arXiv subjects

Ken-ichi Uchida

Publications and source records attributed to Ken-ichi Uchida.

At least 37 records · Page 2Linked to original sources

arXiv2025

Huge anisotropic magneto-thermal switching in high-purity polycrystalline compensated metals

Magneto-thermal transport is a promising physical property for thermal management applications. Magneto-thermal switching enables active control of heat flows, and a high switching ratio is desirable for improving performance. Here, we report on the observation of a huge magneto-thermal switching (MTS) effect in high-purity (5N) Pb polycrystalline wires, where magnetic fields perpendicular to the heat current direction are applied at low temperatures. At T = 3 K and B = 0.1 T, the measured thermal conductivity (\k{appa}) of the Pb wire is about 2500 W m-1 K-1 but is reduced to ~150 and ~5 W m-1 K-1 at B = 1 and 9 T, respectively. This strong suppression is attributed to magnetoresistance in compensated metals. Although the huge magnetoresistance has been studied in single crystals with field along the selected orbitals, our results demonstrate that a huge MTS can similarly be realized even in flexible polycrystalline wires. This finding highlights the practical potential of magneto-thermal control in low-temperature thermal management, including applications in space environments where temperatures are around 3 K.

cond-mat.mtrl-sci↗

arXiv2025

Study of magneto-thermal resistance effect in a Co50Fe50/Cu multilayer through the analysis of electron and lattice thermal conductivities

This study investigates the giant magneto-thermal resistance (GMTR) effect in a fully-bcc epitaxial Co50Fe50/Cu multilayer through both experimental and theoretical approaches. The applied magnetic field results in a giant change of the cross-plane thermal conductivity (Δ\k{appa}) of 37 W m-1 K-1, which reaches 1.5 times larger than the previously reported value for a magnetic multilayer and record the highest value at room temperature among the other solid-state thermal switching materials working on different principles. We investigated the electron thermal conductivity for exploring the remarkable Δ\k{appa} by the two-current-series-resistor model combined with the Wiedemann-Franz (WF) law. However, the result shows the electron contribution accounts for only 35% of the Δ\k{appa}, indicating the presence of additional spin-dependent heat carriers. Further investigation of the lattice thermal conductivity, which is expected to be spin-independent, using non-equilibrium molecular dynamics (NEMD) simulations suggests a striking contrast: the additional spin-dependent heat carrier contribution is significantly enhanced in the parallel magnetization configuration but nearly negligible in the antiparallel configuration. These findings provide a fundamental insight into the origin of large GMTR effect and highlight its potential of active thermal management technologies for future electronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Multifunctional composite magnet realizing record-high transverse thermoelectric generation

Permanent magnets are used in various products and essential for human society. If omnipresent permanent magnets could directly convert heat into electricity, they would lead to innovative energy harvesting and thermal management technologies. However, achieving such "multifunctionality" has been difficult due to poor thermoelectric performance of conventional magnets. In this work, we develop a multifunctional composite magnet (MCM) that enables giant transverse thermoelectric conversion while possessing permanent magnet features. MCM comprising alternately and obliquely stacked SmCo$_{5}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$ multilayers exhibits an excellent transverse thermoelectric performance at room temperature by optimizing its anisotropic structure. Owing to the extremely low interfacial electrical and thermal resistivities, the experimentally determined figure of merit $z_{\rm xy}T$ reaches 0.20 close to the analytically calculated ideal value. The MCM-based thermopile module generates 204 mW in maximum at a temperature difference of 152 K, whose power density normalized by heat transfer area and temperature gradient is not only record-high among transverse thermoelectric modules but also comparable to that of commercial thermoelectric modules utilizing the Seebeck effect. The multifunctionality of our MCM provides unprecedented opportunities for energy harvesting and thermal management everywhere permanent magnets are currently used.

cond-mat.mtrl-sci↗

arXiv2025

Observation of transverse Thomson effect

The thermoelectric Thomson effect, predicted in the 1850s by William Thomson, produces volumetric heating/cooling in a conductor due to the concerted action of the Seebeck and Peltier effects. Recently, transverse thermoelectrics studies on the Nernst and Ettingshausen effects have progressed rapidly to enable versatile thermal management technologies and to explore topological transport properties. However, a transverse Thomson effect, arising from the concerted action of the Nernst and Ettingshausen effects, has not yet been observed. Here, we report the observation of the transverse Thomson effect in a conductor. We observed volumetric heating/cooling in a semimetallic Bi$_{88}$Sb$_{12}$ alloy induced by a charge current, temperature gradient, and magnetic field applied orthogonally to each other using thermoelectric imaging techniques. We found that the heating/cooling can be switched by the field direction. Our experiments and analyses reveal the essential difference between the conventional and transverse Thomson effects; the former depends sorely on the temperature derivative of the Seebeck coefficient, while the latter depends not only on the temperature derivative of the Nernst coefficient but also on its magnitude. The observation of the transverse Thomson effect fills a missing piece in the history of thermoelectrics and provides a new principle for active thermal management technologies.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative noncontact measurement of thermal Hall angle and transverse thermal conductivity by lock-in thermography

We propose and demonstrate a quantitative noncontact measurement method for the thermal Hall effect (THE) based on magnetic-field-modulated lock-in thermography. This method enables visualization of THE-induced temperature change and quantitative estimation of the thermal Hall angle $θ_{\rm THE}$ by applying periodic magnetic fields to a sample and obtaining the first harmonic response of thermal images. By combining this method with LIT-based measurement techniques for the longitudinal thermal conductivity $κ_{xx}$, we also quantify the transverse thermal conductivity $κ_{xy}$. We validate our measurement methods by estimating $θ_{\rm THE}$, $κ_{xx}$, and $κ_{xy}$ in a ferromagnetic Heusler alloy Co$_2$MnGa slab showing large THE.

cond-mat.mes-hall↗

arXiv2025

Adiabatic transverse thermoelectric conversion enhanced by heat current manipulation in artificially tilted multilayers

We phenomenologically formulate and experimentally observe an adiabatic transverse thermoelectric conversion enhanced by a heat current re-orientation in artificially tilted multilayers (ATMLs). By alternately stacking two materials with different thermal conductivities and rotating its multilayered structure with respect to a longitudinal temperature gradient, off-diagonal components in the thermal conductivity tensor are induced. This off-diagonal thermal conduction (ODTC) generates a finite transverse temperature gradient and Seebeck-effect-induced thermopower in the adiabatic condition, which is superposed on the isothermal transverse thermopower driven by the off-diagonal Seebeck effect (ODSE). In this study, we calculate and observe the two-dimensional temperature distribution and the resultant transverse thermopower in ATMLs comprising thermoelectric Co$_{2}$MnGa Heusler alloys and Bi$_{2-a}$Sb$_{a}$Te$_{3}$ compounds. By changing the tilt angle from 0° to 90°, the transverse temperature gradient obviously appeared in the middle angles and the transverse thermopower increases up to -116.1 $μ$V/K in Co$_{2}$MnGa/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$-based ATML at the tilt angle of 45° whereas the isothermal contribution is estimated to be -82.6 $μ$V/K from the analytical calculation. This hybrid action derived from ODTC results in the significant variation of the maximum reduced efficiency for transverse thermoelectric conversion from 3.1% in the isothermal limit to 8.1% in the adiabatic limit.

cond-mat.mtrl-sci↗

arXiv2024

Designing flexible hard magnetic materials for zero-magnetic-field operation of the anomalous Nernst effect

The global shift towards a carbon-neutral society has accelerated the demand for green energy, driving research into efficient technologies for harvesting energy from low-grade waste heat. Recently, transverse thermoelectrics based on the anomalous Nernst effect (ANE) has gained attention due to their simple device structure, scalability, and manufacturing-friendly nature. While topological single crystals and epitaxial films have been focused for enhancing the ANE-driven thermoelectric performance, further improvements in material design are necessary for practical applications. Here, we report an easy-to-implement strategy for designing mechanically flexible and magnetically hard transverse thermoelectric materials by creating amorphous-crystalline heterogeneous composites. We fabricated and optimized the heterogeneous composites through controlled heat treatment, achieving significant enhancements in the coercivity and anomalous Nernst coefficient, while maintaining flexibility. Additionally, using the developed material, we constructed a single-material-based coiled device and demonstrated the zero-field operation of the ANE-based energy harvesting from curved heat sources. These results validate the feasibility of using the ANE-based flexible materials for energy harvesting applications.

cond-mat.mtrl-sci↗

arXiv2024

Magnon-Driven Magnetothermal Transport in Magnetic Multilayers

All-solid-state nanoscale devices capable of efficiently controlling a heat flow are crucial for advanced thermal management technologies. Here we predict a magnon-driven magnetothermal resistance (mMTR) effect in multilayers of ferromagnets and normal metals, i.e. a thermal resistance that varies when switching between parallel and antiparallel magnetization orientations of the ferromagnetic layers, even in the absence of conduction electrons in the ferromagnets. The mMTR arises from an interfacial temperature drop caused by magnon spin accumulations and can be engineered by the layer thicknesses, spin diffusion lengths, and spin conductances. The mMTR predicted here enables magnetothermal switching in insulator-based systems; we already predict large mMTR ratios up to 40$\%$ for superlattices of the electrically insulating magnet yttrium iron garnet and elemental metals.

cond-mat.mes-hall↗

arXiv2024

Hybridizing anomalous Nernst effect in artificially tilted multilayer based on magnetic topological material

Transverse thermoelectric conversion holds significant potential in addressing complex challenges faced by classical Seebeck/Peltier modules. A promising transverse thermoelectric phenomenon is the anomalous Nernst effect (ANE) originating from nontrivial band structures in magnetic topological materials. However, the currently reported performance for ANE in topological materials, e.g., Co$_2$MnGa, remains insufficient for practical thermoelectric applications. Here, we unveil a new availability for ANE by integrating magnetic topological materials into artificially tilted multilayers (ATMLs), known to exhibit the structure-induced transverse thermoelectric conversion due to the off-diagonal Seebeck effect. Our experiments reveal that the transverse thermoelectric performance in Co$_2$MnGa-based ATMLs is improved through the hybrid action of ANE and the off-diagonal Seebeck effect, with the modulation of performance dependent on magnetization being significantly greater than the performance achievable with ANE alone. This unconventional synergy underscores the importance of hybrid transverse thermoelectric conversion and paves a way for advancing thermoelectric applications using magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Quantitative measurement of figure of merit for transverse thermoelectric conversion in Fe/Pt metallic multilayers

This study presents a measurement method for determining the figure of merit for transverse thermoelectric conversion ($ z_\mathrm{T}T $) in thin film forms. Leveraging the proposed methodology, we comprehensively investigate the transverse thermoelectric coefficient ($ S_\mathrm{T} $), in-plane electrical conductivity ($ σ_{yy} $), and out-of-plane thermal conductivity ($ κ_{xx} $) in epitaxial and polycrystalline Fe/Pt metallic multilayers. The $ κ_{xx} $ values of multilayers with a number of stacking repetitions ($ N $) of 200 are lower than those of FePt alloy films, indicating that the multilayer structure effectively contributes to the suppression of $ κ_{xx} $. $ z_\mathrm{T}T $ is found to increase with increasing $ N $, which remarkably reflects the $ N $-dependent enhancement of the $ S_\mathrm{T} $ values. Notably, $ S_\mathrm{T} $ and $ σ_{yy} $ are significantly larger in the epitaxial multilayers than those in the polycrystalline counterparts, whereas negligible differences in $ κ_{xx} $ are observed between the epitaxial and polycrystalline multilayers. This discrepancy in $ σ_{yy} $ and $ κ_{xx} $ with respect to crystal growth is due to the different degree of anisotropy in electron transport between epitaxial and polycrystalline multilayers, and epitaxial growth can lead to an enhancement of $ z_\mathrm{T}T $ in the multilayers. This study is the first demonstration in the evaluation of $ z_\mathrm{T}T $ in thin film forms, and our proposed measurement technique reveals the transverse thermoelectric properties inherent to multilayers.

cond-mat.mtrl-sci↗

arXiv2023

Permanent-magnet-based transverse thermoelectric generator with high fill factor driven by anomalous Nernst effect

A transverse thermoelectric generator for magnetic-field-free and high-density power generation utilizing the anomalous Nernst effect is constructed and its performance is characterized. By alternately stacking two different permanent magnets with the large coercivity and anomalous Nernst coefficients of opposite sign, transverse thermoelectric voltage and power can be generated in the absence of external magnetic fields and enhanced owing to a thermopile structure without useless electrode layers. In the permanent-magnet-based stack, the magnetic attractive force enables easy construction of the thermopile structure with a high fill factor. In this study, we construct a bulk module consisting of twelve pairs of SmCo$_5$- and Nd$_2$Fe$_{14}$B-type permanent magnets with respectively having the positive and negative anomalous Nernst coefficients, whose fill factor reaches ~80%, whereas that of conventional thermoelectric modules based on the Seebeck effect is typically 30-60%. We demonstrate magnetic-field-free anomalous Nernst power generation up to 177 $μ$W at a temperature difference of 75 K around room temperature, which corresponds to the largest anomalous Nernst power density of 65 $μ$W/cm$^2$. The presented module structure concept will provide a design guideline for high-performance transverse thermoelectric power generation.

cond-mat.mtrl-sci↗

arXiv2023

Creation of flexible spin-caloritronic material with giant transverse thermoelectric conversion by nanostructure engineering

Functional materials such as magnetic, thermoelectric, and battery materials have been revolutionized through nanostructure engineering. However, spin caloritronics, an advancing field based on spintronics and thermoelectrics with fundamental physics studies, has focused only on uniform materials without complex microstructures. Here, we show how nanostructure engineering enables transforming simple magnetic alloys into spin-caloritronic materials displaying significantly large transverse thermoelectric conversion properties. The anomalous Nernst effect (ANE), a promising transverse thermoelectric phenomenon for energy harvesting and heat sensing, has been challenging to utilize due to the scarcity of materials with large anomalous Nernst coefficients. We demonstrate a remarkable improvement in the anomalous Nernst coefficients in flexible Fe-based amorphous materials through nanostructural engineering, without altering their composition. This surpasses all reported amorphous alloys and is comparable to single crystals showing large ANE. The enhancement is attributed to Cu nano-clustering, facilitating efficient transverse thermoelectric conversion. This discovery advances the materials science of spin caloritronics, opening new avenues for designing high-performance transverse thermoelectric devices for practical applications.

cond-mat.mtrl-sci↗

arXiv2023

Observation of nonvolatile magneto-thermal switching in superconductors

Applying a magnetic field to a solid changes its thermal-transport properties. Although such magneto-thermal-transport phenomena are usually small effects, giant magneto-thermal resistance has recently been observed in spintronic materials1,2 and superconductors3,4, opening up new possibilities in thermal management technologies. However, the thermal conductivity conventionally changes only when a magnetic field is applied due to the absence of nonvolatility, which limits potential applications of thermal switching devices5,6. Here, we report the observation of nonvolatile thermal switching that changes the thermal conductivity when a magnetic field is applied and retains the value even when the field is turned off. This unconventional magneto-thermal switching, surprisingly, arises in commercial Sn-Pb solders and is realized by phase-separated superconducting states and resultant nonuniform magnetic flux distributions. This result confirms the versatility of the observed phenomenon and aids the development of active solid-state thermal management devices.

cond-mat.mtrl-sci↗

arXiv2023

Direct electrical probing of anomalous Nernst conductivity

Despite the usefulness of the anomalous Nernst conductivity ($α^{A}_{xy}$) for studying electronic band structures and exploring magnetic materials with large transverse thermopower, there has not been a straightforward way to obtain $α^{A}_{xy}$ in the experiment. Here, we propose a simple and versatile method enabling direct electrical probing of $α^{A}_{xy}$, which is realized by creating a closed circuit consisting of a target magnetic material and a non-magnetic conductor. This method was experimentally demonstrated on a thin film of magnetic Weyl semimetal Co$_{2}$MnGa, where the closed circuit was formed simply by connecting both ends of the Co$_{2}$MnGa film with a Au wire. A good approximation of $α^{A}_{xy}$ was obtained in a wide temperature range, validating the proposed method and exhibiting its potential for aiding the further development of topological materials science and transverse thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2023

Large magneto-thermal-switching ratio in superconducting Pb wires

Thermal switching by magnetic fields is one of the important functionalities in thermal management technologies. In low-temperature devices, superconducting states can be used as a magneto-thermal-switching (MTS) component, because carrier thermal conductivity (\k{appa}) is strongly suppressed in superconducting states. Recently, we demonstrated that the MTS ratio (MTSR) of pure Nb reached 650% at a temperature (T) of 2.5 K under a magnetic field (H) of 4.0 kOe [M. Yoshida et al., Appl. Phys. Express 16, 033002 (2023)]. In this study, to enrich knowledge on MTS of superconductors, the MTSRs of pure Pb wires with 5N and 3N purities were investigated by measuring the temperature or magnetic field dependences of \k{appa}. For 5N-Pb, a large MTSR exceeding 1000% was observed below 3.6 K under H > 600 Oe. Although higher MTSRs were expected at lower temperatures under H > 600 Oe, the obtained data under those conditions were accompanied by large errors due to magnetic-field-induced huge \k{appa} at low temperatures. In contrast, the \k{appa} for 3N-Pb were observed to be clearly lower than that for 5N-Pb. Although the magnetic-field-induced change in \k{appa} was small, the MTSR at T = 2.5 K was 300%. These results suggest that Pb is a promising material for low-temperature magneto-thermal switching because of wide-range \k{appa} tunable by magnetic field and the purity.

cond-mat.mtrl-sci↗

arXiv2023

Temperature profile of the Thomson-effect-induced heat release/absorption in junctionless single conductors

The Thomson effect induces heat release or absorption under the simultaneous application of a charge current and a temperature gradient to conductors. Here, we theoretically investigate the temperature profile due to the Thomson-effect-induced heat release/absorption in junctionless single conductors which can be a simple temperature modulator. We also perform analysis of the temperature profile for realistic conductors. As a result, we find that, for a conductor with a large Thomson coefficient, the temperature derivative of the Seebeck coefficient, the Thomson-effect-induced heat absorption overcomes the Joule heating, resulting in current-induced cooling in the bulk region. We also elucidate that a feedback effect of the Thomson effect stabilizes the system temperature to one-side of the heat bath, which reflects the fact that the Thomson effect is dependent on the position and proportional to the local temperature gradient. This work will be the basis for thermal management utilizing the Thomson effect.

cond-mat.mtrl-sci↗

arXiv2023

Magneto-thermal-switching properties of superconducting Nb

Recently, thermal switching has been extensively studied because it is a key component for thermal management in electronic devices. Here, we show a huge magneto-thermal-switching ratio (MTSR) in pure Nb at temperatures lower than its superconducting transition temperature (Tc = 9.2 K). The MTSR increases with decreasing temperature, and MTSR of 650% was observed at T = 2.5 K under H = 4.0 kOe. The thermal switching in superconductors with the huge MTSR will be useful for improvement of the performance of low-temperature electronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Seebeck-driven transverse thermoelectric generation in magnetic hybrid bulk materials

The Seebeck-driven transverse thermoelectric generation in magnetic/thermoelectric hybrid materials (STTG) has been investigated in all-bulk hybrid materials. The transverse thermopower in a ferromagnetic Co$_2$MnGa/thermoelectric $n$-type Si hybrid bulk material with the adjusted dimensions reaches 16.0 $μ$V/K at room temperature with the aid of the STTG contribution, which is much larger than the anomalous Nernst coefficient of the Co$_2$MnGa slab (6.8 $μ$V/K). Although this transverse thermopower is smaller than the value for previously reported thin-film-based hybrid materials, the hybrid bulk materials exhibit much larger electrical power owing to their small internal resistance. This demonstration confirms the validity of STTG in bulk materials and clarifies its potential as a thermal energy harvester.

cond-mat.mtrl-sci↗