Magneto-thermoelectric Effects Research Articles

Measuring lattice thermal conductivity of Bi1-xSbx enabled by external magnetic field

Minimizing lattice thermal conductivity (κL) is one of the most successful strategies for enhancing thermoelectrics. Existing experimental approach for κL-determination largely depend on model approximations for the electronic component (κE) with a subtraction from the measured total thermal conductivity (κ = κL+κE). However, the approximations (e.g. Wiedemann-Franz law) may not always be accurate particularly for materials with complicated band structures. Accurate κL-determination, not only characterizing the fundamental capability in heat conduction but also providing the insight of thermal functional materials, is still challenging. In this work, we utilize the magneto-thermoelectric effect under a magnetic field to suppress κE whereas κL is nearly unaffected. This allows for direct κL-measurement in materials that particularly have high carrier mobility. It is demonstrated in Bi1-xSbx single crystals that longitudinal κE can be sufficiently minimized to enable direct measurement in longitudinal κL at T < 100 K under a magnetic field of 5 T. It is revealed that the commonly used single Kane band (SKB) model approximations would lead to a 30 % overestimation in κL. The method proposed here enables a pathway of κL-determination for understanding the phonon transport in materials with complicated band structure.

Demonstration and imaging of cryogenic magneto-thermoelectric cooling in a van der Waals semimetal

Attaining viable thermoelectric cooling at cryogenic temperatures is of considerable fundamental and technological interest for electronics and quantum materials applications. In-device temperature control can provide more efficient and precise thermal environment management compared with conventional global cooling. The application of a current and perpendicular magnetic field gives rise to cooling by generating electron–hole pairs on one side of the sample and to heating due to their recombination on the opposite side, which is known as the Ettingshausen effect. Here we develop nanoscale cryogenic imaging of the magneto-thermoelectric effect and demonstrate absolute cooling and an Ettingshausen effect in exfoliated WTe2 Weyl semimetal flakes at liquid He temperatures. In contrast to bulk materials, the cooling is non-monotonic with respect to the magnetic field and device size. Our model of magneto-thermoelectricity in mesoscopic semimetal devices shows that the cooling efficiency and the induced temperature profiles are governed by the interplay between sample geometry, electron–hole recombination length, magnetic field, and flake and substrate heat conductivities. The observations open the way for the direct integration of microscopic thermoelectric cooling and for temperature landscape engineering in van der Waals devices.

Magneto-thermoelectric effects mapping using tip-induced temperature gradient in atomic force microscopy

Mapping magneto-thermoelectric effects, such as the anomalous Nernst effect, are crucial to optimize devices that convert thermal energy to electric energy. In this article, we show the methodology to realize this based on a technique we recently established using atomic force microscopy, in which a tip contact on the surface locally creates the temperature gradient. We can map the non-magnetic Seebeck and anomalous Nernst effects separately by investigating the magnetic field dependence. The simulation based on a simple heat transfer model between the tip and sample quantitatively explains our results. We estimated the magnitude of the anomalous Nernst effect in permalloy from the experiment and simulation to be ∼0.10 μV/K.

Doping as a tuning mechanism for magnetothermoelectric effects to improve zT in polycrystalline NbP

Weyl semimetals combine topological and semimetallic effects, making them candidates for interesting and effective thermoelectric transport properties. Here, we present experimental results on polycrystalline NbP, demonstrating the simultaneous existence of a large Nernst effect and a large magneto-Seebeck effect, which is typically not observed in a single material at the same temperature. We compare transport results from two polycrystalline samples of NbP with previously published work, observing a shift in the temperature at which the maximum Nernst and magneto-Seebeck thermopowers occur, while still maintaining thermopowers of similar magnitude. Theoretical modeling shows how doping strongly alters both the Seebeck and Nernst magnetothermopowers by shifting the temperature-dependent chemical potential, and the corresponding calculations provide a consistent interpretation of our results. Thus, we offer doping as a tuning mechanism for shifting magnetothermoelectric effects to temperatures appropriate for device applications, improving $zT$ at desirable operating temperatures. Furthermore, the simultaneous presence of both a large Nernst and magneto-Seebeck thermopower is uncommon and offers unique device advantages if the thermopowers are used additively. Here, we also propose a unique thermoelectric device that would collectively harness the large Nernst and magneto-Seebeck thermopowers to greatly enhance the output and $zT$ of conventional thermoelectric devices.

Magneto thermoelectric effect of nickel thin film synthesized by RF magnetron sputtering

Magneto thermoelectric effect of nickel thin film synthesized by RF magnetron sputtering

Thermal effects in spintronic materials and devices: An experimentalist’s guide

Research in spintronics often involves generation of heat in nanoscale magnetic systems. This heat generation can be intentional, as when studying effects created by an external applied temperature difference, or unintentional, coming as a consequence of driving relatively large charge currents through tiny structures. Understanding and controlling these thermal gradients can present challenges to experimentalists, which are related at some level to the fact that heat flow is much more difficult to isolate and manipulate than charge flow. This paper aims to provide a simple, intuitive framework to understand the fundamental issues that arise in spintronic materials and devices involving thermal gradients. The first goal is to provide simple tools to demonstrate how thermal gradients arise in systems with thin conducting films on bulk substrates. The main results are that a thermal gradient pointing perpendicular to the plane of a thin film supported on a macroscopic substrate is very common, even while the largest temperature drop in the system will exist across the bulk substrate itself. These results point to the need to understand the range of thermoelectric and magnetothermoelectric effects that can generate voltage signals and other responses to thermal gradients. I provide a brief review of these, along with relevant spin effects. The review concludes with examples and comments on several important ongoing issues in spintronics where thermal gradients play key roles.

Measurements of thermoelectric figure of merit based on multi-harmonic thermal analysis of thermographic images

We demonstrate a versatile measurement method for the thermoelectric figure of merit and related transport properties by means of a multi-harmonic thermal analysis of a thermographic movie. The method is based on the thermal analyses of the charge-current-induced temperature distribution generated by the Peltier effect at the first harmonic and by Joule heating at the second harmonic, measured with an infrared camera. This allows simultaneous estimation of the thermal diffusivity, thermal conductivity, volumetric heat capacity, and Peltier/Seebeck coefficient of conductors without attaching an external heater. The thermal analysis developed here is applicable to a system with the interfacial thermal resistance between the target conductor and reference material. Our method enables the measurements while applying an external magnetic field, opening the way for investigating the magnetic field and/or magnetization dependences of the figures of merit and associated properties for the magneto-thermoelectric effects. We demonstrate the usability of this method by estimating the figures of merit for the Peltier/Seebeck, magneto-Peltier/Seebeck, and Ettingshausen/Nernst effects in a Bi–Sb alloy as a promising material for thermoelectric applications. The multi-harmonic thermal analysis method will, thus, aid in developing highly efficient thermoelectric materials and further investigations of magneto-thermal and magneto-thermoelectric transport properties.

Large magneto-thermoelectric effect on the verge of metal–insulator and topological transitionsin pyrochlore iridates

We investigate the Seebeck and Nernst effects of pyrochlore iridium oxides (Nd1−xPrx)2Ir2O7 across the thermally induced and magnetic field-induced metal–insulator and topological transitions. Nd2Ir2O7 exhibits the salient temperature dependence of the Seebeck coefficient accompanied by the enhancement of the Nernst effect in the vicinity of the thermal magnetic transitions. Moreover, the Seebeck coefficient shows a remarkable magnetic field hysteresis with the differential magnitude reaching as large as 110 μV/K, as the conductive magnetic domain walls are generated/annihilated by the external field. For x = 0.5, the Nernst signal increases rapidly across the field-induced metal–insulator transitions, exceeding the values reported in existing ferromagnetic oxides. These findings indicate that the thermoelectric effects increase significantly near the topological electronic phase transitions in strongly correlated systems, providing a new guideline for thermoelectric material design.

Anomalous thermoelectric effects and quantum oscillations in the kagome metal CsV3Sb5

The kagome metal compounds $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A=\text{K}$, Rb, and Cs) feature a wealth of phenomena including nontrivial band topology, charge density wave (CDW), and superconductivity. One intriguing property is the time-reversal symmetry breaking in the CDW state without local moments, which leads to anomalous transport responses. Here, we report the investigation of magnetothermoelectric effects on high-quality ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ single crystals. A large anomalous Nernst effect (ANE) is observed at temperatures below 30 K and can be enhanced by the high mobility. Multiple Fermi surfaces with small effective masses are revealed by quantum oscillations in the Nernst and Seebeck signals. Furthermore, we discover a magnetic breakdown effect across the two smallest Fermi surfaces, with a gap around 20 meV between them. We propose that the two Fermi surfaces are split from a Dirac band by the CDW gap. These results indicate the large ANE originates from the CDW modulated nontrivial band structure as well as the extrinsic contributions. A second phase transition below the CDW transition temperature is also suggested by the strange temperature dependence of the ANE.

Deposition temperature dependence of thermo-spin and magneto-thermoelectric conversion in Co2MnGa films on Y3Fe5O12 and Gd3Ga5O12

We have characterized Co2MnGa (CMG) Heusler alloy films grown on Y3Fe5O12 (YIG) and Gd3Ga5O12 (GGG) substrates at different deposition temperatures and investigated thermo-spin and magneto-thermoelectric conversion properties by means of a lock-in thermography technique. X-ray diffraction, magnetization, and electrical transport measurements show that the deposition at high substrate temperatures induces the crystallized structures of CMG, while the resistivity of the CMG films on YIG (GGG) prepared at and above 500 °C (550 °C) becomes too high to measure the thermo-spin and magneto-thermoelectric effects due to large roughness, highlighting the difficulty of fabricating highly ordered continuous CMG films on garnet structures. Our lock-in thermography measurements show that the deposition at high substrate temperatures results in an increase in the current-induced temperature change for CMG/GGG and a decrease in that for CMG/YIG. The former indicates the enhancement of the anomalous Ettingshausen effect in CMG through crystallization. The latter can be explained by the superposition of the anomalous Ettingshausen effect and the spin Peltier effect induced by the positive (negative) charge-to-spin conversion for the amorphous (crystallized) CMG films. These results provide a hint to construct spin-caloritronic devices based on Heusler alloys.

Large Transverse and Longitudinal Magneto‐Thermoelectric Effect in Polycrystalline Nodal‐Line Semimetal Mg3Bi2

Topological semimetals provide new opportunities for exploring novel thermoelectric phenomena, owing to their exotic and nontrivial electronic structure topology around the Fermi surface. Herein, the discovery of large transverse and longitudinal magneto-thermoelectric (MTE) effects in Mg3 Bi2 is reported and predicted to be a type-II nodal-line semimetal in the absence of spin-orbit coupling (SOC). The maximum transverse power factor is 2182 μW m-1 K-2 at 13.5 K and 6 Tesla. The longitudinal power factor reaches up to 3043 μW m-1 K-2 , which is 20 times higher than that in a zero-strength magnetic field and is also comparable to state-of-the-art MTE materials. By compensating the Mg loss in Mg-rich conditions for tuning the carrier concentration close to intrinsic state, the sample fabricated in this study exhibits a large linear non-saturating magnetoresistance of 940% under a field of 14 Tesla. Using density functional calculations, the authors attribute the underlying mechanism to the parent linear-dispersed nodal-line electronic structure without SOC and the anisotropic Fermi surface shape with SOC, highlighting the essential role of high carrier mobility and open electron orbits in the moment space. This work offers a new avenue toward highly efficient MTE materials through defect engineering in polycrystalline topological semimetals.

Giant Photo‐Magneto‐Thermoelectric Effect of End‐On Oriented PEDOT Grown from Self‐Assembled 3D Tectons

AbstractConductive polymers (CPs) are arranged in the end‐on direction on graphene surfaces using 3D tectons of ZnPc assemblies, to control carrier transport from hot to cold via external magnetic fields. Poly(3,4‐ethylenedioxythiophene)s doped with tosylate are synthesized using monomer seeds that are complexed through pyridine units to 3D tectons on graphene via surface‐confined host‐guest chemistry, to provide an end‐on‐oriented polymer film on graphene with a high degree of crystallinity. The film shows free‐electron‐like charge transport properties, with an enhanced Hall mobility, and an increased Seebeck coefficient compared to the CP on graphene that is prepared without 3D tectons. A magneto‐thermoelectric (MTE) effect is reproducibly observed for the film device. Taking advantage of a large photothermal effect of the polymer film, a photon‐mode MTE effect is demonstrated for the first time. Both dark and photo‐MTE effects correlate well to the generated Lorentz force. These results provide a unique tool to program carrier transport in thin‐film via external magnetic stimuli with a temperature gradient.

Giant Magnetoresistance and Magneto-Thermopower in 3D Interconnected NixFe1-x/Cu Multilayered Nanowire Networks.

The versatility of the template-assisted electrodeposition technique to fabricate complex three-dimensional networks made of interconnected nanowires allows one to easily stack ferromagnetic and non-magnetic metallic layers along the nanowire axis. This leads to the fabrication of unique multilayered nanowire network films showing giant magnetoresistance effect in the current-perpendicular-to-plane configuration that can be reliably measured along the macroscopic in-plane direction of the films. Moreover, the system also enables reliable measurements of the analogous magneto-thermoelectric properties of the multilayered nanowire networks. Here, three-dimensional interconnected NiFe/Cu multilayered nanowire networks (with ) are fabricated and characterized, leading to large magnetoresistance and magneto-thermopower ratios up to 17% and −25% in NiFe/Cu, respectively. A strong contrast is observed between the amplitudes of magnetoresistance and magneto-thermoelectric effects depending on the Ni content of the NiFe alloys. In particular, for the highest Ni concentrations, a strong increase in the magneto-thermoelectric effect is observed, more than a factor of 7 larger than the magnetoresistive effect for NiFe/Cu multilayers. This sharp increase is mainly due to an increase in the spin-dependent Seebeck coefficient from −7 µV/K for the NiFe/Cu and NiFe/Cu nanowire arrays to −21 µV/K for the NiFe/Cu nanowire array. The enhancement of the magneto-thermoelectric effect for multilayered nanowire networks based on dilute Ni alloys is promising for obtaining a flexible magnetic switch for thermoelectric generation for potential applications in heat management or logic devices using thermal energy.

Transport phenomena in spin caloritronics.

The interconversion between spin, charge, and heat currents is being actively studied from the viewpoints of both fundamental physics and thermoelectric applications in the field of spin caloritronics. This field is a branch of spintronics, which has developed rapidly since the discovery of the thermo-spin conversion phenomenon called the spin Seebeck effect. In spin caloritronics, various thermo-spin conversion phenomena and principles have subsequently been discovered and magneto-thermoelectric effects, thermoelectric effects unique to magnetic materials, have received renewed attention with the advances in physical understanding and thermal/thermoelectric measurement techniques. However, the existence of various thermo-spin and magneto-thermoelectric conversion phenomena with similar names may confuse non-specialists. Thus, in this Review, the basic behaviors, spin-charge-heat current conversion symmetries, and functionalities of spin-caloritronic phenomena are summarized, which will help new entrants to learn fundamental physics, materials science, and application studies in spin caloritronics.

Magneto-thermoelectric Effects in Cylindrical Quantum Wire under the Influence of Electromagnetic Wave for Electron-optical Phonon Scattering

Based on the quantum kinetic equation for electrons, we have theoretically studied the influence of a Strong Electromagnetic Wave (EMW) on the Magneto-Thermoelectric Effects in a cylindrical quantum wire with an infinite potential (CQWIP) (for electron - optical phonon scattering) under the influence of electric field E1=(0,0,E1), magnetic field B=(0,B,0) and a strong EMW (laser radiation) E= (0,0,E0sinΩt) (where and are amplitude and frequency of EMW, respectively). We obtain the analytic expressions for the kinetic tensors sik, bik, gik, xik and the Ettingshausen coefficient (EC) in the CQWIP with the dependence on the frequency, the amplitude of EMW, the Quantum Wire (CQWIP) parameters, the magnetic field and the temperature. The results are numerically evaluated and graphed for GaAs/GaAsAl quantum wire. Then, the results in this case are compared with the case of the bulk semiconductors and others low-dimension system in order to show the difference and the novelty of the results. Moreover, we realize that as the EMW frequency increases, the EC fluctuates with a stable trend and the appearance of the Shubnikov-de Haas (SdH) oscillations pattern when we survey the dependence of EC on the magnetic field.

Study of the Quantum Magneto-Thermoelectric Effect in the Two-Dimensional Compositional Superlattice GaAs/AlGaAs under the influence of an Electromagnetic Wave by Using the Quantum Kinetic Equation

The quantum magneto-thermoelectric effect in a two-dimensional compositional superlattice under the influence of an electromagnetic wave (EMW) in two cases is investigated. Two cases of the electron scattering mechanism are considered: the electron-acoustic phonon scattering and electron-optical phonon scattering. Analytical expressions for the quantum Ettingshausen coefficient (EC), the thermopower tensor, the thermoelectric tensor and the kinetic tensor are obtained by using a quantum kinetic equation. These expressions are numerically solved for the two-dimensional compositional superlattice GaAs/AlGaAs and the results are discussed. The results show that in the case of electron-acoustic phonon scattering, the Shubnikov-de Haas oscillations appear when we examine the dependences of the quantum EC, the thermopower tensor and the thermoelectric tensor on the magnetic field. In the case of electron-optical phonon scattering, resonance peaks that satisfy the condition of the inter-subband magneto-phonon resonance appear. In the two cases, the superlattice period (a parameter specific to the material) strongly affects the quantum magneto-thermoelectric effect. When the superlattice period is small, quantum EC oscillations (in the case of electron-acoustic phonon interaction) and quantum EC resonance peaks (in the case of electron-optical phonon interaction) appear. However, when the superlattice period is large, these oscillations and resonance peaks are not observed. Especially, the influence of electromagnetic waves on the quantum magneto-thermoelectric effect is also clarified. The quantum theory of the magneto-thermoelectric effect has been studied from low temperature to high temperature. This overcomes the limitations of the Boltzmann kinetic equation which was studied at high temperature. The results are new and can serve as a basis for further development of the theory of quantum magneto-thermoelectric effects in low-dimensional semiconductor systems.

Enhanced Figure of Merit in Bismuth-Antimony Fine-Grained Alloys at Cryogenic Temperatures

Thermoelectric (TE) materials research plays a vital role in heat-to-electrical energy conversion and refrigeration applications. Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures (100–150 K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples consisting of micron-size grains. The achieved ZT is nearly 50% higher than polycrystalline averaged single crystal ZT of ~0.4, and it is also significantly higher than ZT of less than ~0.3 measured below 150 K in Bi-Te alloys commonly used for cryogenic cooling applications. The improved thermoelectric properties can be attributed to the fine-grained microstructure achieved from rapid solidification, which not only significantly reduced the thermal conductivity but also mitigated a segregation effect. A record low thermal conductivity of ~1.5 W m−1 K−1 near 100 K was measured using the hot disk method. The thermoelectric properties for this intriguing semimetal-semiconductor alloy system were analyzed within a two-band effective mass model. The study revealed a gradual narrowing of the band gap at increasing temperature in Bi-Sb alloy for the first time. Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE properties, leading to ZT of about 0.7. The magneto-TE effect was further demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in thermoelectric cooling applications.

Strain-induced switching of heat current direction generated by magneto-thermoelectric effects

Since the charge current plays a major role in information processing and Joule heating is inevitable in electronic devices, thermal management, i.e., designing heat flows, is required. Here, we report that strain application can change a direction of a heat current generated by magneto-thermoelectric effects. For demonstration, we used metallic magnets in a thin-film form, wherein the anomalous Ettingshausen effect mainly determines the direction of the heat flow. Strain application can alter the magnetization direction owing to the magnetoelastic effect. As a result, the heat current, which is in the direction of the cross product of the charge current and the magnetization vector, can be switched or rotated simply by applying a tensile strain to the metallic magnets. We demonstrate 180° switching and 90° rotation of the heat currents in an in-plane magnetized Ni sample on a rigid sapphire substrate and a perpendicularly magnetized TbFeCo film on a flexible substrate, respectively. An active thermography technique was used to capture the strain-induced change in the heat current direction. The method presented here provides a novel method for controlling thermal energy in electronic devices.

Thermoelectric figure of merit in transverse magnetic field under adiabatic and isothermal conditions

The influence of the magnetic field on the thermoelectric figure of merit in semiconducting materials is studied theoretically. One of the important issues is the urgency to distinguish the various conditions under which the magneto-thermoelectric effects can be observed. Isothermal effects take place when there is no temperature gradient in the direction perpendicular to the plane containing the magnetic field and applied temperature gradient. Adiabatic effects occur when there is no heat current in the same transverse direction while a temperature gradient arises due to the Righi-Leduc effect. In this paper we examine how the application of an external magnetic field can vary the figure of merit ZT under isothermal and adiabatic conditions and establish relations among them for both longitudinal and transverse Nernst-Ettingshausen effects. Analytical expressions for the figures of merit are obtained in semiconductors with both non-degenerate and strongly degenerate statistics of charge carriers, in limiting cases of weak and classically strong magnetic fields.

Magneto-thermoelectric effects in rectangular quantum wire with an infinitely high potential in the presence of electromagnetic wave (electron - acoustic phonon interaction)

Based on the quantum kinetic equation for electrons, we have theoretically studied the influence of a Strong Electromagnetic Wave on the Ettingshausen Effect (EC) in a rectangular quantum wire with an infinite potential (RQWIP). We obtain the analytic expressions for the kinetic tensor as well as the Ettingshausen coefficient in the rectangular quantum wire with an infinite potential with the dependence on B and Ω…… The results are numerically evaluated and graphed for GaAs/GaAs:Al quantum wire. We survey the electrical and thermal conductivity tensor depend on Electromagnetic Wave frequency and temperature. The results give us appearance of the Shubnikov–de Haas oscillations when we survey the dependence of Ettingshausen coefficient on the magnetic field. Then, we realize that as the temperature increases, the Ettingshausen coefficient decreases. This shows that the Electromagnetic Wave have a clear impact on the effects. These are latest results which have been studied in terms of Ettingshausen effect in rectangular quantum wire.