Geometrically stabilized skyrmionic vortex in FeGe tetrahedral nanoparticles.

The concept of topology has dramatically expanded the research landscape of magnetism, leading to the discovery of numerous magnetic textures with intriguing topological properties. A magnetic skyrmion is an emergent topological magnetic texture with a string-like structure in three dimensions and a disk-like structure in one and two dimensions. Skyrmions in zero dimensions have remained elusive due to challenges from many competing orders. Here, by combining electron holography and micromagnetic simulations, we uncover the real-space magnetic configurations of a skyrmionic vortex structure confined in a B20-type FeGe tetrahedral nanoparticle. An isolated skyrmionic vortex forms at the ground state and this texture shows excellent robustness against temperature without applying a magnetic field. Our findings shed light on zero-dimensional geometrical confinement as a route to engineer and manipulate individual skyrmionic metastructures.

Phase equilibria among η-Fe2Al5 and its higher-ordered phases.

Phase equilibria among the η-Fe2Al5 phase and its higher-ordered phases with the η framework structure were determined experimentally. The solubility range of the η phase at elevated temperature does not differ remarkably from that in previous studies, but this phase is found to undergo complicated phase transformations upon cooling. Four phases are present, namely η', η", η"' and η m, with higher-order atomic orderings in the c-axis chain sites of the orthorhombic crystal structure of the parent η phase. The η" and η"' phases form on the Al-poor and Al-rich sides, respectively, in equilibrium with the ζ-FeAl2 phase below ~415°C and θ-Fe4Al13 phase below ~405°C. The η' and η m phases become stable below 312°C and 343°C with the peritectoid reactions η' → η m + Î·"' and η m → η + Î·", respectively. The η phase is not stable below 331°C with the eutectoid reaction of η m + Î·"' → η. On the basis of these findings, we unraveled the phase equilibria among the η-Fe2Al5 phase and its higher-ordered phases with the η framework structure.

Inorganic process for wet silica-doping of calcium phosphate.

Silica is not only a biocompatible trace element but also an essential element for bone formation and metabolism. Therefore, it is often doped into bioceramics such as calcium phosphate and calcium carbonate for enhancing biomaterial ability. Heretofore, organic silica materials are employed as silica sources, but the residual organic matter is a significant drawback in biomaterial applications. Therefore, in this study, we introduce a one-pot inorganic synthesis method for the formation of silica-doped octacalcium phosphate (OCP) using Na2SiO3 as the silica source. Silica was intercalated into the OCP unit lattice, replacing its hydrous layer structure, and then a layer-by-layer structure of apatite and silica was formed. Furthermore, by immersing the fabricated silica-doped OCP into suitable solutions, both silica-doped hydroxyapatite and carbonate apatite were fabricated through a one-step inorganic processes.

Aesthetic Silver-Doped Octacalcium Phosphate Powders Exhibiting Both Contact Antibacterial Ability and Low Cytotoxicity.

Since the introduction of biomaterials, infection has been a serious problem in clinical operations. Although several studies have introduced hybrid materials of calcium phosphate and Ag0 nanoparticles (NPs) that exhibit antibacterial activity, released Ag+ ions and Ag0 NPs are highly cytotoxic and the materials require complex fabrication techniques such as laser irradiation. In this study, we introduce a simple one-pot synthesis method based on crystal-engineering techniques to prepare Ag+-substituted octacalcium phosphate (OCP-Ag) powder that simultaneously exhibits antibacterial activity, little change in color, and low cytotoxicity, thereby overcoming the shortcomings of calcium phosphate as a biomaterial. We used AgNO3-containing (NH4)2HPO4 aqueous solutions as reaction solutions in which Ag+ forms soluble complex [Ag(NH3)2]+ ions that are stable at Ag+ concentrations less than ∼30 mmol/L. Hydrolysis of soluble calcium phosphate in this solution led to pure OCP-Ag when the Ag+ concentration was less than ∼30 mmol/L. Crystallographic analysis showed that Ag+ substituted at the P5 PO4-conjugated sites and was uniformly distributed. When the concentration of Ag+ in the reaction solution was varied, the Ag+ content of the OCP-Ag could be controlled. The obtained OCP-Ag exhibited little color change or Ag+ release when immersed in various media; however, it exhibited contact antibacterial ability toward resident oral bacteria. The prepared OCP-Ag showed no substantial cytotoxicity toward undifferentiated and differentiated MC3T3-E1 cells in assays. Notably, when the Ag+ content in OCP-Ag was optimized (Ag: ∼1 at %), it simultaneously exhibited contact antibacterial ability, little color change, and low cytotoxicity.

Electron Holography and Magnetotransport Measurements Reveal Stabilized Magnetic Skyrmions in Fe1-xCoxSi Nanowires.

Magnetic skyrmions are topological spin textures that have shown promise for future nonvolatile memory devices. Herein, we report on the stability of magnetic skyrmions in alloyed cubic B20 Fe1-xCoxSi nanowires (NWs) determined using off-axis electron holography and magnetotransport measurements. This study presents the real space observation of one-dimensional skyrmion lattice in a NW of Fe1-xCoxSi which shows that the skyrmion phase in a Fe0.75Co0.25Si NW exists at lower applied magnetic fields (200 Oe) with a reduced domain size (28 ± 2 nm) in comparison to bulk and thin film samples. Magnetotransport measurements were used to observe the helimagnetic transition temperature dependence on the cobalt concentration in the Fe1-xCoxSi NWs. Field-dependent magnetoresistance measurements of Fe1-xCoxSi NWs under applied magnetic field parallel to the NW axis and their second derivative plots reveal the critical fields for the magnetic state transition at different temperatures. A representative magnetic phase diagram constructed with the results from transport measurements of a Fe0.81Co0.19Si NW clearly shows expanded stability region for magnetic skyrmions in the Fe1-xCoxSi NWs.

Electron holography on Fraunhofer diffraction.

Electron holography in Fraunhofer region was realized by using an asymmetric double slit. A Fraunhofer diffraction wave from a wider slit worked as an objective wave interfered with a plane wave from a narrower slit as a reference wave under the pre-Fraunhofer condition and recorded as a hologram. Here, the pre-Fraunhofer condition means that the following conditions are simultaneously satisfied: single-slit observations are performed under the Fraunhofer condition and the double-slit observations are performed under the Fresnel condition. Amplitude and phase distributions of the Fraunhofer diffraction wave were reconstructed from the hologram by the Fourier transform reconstruction method. The reconstructed amplitude and phase images corresponded to Fraunhofer diffraction patterns; in particular, the phase steps of π at each band pattern in the phase image were confirmed. We hope that the developed Fraunhofer electron holography can be extended to a direct phase detection method in the reciprocal space.

Interference experiment with asymmetric double slit by using 1.2-MV field emission transmission electron microscope.

Advanced electron microscopy technologies have made it possible to perform precise double-slit interference experiments. We used a 1.2-MV field emission electron microscope providing coherent electron waves and a direct detection camera system enabling single-electron detections at a sub-second exposure time. We developed a method to perform the interference experiment by using an asymmetric double-slit fabricated by a focused ion beam instrument and by operating the microscope under a "pre-Fraunhofer" condition, different from the Fraunhofer condition of conventional double-slit experiments. Here, pre-Fraunhofer condition means that each single-slit observation was performed under the Fraunhofer condition, while the double-slit observations were performed under the Fresnel condition. The interference experiments with each single slit and with the asymmetric double slit were carried out under two different electron dose conditions: high-dose for calculation of electron probability distribution and low-dose for each single electron distribution. Finally, we exemplified the distribution of single electrons by color-coding according to the above three types of experiments as a composite image.

Current-Driven Motion of Domain Boundaries between Skyrmion Lattice and Helical Magnetic Structure.

To utilize magnetic skyrmions, nanoscale vortex-like magnetic structures, experimental elucidation of their dynamics against current application in various circumstances such as in confined structure and mixture of different magnetic phases is indispensable. Here, we investigate the current-induced dynamics of the coexistence state of magnetic skyrmions and helical magnetic structure in a thin plate of B20-type helimagnet FeGe in terms of in situ real-space observation using Lorentz transmission electron microscopy. Current pulses with various heights and widths were applied, and the change of the magnetic domain distribution was analyzed using a machine-learning technique. The observed average driving direction of the two-magnetic-state domain boundary is opposite to the applied electric current, indicating ferromagnetic s-d exchange coupling in the spin-transfer torque mechanism. The evaluated driving distance tends to increase with increasing the pulse duration time, current density (>1 × 109 A/m2), and sample temperature, providing valuable information about hitherto unknown current-induced dynamics of the skyrmion-lattice ensemble.

Magnetic field observations in CoFeB/Ta layers with 0.67-nm resolution by electron holography.

Nanometre-scale magnetic field distributions in materials such as those at oxide interfaces, in thin layers of spintronics devices, and at boundaries in magnets have become important research targets in materials science and applied physics. Electron holography has advantages in nanometric magnetic field observations, and the realization of aberration correctors has improved its spatial resolution. Here we show the subnanometre magnetic field observations inside a sample at 0.67-nm resolution achieved by an aberration-corrected 1.2-MV holography electron microscope with a pulse magnetization system. A magnetization reduction due to intermixing in a CoFeB/Ta multilayer is analyzed by observing magnetic field and electrostatic potential distributions simultaneously. Our results demonstrate that high-voltage electron holography can be widely applied to pin-point magnetization analysis with structural and composition information in physics, chemistry, and materials science.

Secondary electron effect on electron beam induced charging of SiO2 particle analyzed by electron holography.

Charging of a SiO2 particle induced by electron illumination was investigated by changing the illuminated area of the particle and its support film through control of the position of the mask plate inserted in a transmission electron microscope illumination system. The electric fields around the charged SiO2 particle were analyzed using electron holography. The amount of charge was evaluated quantitatively by comparing the reconstructed phase images with the simulated phase images. When the support film was not covered against the incident electron beam, secondary electrons emitted from the conductive support film were attracted to the charged particle, resulting in particle discharge. In contrast, when the support film was completely covered, secondary electrons were not emitted from the film, so that the particle remained positively charged.