinder magnetization.

We report for the first time quasi-phase-matching (QPM) structures induced by ferroelastic domains in RbTiOAsO4 crystals, which overcome the current limit of ~3 μm period and ~3 mm thickness in electrically poled QPM structures.