arXiv subjects

Keisuke Yamada

Publications and source records attributed to Keisuke Yamada.

11 recordsLinked to original sources

arXiv2025

Comparison calibration system for digital-output infrasound sensors

Recent advancements in micro electro-mechanical systems (MEMS) have enabled the application of digital-output MEMS modules in infrasound monitoring. These modules, combining MEMS pressure sensors and microcontrollers, provide timestamped digital pressure data. Compared with conventional analog infrasound sensors, the affordability and compactness of MEMS modules allow the construction of infrasound monitoring networks with a high density of measurement stations. However, dynamic frequency response characteristics of the MEMS modules, including both sensitivity modulus and phase, remain unassessed. In this study, we developed a comparison calibration system for digital-output infrasound sensors, with special attention paid to ensuring phase synchronization between the analog-output of reference standards and the digital-output of MEMS modules. Using a pulse per second signal synchronized with a time frequency standard, we successfully timestamped the reference analog signals, achieving synchronization between reference analog standards and digital-output sensors. Example calibrations were conducted on a digital-output MEMS module consisting of a DPS310 MEMS pressure sensor and an ESP32 microcontroller, in the 0.2 Hz to 4 Hz range. The sensitivity modulus matched the reference within a few percent, but the phase delayed by approximately 10 ms. We anticipate that the appliying corrections based on the results reported herein will enhance the reliability of infrasound measurements with digital-output sensors.

physics.ins-det↗

arXiv2025

Optical sound pressure measurement using Fabry-P\'erot cavity for primary acoustic standards

Optical sound pressure measurement is a promising technology to establish primary acoustic standards without reliance on specific types of microphones. We developed a precision optical sound pressure measurement system by combining a Fabry-P\'erot optical cavity, a phase-stabilized optical frequency comb, and a custom-made phasemeter. The optical cavity detects changes in the air's refractive index induced by sound waves as changes in its resonance frequency. A continuous-wave laser frequency is stabilized at the resonance, and the frequency comb detects the changes in the laser frequency. The frequency changes are measured with high sensitivity and accuracy using a phasemeter that we developed. The sound pressures measured by this system agreed with the measurement value obtained using a reference microphone within 5% at sound pressure levels of 78 dB and 84 dB, within a frequency range of 100 Hz to 1 kHz. A systematic deviation of 2.6% was observed, with the optical system yielding higher values than the microphone. To identify the cause of this deviation, we performed vibration displacement measurements of the cavity mirrors and finite element analysis, which revealed that fluctuations in the optical path length due to insufficient fixation of the mirrors were responsible.

physics.ins-det↗

arXiv2025

Reduction of current for magnetization switching in a nanomagnet with perpendicular anisotropy by spin-splitter torque

Recently, spin-transfer torque (STT) based magnetization switching has been widely utilized in magnetic resistance-based memories, which have broad applications in microcontroller units and other devices. This study utilizes a macrospin model to simulate magnetization switching in nanoscale magnets with perpendicular anisotropy through spin-splitter torque (SST). The study primarily addresses minimizing the current for magnetization switching and identifying the conditions necessary for achieving high switching probabilities. Notably, the threshold current density for SST-induced magnetization switching is reduced by approximately 75-80% compared to conventional STT and spin-orbit torque mechanisms, provided the spin torque polar angle is optimized. For practical implementation in magnetic random-access memory (MRAM), a polar angle exceeding roughly 128 degrees must be maintained to ensure sufficient switching probability. Additionally, optimizing the shape of the applied current pulse significantly lowers the switching per rate by approximately 18 times. These findings underscore the effectiveness of SST in reducing magnetization switching currents and offer valuable insights into its potential application in SST-MRAM technology.

cond-mat.mtrl-sci↗

arXiv2023

Surface transfer doping of hydrogen-terminated diamond probed by shallow nitrogen-vacancy centers

The surface conductivity of hydrogen-terminated diamond is a topic of great interest from both scientific and technological perspectives. This is primarily due to the fact that the conductivity is exceptionally high without the need for substitutional doping, thus enabling a wide range of electronic applications. Although the conductivity is commonly explained by surface transfer doping due to air-borne surface acceptors, there remains uncertainty regarding the main determining factors that govern the degree of band bending and hole density, which are crucial for the design of electronic devices. Here, we elucidate the dominant factor influencing band bending by creating shallow nitrogen-vacancy (NV) centers beneath the hydrogen-terminated diamond surface through nitrogen ion implantation at varying fluences. We measured the photoluminescence and optically detected magnetic resonance (ODMR) of the NV centers, as well as the surface conductivity, as a function of the nitrogen implantation fluence. The disappearance of the conductivity with increasing nitrogen implantation fluence coincides with the appearance of photoluminescence and ODMR signals from negatively charged NV centers. This finding indicates that band bending is not exclusively determined by the work-function difference between diamond and the surface acceptor material, but by the finite density of surface acceptors. This work emphasizes the importance of distinguishing work-function-difference-limited band bending and surface-acceptor-density-limited band bending when modeling the surface transfer doping, and provides useful insights for the development of devices based on hydrogen-terminated diamond.

cond-mat.mtrl-sci↗

arXiv2021

Determination of the Dzyaloshinskii-Moriya interaction using pattern recognition and machine learning

Machine learning is applied to a large number of modern devices that are essential in building energy efficient smart society. Audio and face recognition are among the most well-known technologies that make use of such artificial intelligence. In materials research, machine learning is adapted to predict materials with certain functionalities, an approach often referred to as materials informatics. Here we show that machine learning can be used to extract material parameters from a single image obtained in experiments. The Dzyaloshinskii-Moriya (DM) interaction and the magnetic anisotropy distribution of thin film heterostructures, parameters that are critical in developing next generation storage class magnetic memory technologies, are estimated from a magnetic domain image. Micromagnetic simulation is used to generate thousands of random images for training and model validation. A convolutional neural network system is employed as the learning tool. The DM exchange constant of typical Co-based thin film heterostructures is studied using the trained system: the estimated values are in good agreement with experiments. Moreover, we show that the system can independently determine the magnetic anisotropy distribution, demonstrating the potential of pattern recognition. This approach can considerably simplify experimental processes and broaden the scope of materials research.

cond-mat.mes-hall↗

arXiv2017

Electrical detection of magnetic states in crossed nanowires using the topological Hall effect

We used micromagnetic simulations to investigate the spatial distributions of the effective magnetic fields induced by spin chirality in crossed nanowires with three characteristic magnetic structures: a radiated-shape, an antivortex, and a uniform-like states. Our results indicate that, unlike the anomalous Hall effect, the topological Hall effect (which is related to the spin chirality) depends on both the polarity and the vorticity. Therefore, measuring the topological Hall effect can detect both the polarity and the vorticity simultaneously in crossed nanowires. This approach may be suitable for use as an elemental technique in the quest for a next-generation multi-value memory.

cond-mat.mes-hall↗

arXiv2015

Soliton-like magnetic domain wall motion induced by the interfacial Dzyaloshinskii-Moriya interaction

Topological defects such as magnetic solitons, vortices, Bloch lines, and skyrmions have started to play an important role in modern magnetism because of their extraordinary stability, which can be exploited in the production of memory devices. Recently, a novel type of antisymmetric exchange interaction, namely the Dzyaloshinskii-Moriya interaction (DMI), has been uncovered and found to influence the formation of topological defects. Exploring how the DMI affects the dynamics of topological defects is therefore an important task. Here we investigate the dynamic domain wall (DW) under a strong DMI and find that the DMI induces an annihilation of topological vertical Bloch lines (VBLs) by lifting the four-fold degeneracy of the VBL. As a result, velocity reduction originating from the Walker breakdown is completely suppressed, leading to a soliton-like constant velocity of the DW. Furthermore, the strength of the DMI, which is the key factor for soliton-like DW motion, can be quantified without any side effects possibly arising from current-induced torques or extrinsic pinnings in magnetic films. Our results therefore shed light on the physics of dynamic topological defects, which paves the way for future work in topology-based memory applications.

cond-mat.mtrl-sci↗

arXiv2013

Switching of the Magnetic Vortex Core in a Pac-man Disk using a Single Field-Pulse

We report on the switching of the magnetic vortex core in a Pac-man disk using a magnetic field pulse, investigated via micromagnetic simulations. The minimum core switching field is reduced by 72 % compared to that of a circular disk with the same diameter and thickness. However, the core switches irregularly with respect to both the field pulse amplitude and duration. This irregularity is induced by magnetization oscillations which arise due to excitation of the spin waves when the core annihilates. We show that the core switching can be controlled with the assist magnetic field and by changing the waveform.

cond-mat.mes-hall↗

arXiv2008

Switching magnetic vortex core by a single nanosecond current pulse

In a ferromagnetic nanodisk, the magnetization tends to swirl around in the plane of the disk and can point either up or down at the center of this magnetic vortex. This binary state can be useful for information storage. It is demonstrated that a single nanosecond current pulse can switch the core polarity. This method also provides the precise control of the core direction, which constitutes fundamental technology for realizing a vortex core memory.

cond-mat.other↗

arXiv2008

Probing the spin polarization of current by soft X-ray imaging of current-induced magnetic vortex dynamics

Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque.

cond-mat.mtrl-sci↗

arXiv2007

Electrical switching of vortex core in a magnetic disk

A magnetic vortex is a curling magnetic structure realized in a ferromagnetic disk, which is a promising candidate of a memory cell for future nonvolatile data storage devices. Thus, understanding of the stability and dynamical behaviour of the magnetic vortex is a major requirement for developing magnetic data storage technology. Since the experimental proof of the existence of a nanometre-scale core with out-of-plane magnetisation in the magnetic vortex, the dynamics of a vortex has been investigated intensively. However, the way to electrically control the core magnetisation, which is a key for constructing a vortex core memory, has been lacking. Here, we demonstrate the electrical switching of the core magnetisation by utilizing the current-driven resonant dynamics of the vortex; the core switching is triggered by a strong dynamic field which is produced locally by a rotational core motion at a high speed of several hundred m/s. Efficient switching of the vortex core without magnetic field application is achieved thanks to resonance. This opens up the potentiality of a simple magnetic disk as a building block for spintronic devices like a memory cell where the bit data is stored as the direction of the nanometre-scale core magnetisation.

cond-mat.mtrl-sci↗