arXiv subjects

Kenji Tanabe

Publications and source records attributed to Kenji Tanabe.

At least 19 recordsLinked to original sources

arXiv2026

Amorphous Fe-Sn nanofilms for anomalous-Nernst heat-flux sensing

Amorphous magnetic films are promising for anomalous-Nernst heat-flux sensing because their low thermal conductivity can enhance the temperature gradient generated by an applied heat flux. However, amorphization often degrades electronic transport and thermoelectric properties, making it challenging to obtain a large anomalous Nernst response in structurally disordered films. Here, we demonstrate nanometer-thick amorphous Fe-Sn films as high-sensitivity anomalous-Nernst heat-flux sensing materials. By systematically controlling composition and thickness, we find that amorphous Fe-Sn nanofilms combine a large anomalous Nernst response with low thermal conductivity, resulting in a heat-flux sensitivity of 0.37 um/A. This value exceeds the sensitivities reported for both amorphous magnetic thin films and representative crystalline topological magnets. X-ray diffraction and Mossbauer spectroscopy show that the optimized films lack long-range crystallinity while retaining local Fe-Sn environments, suggesting that short-range atomic order contributes to the anomalous Nernst response in the amorphous matrix. The sensitivity is also reproduced on flexible polymer substrates, indicating compatibility with mechanically compliant device architectures. These results establish amorphous Fe-Sn nanofilms as a platform for anomalous-Nernst heat-flux sensing and provide a materials design route based on local-structure control and thermal-conductivity reduction.

cond-mat.mtrl-sci↗

arXiv2026

Ternary-composition tuning of the anomalous Nernst effect in amorphous-like Gd-Co-Pt films

We report composition tuning of the anomalous Nernst effect in amorphous-like Gd-Co-Pt ternary films. Pt incorporation into Gd-Co films modifies the anomalous Nernst coefficient and induces a sign reversal of S_ANE, whereas Gd incorporation into Co-Pt films suppresses S_ANE but reduces thermal conductivity. Owing to the balance between transverse thermoelectric response and thermal transport, the heat-flux sensitivity reaches approximately 0.24 {\mu}m/A. Composition maps reveal that the magnitude and heat-flux sensitivity of the anomalous Nernst effect can be systematically tuned in the Gd-Co-Pt ternary composition space. This work extends ANE material design from binary-alloy optimization to ternary-composition engineering.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Nernst effect in amorphous Tb-Fe-Co thin films

We conducted a comprehensive study on the compositional dependence of the anomalous Nernst effect (ANE) in amorphous (amo.) Tb-Fe-Co thin films. The anomalous Nernst coefficient strongly depends not only on the Tb composition but also on the transition metal composition, reaching a maximum of 1.8 uV/K for amo. Tb11.0(Fe50.0Co50.0) 89.0. By evaluating the electrical and thermoelectric properties, it was clarified that this maximum is achieved by the superposition of two large contributions: S_1 arising from direct transverse electron conduction due to a temperature gradient, and S_2 resulting from the combined Seebeck and anomalous Hall effects. We discovered that the anomalous Nernst conductivity, which is attributed to Berry curvature, varied significantly with the transition metal, even in an amorphous material lacking long-range crystalline order. Our research indicates that it is possible to control the electronic states that influence thermoelectric properties, even in the amorphous state.

cond-mat.mtrl-sci↗

arXiv2024

Machine learning-based determination of magnetic parameters from magnetic images with different imaging scales

The determination of material parameters is significantly important in material science, which is often a challenging task. Recently, advancements have shown that magnetic parameters, such as the Dzyaloshinskii-Moriya interaction (DMI), can be estimated from a magnetic domain image using machine learning (ML). This development suggests a potential shift in how magnetic parameters are determined, moving away from traditional measurement techniques to more innovative methods involving image-based inputs processed by ML. In previous studies, the test images used for estimation always matched the training images in size. However, since image size is contingent on the microscopy technique used, the ability to accurately estimate parameters from images of varying sizes is essential. Here, we investigated the feasibility of estimating the DMI constant and saturation magnetization from magnetic domain images of different sizes using ML. We successfully demonstrated that it is possible to estimate these parameters even when the imaging sizes differ between training and test datasets. Additionally, our comparison of the estimation accuracy for the DMI constant and saturation magnetization revealed that the tolerance for differences in image size varies depending on the specific parameter being estimated. These findings could have a significant impact on the future methods of determining magnetic parameters.

cond-mat.mtrl-sci↗

arXiv2024

Significant improvement in sensitivity of an anomalous Nernst heat flux sensor by composite structure

Heat flux sensors (HFS) have attracted significant interest for their potential in managing waste heat efficiently. A recently proposed HFS, that works on the basis of the anomalous Nernst effect (ANE), offers several advantages in its simple structure leading to easy fabrication, low cost, and reduced thermal resistance. However, enhancing sensitivity through traditional material selection is now challenging due to a small number of materials satisfying the required coexistence of a large transverse Seebeck coefficient and low thermal conductivity. In this study, by utilizing composite structures and optimizing the device geometry, we have achieved a substantial improvement in the sensitivity of an ANE-based HFS. We developed composite structures comprised of a plastic substrate with an uneven surface and three-dimensional (3D) uneven TbCo films, fabricated using nanoimprint techniques and sputtering. This approach resulted in a sensitivity that is approximately four times greater than that observed in previous studies. Importantly, this method is independent of the material properties and can significantly enhance the sensitivity. Our findings could lead to the development of highly sensitive HFS devices and open new avenues for the fabrication of 3D devices.

cond-mat.mes-hall↗

arXiv2024

Maximizing bipolar sensitivity for anomalous Nernst thermopiles in heat flux sensing in amorphous GdCo alloys

A Heat Flux Sensor (HFS) facilitates the visualization of heat flow, unlike a temperature sensor, and is anticipated to be a key technology in managing waste heat. Recently, an HFS utilizing the Anomalous Nernst Effect (ANE) has been proposed garnering significant interest in enhancing the transverse Seebeck coefficient. However, ideal materials for HFS not only require a large transverse Seebeck coefficient but also meet several criteria including low thermal conductivity and a bipolar nature of the transverse Seebeck coefficient, especially a negative coefficient. In this study, we have investigated ANE in amorphous ferrimagnetic GdCo alloys, revealing their numerous advantages as HFS materials. These include a large transverse Seebeck coefficient, extremely low thermal conductivity, large negative sensitivity, unparalleled bipolar sensitivity, versatility for deposition on various substrates, and a small longitudinal Seebeck coefficient. These qualities position GdCo films as promising candidates for the advancement of HFS technology.

cond-mat.mtrl-sci↗

arXiv2024

Coexistence of large anomalous Nernst effect and large coercive force in amorphous ferrimagnetic TbCo alloy films

The Anomalous Nernst Effect (ANE) has garnered significant interest for practical applications, particularly in energy harvesting and heat flux sensing. For these applications, it is crucial for the module to operate without an external magnetic field, necessitating a combination of a large ANE and a substantial coercive force. However, most materials exhibiting a large ANE typically have a relatively small coercive force. In our research, we have explored the ANE in amorphous ferrimagnetic TbCo alloy films, noting that the coercive force peaks at the magnetization compensation point (MCP). We observed that transverse Seebeck coefficients are amplified with Tb doping, reaching more than 1.0 uV/K over a wide composition range near the MCP, which is three times greater than that of pure Co. Our findings indicate that this enhancement is primarily due to direct conversion, a product of the transverse thermoelectric component and electrical resistivity. TbCo films present several significant advantages for practical use: a large ANE, the capability to exhibit both positive and negative ANE, the flexibility to be deposited on any substrate due to their amorphous nature, a low thermal conductivity, and a large coercive force. These attributes make TbCo films a promising material for advancing ANE-based technologies.

cond-mat.mes-hall↗

arXiv2023

Observation of anisotropic magneto-inductance effect

We have discovered a new phenomenon that inductance oscillates as a function of the angle between an in-plane magnetic field and an electric current direction in permalloy films, which we have named "the anisotropic magneto-inductance (AML) effect". We have investigated the dependences of the AML effect on the size and voltage. The length, frequency, and amplitude dependences suggest that the AML effect should be evaluated in terms of "inductivity". Inductors based on this AML effect have the potential to be variable, on-chip, and one billion times smaller than the small commercial inductor.

cond-mat.mes-hall↗

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↗

arXiv2020

Observation of spin-motive force in ferrimagnetic GdFeCo alloy films

Non-uniform magnetic structures produce emergent electromagnetic phenomena such as the topological Hall effect and the spin-motive force (SMF). The experimental reports on the SMF, however, are very few and the relationship between the SMF and material parameters is still unclear. In this study, we investigated the SMF in ferrimagnetic GdFeCo alloy films using the spin-torque-induced ferromagnetic resonance method and clarified the relationship. The amplitude of the detected SMF becomes larger than that of the transition metal alloy FeCo by the Gd doping and reaches the maximum near a Gd composition of the boundary between in-plane and perpendicularly magnetized films. According to the analytical calculation, the enhancement is related to the trajectory of the magnetization precession. Moreover, we find that the SMF induced by the magnetic resonance is inversely proportional to the square of the damping constant.

cond-mat.mtrl-sci↗

arXiv2018

On the Nature of Long-Period Dwarf Novae with Rare and Low-Amplitude Outbursts

There are several peculiar long-period dwarf-nova like objects, which show rare, low-amplitude outbursts with highly ionized emission lines. 1SWASP J162117$+$441254, BD Pav, and V364 Lib belong to this kind of objects. Some researchers even doubt whether 1SWASP J1621 and V364 Lib have the same nature as normal dwarf novae. We studied the peculiar outbursts in these three objects via our optical photometry and spectroscopy, and performed numerical modeling of their orbital variations to investigate their properties. We found that their outbursts lasted for a long interval (a few tens of days), and that slow rises in brightness were commonly observed during the early stage of their outbursts. Our analyses and numerical modeling suggest that 1SWASP J1621 has a very high inclination, close to 90 deg, plus a faint hot spot. Although BD Pav seems to have a slightly lower inclination ($\sim$75 deg), the other properties are similar to those in 1SWASP J1621. On the other hand, V364 Lib appears to have a massive white dwarf, a hot companion star, and a low inclination ($\sim$35 deg). In addition, these three objects possibly have low transfer rate and/or large disks originating from the long orbital periods. We find that these properties of the three objects can explain their infrequent and low-amplitude outbursts within the context of the disk instability model in normal dwarf novae without strong magnetic field. In addition, we suggest that the highly-ionized emission lines in outburst are observed due to a high inclination and/or a massive white dwarf. More instances of this class of object may be unrecognized, since their unremarkable outbursts can be easily overlooked.

astro-ph.SR↗

arXiv2017

Multilayered cuprate superconductor Ba$_2$Ca$_5$Cu$_6$O$_{12}$(O$_{1-x}$,F$_x$)$_2$ studied by temperature-dependent scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multi-layered cuprate superconductor Ba$_2$Ca$_5$Cu$_6$O$_{12}$(O$_{1-x}$,F$_x$)$_2$. STM topography revealed random spot structures with the characteristic length $\le 0.5$ nm. The conductance spectra dI/dV(V) show the coexistence of smaller gaps $\Delta_S$ and large gaps (pseudogaps) $\Delta_L$. The pseudogap-related features in the superconducting state were traced with the spatial resolution of $\sim$ 0.07 nm. Here, $I$ and $V$ are the tunnel current and bias voltage, respectively. The temperature, $T$, dependence of $\Delta_S$ follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2$\Delta_{S}(T=0)/k_B T_c$ equals to 4.9, which is smaller than those of other cuprate superconductors. Here, $T_c$ is the superconducting critical temperature and $k_B$ is the Boltzmann constant. The larger gap $\Delta_L$ survives in the normal state and even increases with $T$ above $T_c$. The $T$ dependences of the spatial distributions for both relevant gaps ($\Delta$ map), as well as for each gap separately ($\Delta_S$ and $\Delta_L$) were obtained. From the histogram of $\Delta$ map, the averaged gap values were found to be $\bar \Delta_S = \sim 24$ meV and $\bar \Delta_L = \sim 79$ meV. The smaller gap $\Delta_S$ shows a spatially homogeneous distribution while the larger gap $\Delta_L$ is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length $\xi_{\Delta_L}$ of $\Delta_L$ correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that $\Delta_L$ is strongly affected by the disorder of the apical O/F.

cond-mat.supr-con↗

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↗

arXiv2017

Absence of magnetic long range order in Ba$_3$ZnRu$_2$O$_9$: A spin-liquid candidate in the $S=3/2$ dimer lattice

We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of $S = $ 3/2 spins of Ru$^{5+}$,Ba$_3$ZnRu$_2$O$_9$. This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter- and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.

cond-mat.str-el↗

arXiv2016

Intrinsic Enhancement of Dielectric Permittivity in (Nb + In) co-doped TiO2 single crystals

The development of dielectric materials with colossal permittivity is important for the miniaturization of electronic devices and fabrication of high-density energy-storage devices. The electron-pinned defect-dipoles has been recently proposed to boost the permittivity of (Nb + In) co-doped TiO2 to 105. However, the follow-up studies suggest an extrinsic contribution to the colossal permittivity from thermally excited carriers. Herein, we demonstrate a marked enhancement in the permittivity of (Nb + In) co-doped TiO2 single crystals at sufficiently low temperatures such that the thermally excited carriers are frozen out and exert no influence on the dielectric response. The results indicate that the permittivity attains quadruple of that for pure TiO2. This finding suggests that the electron-pinned defect-dipoles add an extra dielectric response to that of the TiO2 host matrix. The results offer a novel approach for the development of functional dielectric materials with large permittivity by engineering complex defects into bulk materials.

cond-mat.mtrl-sci↗

arXiv2016

Improper ferroelectricity in stuffed aluminate sodalites for pyroelectric energy harvesting

Ferroelectricity in stuffed aluminate sodalites is demonstrated in the present study. The present results indicate that stuffed aluminate sodalites exhibit excellent performance as a pyroelectric energy harvester, suggesting that they exhibit great promise as a class of materials for highly efficient energy harvesting devices. (See manuscript for the full abstract)

cond-mat.mtrl-sci↗

arXiv2016

Snell's Law for Spin Waves

We report the experimental observation of Snell's law for magneto-static spin waves in thin ferromagnetic Permalloy films by imaging incident, refracted and reflected waves. We use a thickness step as the interface between two media with different dispersion relation. Since the dispersion relation for magneto-static waves in thin ferromagnetic films is anisotropic, deviations from the isotropic Snell's law known in optics are observed for incidence angles larger than 25\textdegree{} with respect to the interface normal between the two magnetic media. Furthermore, we can show that the thickness step modifies the wavelength and the amplitude of the incident waves. Our findings open up a new way of spin wave steering for magnonic applications.

cond-mat.mes-hall↗

arXiv2016

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. VIII: The Eighth Year (2015-2016)

Continuing the project described by Kato et al. (2009, arXiv:0905.1757), we collected times of superhump maxima for 128 SU UMa-type dwarf novae observed mainly during the 2015-2016 season and characterized these objects. The data have improved the distribution of orbital periods, the relation between the orbital period and the variation of superhumps, the relation between period variations and the rebrightening type in WZ Sge-type objects. Coupled with new measurements of mass ratios using growing stages of superhumps, we now have a clearer and statistically greatly improved evolutionary path near the terminal stage of evolution of cataclysmic variables. Three objects (V452 Cas, KK Tel, ASASSN-15cl) appear to have slowly growing superhumps, which is proposed to reflect the slow growth of the 3:1 resonance near the stability border. ASASSN-15sl, ASASSN-15ux, SDSS J074859.55+312512.6 and CRTS J200331.3-284941 are newly identified eclipsing SU UMa-type (or WZ Sge-type) dwarf novae. ASASSN-15cy has a short (~0.050 d) superhump period and appears to belong to EI Psc-type objects with compact secondaries having an evolved core. ASASSN-15gn, ASASSN-15hn, ASASSN-15kh and ASASSN-16bu are candidate period bouncers with superhump periods longer than 0.06 d. We have newly obtained superhump periods for 79 objects and 13 orbital periods, including periods from early superhumps. In order that the future observations will be more astrophysically beneficial and rewarding to observers, we propose guidelines how to organize observations of various superoutbursts.

astro-ph.SR↗