arXiv subjects

Hiroshi Nakajima

Publications and source records attributed to Hiroshi Nakajima.

At least 55 records · Page 3Linked to original sources

arXiv2021

Crystallization behaviors in superionic conductor Na$_3$PS$_4$

All-solid-state batteries using sodium are promising candidates for next-generation rechargeable batteries due to the limited lithium resources. A practical sodium battery requires an electrolyte with high conductivity. Cubic Na$_3$PS$_4$ exhibiting high conductivity of over 10$^{-4}$ S cm$^{-1}$ is obtained by crystallizing amorphous Na$_3$PS$_4$ synthesized by ball milling. Amorphous Na$_3$PS$_4$ crystallizes in a cubic structure and then is transformed into a tetragonal phase upon heating. In this study, in situ observation by transmission electron microscopy demonstrates that the crystallite size drastically increases during the transition from the cubic phase to the tetragonal phase. Moreover, an electron diffraction analysis reveals that amorphous domains and nano-sized crystallites coexist in the cubic Na$_3$PS$_4$ specimen, while the tetragonal phase contains micro-sized crystallites. The nano-sized crystallites and the composite formed by crystallites and amorphous domains are most likely responsible for the increase in conductivity in the cubic Na$_3$PS$_4$ specimens.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic textures in a hexaferrite thin film and their response to magnetic fields revealed by phase microscopy

We investigated magnetic textures in a Sc-doped hexaferrite film by means of phase microscopy (PM) with a hole-free phase plate in a transmission electron microscope. In a zero magnetic field, the stripe-shaped magnetic domains coexist with magnetic bubbles. The magnetization in both magnetic domains was oriented perpendicular to the film and the domain walls have an in-plane magnetization. In the remnant state at 9.2 mT, several magnetic bubbles were formed with the formation of stripe-shaped magnetic domains, and the out-of-plane component in the stripe-shaped domains gradually appeared as the film thickness increased. As the film thickness increases further, the magnetic bubbles with clockwise or counter-clockwise spin helicities formed a triangular lattice. These results in the remnant state suggest that the domain wall energy in the magnetic bubble domains is lower in the thicker region.

cond-mat.mtrl-sci↗

arXiv2021

Electrostatic potential measurement at the Pt/TiO$_2$ interface using electron holography

The interface of Pt/TiO$_2$ plays an essential role in device engineering and chemical reactions. Here, we report the electrostatic potential distribution of a Pt/TiO$_2$ interface by electron holography. The decrease of the electrostatic potential exists at TiO$_2$ in the vicinity of the interface, indicating the presence of negative charge due to electron transfer from TiO$_2$ and Pt. The decrease of the electrostatic potential can be understood in the difference in work functions between Pt and TiO$_2$. This study reveals the interplay between Pt and TiO$_2$ and the usefulness of electron holography for probing the potential in nanoscale interfaces.

cond-mat.mtrl-sci↗

arXiv2021

Suzaku Detection of Solar Wind Charge Exchange Emission from a Variety of Highly-ionized Ions in an Interplanetary Coronal Mass Ejection

X-ray emission generated through solar-wind charge exchange (SWCX) is known to contaminate X-ray observation data, the amount of which is often significant or even dominant, particularly in the soft X-ray band, when the main target is comparatively weak diffuse sources, depending on the space weather during the observation. In particular, SWCX events caused by interplanetary coronal mass ejections (ICMEs) tend to be spectrally rich and to provide critical information about the metal abundance in the ICME plasma. We analyzed the SN1006 background data observed with Suzaku on 2005 September 11 shortly after an X6-class solar flare, signatures of which were separately detected together with an associated ICME. We found that the data include emission lines from a variety of highly ionized ions generated through SWCX. The relative abundances of the detected ions were found to be consistent with those in past ICME-driven SWCX events. Thus, we conclude that this event was ICME-driven. In addition, we detected a sulfur XVI line for the first time as one from the SWCX emission, the fact of which suggests that it is the most spectrally-rich SWCX event ever observed. We suggest that observations of ICME-driven SWCX events can provide a unique probe to study the population of highly-ionized ions in the plasma, which is difficult to measure in currently-available in-situ observations.

astro-ph.SR↗

arXiv2020

Experimental studies on the charge transfer inefficiency of CCD developed for the soft X-ray imaging telescope Xtend aboard the XRISM satellite

We present experimental studies on the charge transfer inefficiency (CTI) of charge-coupled device (CCD) developed for the soft X-ray imaging telescope, Xtend, aboard the XRISM satellite. The CCD is equipped with a charge injection (CI) capability, in which sacrificial charge is periodically injected to fill the charge traps. By evaluating the re-emission of the trapped charge observed behind the CI rows, we find that there are at least three trap populations with different time constants. The traps with the shortest time constant, which is equivalent to a transfer time of approximately one pixel, are mainly responsible for the trailing charge of an X-ray event seen in the following pixel. A comparison of the trailing charge in two clocking modes reveals that the CTI depends not only on the transfer time but also on the area, namely the imaging or storage area. We construct a new CTI model with taking into account with both transfer-time and area dependence. This model reproduces the data obtained in both clocking modes consistently. We also examine apparent flux dependence of the CTI observed without the CI technique. The higher incident X-ray flux is, the lower the CTI value becomes. It is due to a sacrificial charge effect by another X-ray photon. This effect is found to be negligible when the CI technique is used.

astro-ph.IM↗

arXiv2020

Magnetic bubbles in an M-type hexagonal ferrite observed by hollow-cone Foucault imaging and small-angle electron diffraction

We report hollow-cone imaging and small-angle electron diffraction of nanoscale magnetic textures such as magnetic-striped domains and magnetic bubbles of M-type hexagonal ferrite BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$. The advantage of the hollow-cone Foucault method is that magnetic domains with various directions of magnetization can be visualized under an infocus condition. Moreover, the contrast of magnetic domain walls in magnetic bubbles depends on the inclination angle of the illumination beam. The combination of small-angle electron diffraction and hollow-cone Foucault imaging proves that magnetization at domain walls exhibits in-plane directions in the magnetic-striped domains and magnetic bubbles.

cond-mat.mtrl-sci↗

arXiv2020

Optical Blocking Performance of CCDs Developed for the X-ray Astronomy Satellite XRISM

We have been developing P-channel Charge-Coupled Devices (CCDs) for the upcoming X-ray Astronomy Satellite XRISM, planned to be launched in 2021. While the basic design of the CCD camera (Soft X-ray Imager: SXI) is almost the same as that of the lost Hitomi (ASTRO-H) observatory, we are planning to reduce the "light leakages" that is one of the largest problems recognized in Hitomi data. We adopted a double-layer optical blocking layer on the XRISM CCDs and also added an extra aluminum layer on the backside of them. We develop a newly designed test sample CCD and irradiate it with optical light to evaluate the optical blocking performance. As a result, light leakages are effectively reduced compared with that of the Hitomi CCDs. We thus conclude that the issue is solved by the new design and that the XRISM CCDs satisfy the mission requirement for the SXI.

astro-ph.IM↗

arXiv2020

Observation of magnetic domains in uniaxial magnets via small-angle electron diffraction and Foucault imaging

Observation of magnetic domains is important in understanding the magnetic properties of magnetic materials and devices. In this study, we report that the magnetic domains of M-type hexaferrites with uniaxial anisotropy can be visualized via small-angle electron diffraction and Foucault imaging. The position of the diffraction pattern spots has the same period as that of magnetic domains in a Sc-substituted hexaferrite (BaFe$_{12-x-δ}$Sc$_x$Mg$_δ$O$_{19}$). Conversely, the spots were observed four times longer than the period of magnetic domains in hexaferrite without substitution (BaFe$_{12}$O$_{19}$), demonstrating the long-range order of the Bloch walls. When the specimen was tilted, the magnetic deflection effect, as well as the periodic spots of magnetic domains, occurred. Thus, we were able to visualize the magnetic domains with different magnetization directions and domain orientations by selecting deflection spots. The results indicate that the technique utilized in this study is useful in observing the magnetic materials with uniaxial anisotropy.

cond-mat.mes-hall↗

arXiv2020

Inverse FIP Effects in Giant Solar Flares Found from Earth X-Ray Albedo with Suzaku/XIS

We report X-ray spectroscopic results for four giant solar flares occurred on 2005 September 7 (X17.0), 2005 September 8 (X5.4), 2005 September 9 (X6.2), and 2006 December 5 (X9.0), obtained from Earth albedo data with the X-ray imaging spectrometer (XIS) onboard Suzaku. The good energy resolution of the XIS (FWHM$\sim$100 eV) enables us to separate a number of line-like features and detect the underlying continuum emission. These features include Si He$α$, Si Ly$α$, S He$α$, S Ly$α$, Ar He$α$, and Ca He$α$ originating from solar flares as well as fluorescent Ar K$α$ and Ar K$β$ from the Earth atmosphere. Absolute elemental abundances (X/H) averaged over the four flares are obtained to be $\sim$2.0 (Ca), $\sim$0.7 (Si), $\sim$0.3 (S), and $\sim$0.9 (Ar) at around flare peaks. This abundance pattern is similar to those of active stars' coronae showing inverse first ionization potential (i-FIP) effects, i.e., elemental abundances decrease with decreasing FIP with a turnover at the low end of FIP. The abundances are almost constant during the flares, with an exception of Si which increases by a factor of $\sim$2 in the decay phase. The evolution of the Si abundance is consistent with a picture that the i-FIP plasma originates from the chromosphere evaporation and then mixes with the surrounding low-FIP biased materials. Flare-to-flare abundance varied by a factor of 2, agreeing with past observations of solar flares. Finally, we emphasize that Earth albedo data acquired by X-ray astronomy satellites like Suzaku and XRISM can significantly contribute to studies of solar physics.

astro-ph.SR↗

arXiv2019

Foucault imaging and small-angle electron diffraction in controlled external magnetic fields

We report a method for acquiring Foucault images and small-angle electron diffraction patterns in external magnetic fields using a conventional transmission electron microscope without any modification. In the electron optical system that we have constructed, external magnetic fields parallel to the optical axis can be controlled using the objective lens pole piece under weak excitation conditions in the Foucault mode and the diffraction mode. We observe two ferromagnetic perovskite-type manganese oxides, La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$, in order to visualize magnetic domains and their magnetic responses to external magnetic fields. In rhombohedral-structured La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, pinning of magnetic domain walls at crystallographic twin boundaries was found to have a strong influence on the generation of new magnetic domains in external applied magnetic fields.

cond-mat.mtrl-sci↗

arXiv2019

Electron diffraction covering a wide angular range from Bragg diffraction to small-angle diffraction

We construct an electron optical system to investigate Bragg diffraction (the crystal lattice plane, $10^{-2}$-$10^{-3}$ rad) with the objective lens turned off by adjusting the current in the intermediate lenses. A crossover was located on the selected-area aperture plane. Thus, the dark-field imaging can be performed by using a selected-area aperture to select Bragg diffraction spots. The camera length can be controlled in the range of 0.8 to 4 m without exciting the objective lens. Furthermore, we can observe the magnetic-field dependence of electron diffraction using the objective lens under weak excitation conditions. The diffraction mode for Bragg diffraction can be easily switched to a small-angle electron diffraction mode having a camera length of more than 100 m. We propose this experimental method to acquire electron diffraction patterns that depict an extensive angular range from 10$^{-2}$ to 10$^{-7}$ rad. This method is applied to analyze the magnetic microstructures in three distinct magnetic materials, i.e., a uniaxial magnetic structure of BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$, a martensite of a Ni-Mn-Ga alloy, and a helical magnetic structure of Ba$_{0.5}$Sr$_{1.5}$Zn$_{2}$Fe$_{12}$O$_{22}$.

cond-mat.mtrl-sci↗

arXiv2019

Self-assembled structure of dendronized CdS nanoparticles

Self-assembled dendronized CdS nanoparticles have been attracting considerable attention because of their photoluminescence properties depending on annealing treatments. In this study, their annealing-induced self-assembled structure was investigated via scanning transmission electron microscopy (STEM); thin foil specimens of self-assembled dendronized CdS nanoparticles were prepared by ultramicrotomy and the STEM images revealed their ordered structure and the effect of the annealing treatment. In addition, a structural order belonging to the P213 space group was identified via an autocorrelation analysis. The results indicated that this structural order could be achieved only over a few tens of nanometers.

cond-mat.mtrl-sci↗

arXiv2019

Charged domain walls and crystallographic microstructures in hybrid improper ferroelectric Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$

The charged domain walls in ferroelectric materials exhibit intriguing physical properties. We examine herein the charged-domain-wall structures in Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ using transmission electron microscopy. When viewed along the [001] axis, the wavy charged domain walls are observed over a wide range ($>$5 $μ$m). In contrast, short charged-domain-wall fragments (from 10 to 200 nm long) occur because they are intercepted and truncated by the conventional 180$^°$ domain walls. These results reveal the unusual charged domain structures in Ca$_{3-x}$Sr$_x$Ti$_2$O$_7$ and will be useful for understanding their formation process.

cond-mat.mtrl-sci↗

arXiv2019

Formation process of skyrmion lattice domain boundaries: The role of grain boundaries

We report on the formation process of skyrmion lattice (SkL) domain boundaries in FeGe using Lorentz transmission electron microscopy and small-angle electron diffraction. We observed that grain boundaries and edges play an important role in the formation of SkL domain boundaries; The SkL domain boundary is stabilized at the intersection of two grains. A micromagnetic simulation using the Landau-Lifshitz-Gilbert equation revealed that the SkL domains separated by a boundary represent the lowest energy configuration. Conversely, in a wide area, SkL domain boundaries were not formed and SkL domains with different orientations rotated to form a single SkL domain.

cond-mat.mes-hall↗

arXiv2019

Microscopic magnetization distribution of Bloch lines in a uniaxial magnet

Bloch lines are formed to reduce the magnetostatic energy generated by the Bloch walls in uniaxial magnets. Recently, it is reported that Bloch lines play important roles in the emergence and helicity reversal of magnetic bubbles in Sc-substitute M-type hexaferrites (BaFe$_{12-x-0.05}$Sc$_{x}$Mg$_{0.05}$O$_{19}$). Although Bloch lines have been discussed on the basis of micromagnetic simulations, the detailed structure was not observed directly. In this study, we investigated the microscopic structures of Bloch lines in BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$ uniaxial magnets. Differential-phase contrast scanning transmission microscopy (DPC-STEM) directly revealed that the edges of the Bloch walls were misaligned in the Bloch lines of BaFe$_{10.35}$Sc$_{1.6}$Mg$_{0.05}$O$_{19}$. From the micromagnetic simulations based on the Monte-Carlo technique, we showed that the misaligned Bloch walls were caused by the dipole-dipole interactions in the hexaferrite. Our results will help to understand the microstructures of Bloch lines at nanometer scale.

cond-mat.mes-hall↗

arXiv2019

Radiation hardness of a p-channel notch CCD developed for the X-ray CCD camera onboard the XRISM satellite

We report the radiation hardness of a p-channel CCD developed for the X-ray CCD camera onboard the XRISM satellite. This CCD has basically the same characteristics as the one used in the previous Hitomi satellite, but newly employs a notch structure of potential for signal charges by increasing the implant concentration in the channel. The new device was exposed up to approximately $7.9 \times 10^{10} \mathrm{~protons~cm^{-2}}$ at 100 MeV. The charge transfer inefficiency was estimated as a function of proton fluence with an ${}^{55} \mathrm{Fe}$ source. A device without the notch structure was also examined for comparison. The result shows that the notch device has a significantly higher radiation hardness than those without the notch structure including the device adopted for Hitomi. This proves that the new CCD is radiation tolerant for space applications with a sufficient margin.

astro-ph.IM↗

arXiv2019

X-ray imaging polarimetry with a 2.5-$\mathrmμ$m pixel CMOS sensor for visible light at room temperature

X-ray polarimetry in astronomy has not been exploited well, despite its importance. The recent innovation of instruments is changing this situation. We focus on a complementary MOS (CMOS) pixel detector with small pixel size and employ it as an x-ray photoelectron tracking polarimeter. The CMOS detector we employ is developed by GPixel Inc., and has a pixel size of 2.5 $\mathrmμ$m $\times$ 2.5 $\mathrmμ$m. Although it is designed for visible light, we succeed in detecting x-ray photons with an energy resolution of 176 eV (FWHM) at 5.9 keV at room temperature and the atmospheric condition. We measure the x-ray detection efficiency and polarimetry sensitivity by irradiating polarized monochromatic x-rays at BL20B2 in SPring-8, the synchrotron radiation facility in Japan. We obtain modulation factors of 7.63% $\pm$ 0.07% and 15.5% $\pm$ 0.4% at 12.4 keV and 24.8 keV, respectively. It demonstrates that this sensor can be used as an x-ray imaging spectrometer and polarimeter with the highest spatial resolution ever tested.

astro-ph.IM↗

arXiv2018

Universal detection of high-temperature emission in X-ray Isolated Neutron Stars

Strongly magnetized isolated neutron stars (NSs) are categorized into two families, according mainly to their magnetic field strength. The one with a higher magnetic field of $10^{14}$ - $10^{15}$ G is called magnetar, characterized with repeated short bursts, and the other is X-ray isolated neutron star (XINS) with $10^{13}$ G. Both magnetars and XINSs show thermal emission in X-rays, but it has been considered that the thermal spectrum of magnetars is reproduced with a two-temperature blackbody (2BB), while that of XINSs shows only a single-temperature blackbody (1BB) and the temperature is lower than that of magnetars. On the basis of the magnetic field and temperature, it is often speculated that XINSs may be old and cooled magnetars. Here we report that all the seven known XINSs show a high-energy component in addition to the 1BB model. Analyzing all the XMM-Newton data of the XINSs with the highest statistics ever achieved, we find that their X-ray spectra are all reproduced with a 2BB model, similar to magnetars. Their emission radii and temperature ratios are also similar to those of magnetars except for two XINSs, which show significantly smaller radii than the others. The remarkable similarity in the X-ray spectra between XINSs and magnetars suggests that their origins of the emission are also the same. The lower temperature in XINSs can be explained if XINSs are older than magnetars. Therefore, these results are the observational indication that supports the standard hypothesis on the classification of highly-magnetized NSs.

astro-ph.HE↗