Large Anomalous Nernst Effect Research Articles

Unconventional anomalous Hall effect and large anomalous Nernst effect in antiferromagnet SmMnBi2

The anomalous Hall effect (AHE) and its thermoelectric counterpart, the anomalous Nernst effect (ANE), are two transverse transport coefficients that are intensely studied in condensed matter physics. While conventional wisdom links AHE and ANE to ferromagnetism, recent achievements reveal that they can emerge in nonmagnetic and antiferromagnetic topological materials with a diversity of mechanisms—many of which await further elucidation. Here, both an unconventional AHE (UAHE) that does not scale with the magnetization and a sizable ANE ( ≈ 1.8 μV K−1) are shown to be possessed by the metallic tetragonal antiferromagnet SmMnBi2. Electronic band structure of SmMnBi2 is investigated by angle-resolved photoemission spectroscopy and first-principles calculations. It is demonstrated that the UAHE reflects the intrinsic Berry curvature contribution stemming from the spin-canted antiferromagnetism, whereas the ANE is possibly further amplified by extrinsic mechanisms. These results identify SmMnBi2 as a promising candidate for exploring unusual transverse transport effects and the extremely rich underlying physics.

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 μV/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.

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, 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 ~ 70% improvement in the anomalous Nernst coefficients (reaching ~ 3.7 µVK−1) and a significant ~ 200% enhancement in the power factor (reaching ~ 7.7 µWm−1K−2) in flexible Fe-based amorphous materials by nanostructure engineering without changing their composition. This surpasses all reported amorphous alloys and is comparable to single crystals showing large anomalous Nernst effect. 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.

Bipolarity of large anomalous Nernst effect in Weyl magnet-based alloy films

Bipolarity of large anomalous Nernst effect in Weyl magnet-based alloy films

First-principles study of anomalous Nernst effect in Cr-doped Bi2Se3

We have investigated electronic structures and thermoelectric properties on six quintuple layers of Cr-doped Bi2Se3 as a model of quantized anomalous Hall insulator, Chern insulator. The Chern insulator might be a good transverse thermoelectric material exhibiting a large anomalous Nernst effect by the intrinsic contribution of anomalous Hall effect and large Seebeck effect. Based on rigid band approximation, we have performed first-principles density functional calculations of carrier-dependent anomalous Nernst coefficients. To optimize thermoelectric performance, we discussed the origin of the anomalous Nernst effect, divided into contributions from pure Nernst and Seebeck terms. We found the significant contribution of the Seebeck term in Cr-doped Bi2Se3.

Anomalous Nernst effect in honeycomb and kagome magnet LaCo5 at room temperature

Magnetic order-induced time reversal symmetry breaking in the kagome lattice is predicted to generate Weyl semimetal and Chern insulating phases. The two-dimensional (2D) honeycomb lattices with spin orbit coupling (SOC) and ferromagnetism are also known to host the Chern insulating phase. Here, we study the transport properties of the ferromagnetic topological material LaCo5 crystals with TC = 840 K, which is the highest among the reported magnetic topological materials up to now. It is composed of 2D Co2 kagome layers that are separated by La–Co1 layers, in which the Co1 atoms exhibit a honeycomb structure. Both the 2D kagome and honeycomb layers exhibit out-of-plane magnetization. The large intrinsic anomalous Hall effect and anomalous Nernst effect of about 4.6 μVK−1 are observed in the ab plane with the B//c configuration at room temperature, which can be attributed to non-zero Berry curvature according to the first-principles calculations. Our results enrich the magnetic topological materials with honeycomb and kagome lattices, providing a good platform for the further study of the QAHE. The excellent thermoelectric performance, high TC, nontoxicity, and low cost also make LaCo5 a promising candidate for the applications of thermoelectric generators and heat flow sensors at high temperatures.

Rational design of large anomalous Nernst effect in Dirac semimetals

Anomalous Nernst effect generates a transverse voltage perpendicular to the temperature gradient. It has several advantages compared with the longitudinal thermoelectricity for energy conversion, such as decoupling of electronic and thermal transports, higher flexibility, and simpler lateral structure. However, a design principle beyond specific materials systems for obtaining a large anomalous Nernst conductivity (ANC) is still absent. In this work, we theoretically demonstrate that a pair of Dirac nodes under a Zeeman field manifests an odd-distributed, double-peak anomalous Hall conductivity curve with respect to the chemical potential and a compensated carrier feature, leading to an enhanced ANC compared with that of a simple Weyl semimetal with two Weyl nodes. Based on first-principles calculations, we then provide two Dirac semimetal candidates, i.e., Na3Bi and NaTeAu, and show that under a Zeeman field, they exhibit a sizable ANC value of 0.4 {rm{A}}{{rm{m}}}^{-1}{{rm{K}}}^{-1} and 1.3 {rm{A}}{{rm{m}}}^{-1}{{rm{K}}}^{-1}, respectively, near the Fermi level. Such an approach is also applicable to ferromagnetic materials with intrinsic Zeeman splitting, as exemplified by a hypothetical alloy NaFeTe2Au2, exhibiting an ANC as high as 3.7 {rm{A}}{{rm{m}}}^{-1}{{rm{K}}}^{-1} at the Fermi level. Our work provides a design principle with a prototype band structure for enhanced ANC pinning at the Fermi level, shedding light on the inverse design of other specific functional materials based on electronic structure.

Large anomalous Hall, Nernst effect and topological phases in the 3d-4d/5d-based oxide double perovskites

Magnetism and spin-orbit coupling are two fundamental and interconnected properties of oxide materials, that can give rise to various topological transport phenomena, including anomalous Hall and anomalous Nernst effects. These transport responses can be significantly enhanced by designing an electronic structure with a large Berry curvature. In this context, rocksalt-ordered double perovskites (DP), denoted as A2BB’O6, with two distinct transition metal sites are very powerful platforms for exploration and research. In this work, we present a comprehensive study based on the intrinsic anomalous transport in cubic and tetragonal stable DP compounds with 3d-4d/5d elements. Our findings reveal that certain DP compounds show a large anomalous Hall effect, displaying topological band crossings in the proximity of the Fermi energy.

Large Anomalous Nernst Effects at Room Temperature in Fe3 Pt Thin Films.

Heat current in ferromagnets can generate a transverse electric voltage perpendicular to magnetization, known as anomalous Nernst effect (ANE). ANE originates intrinsically from the combination of large Berry curvature and density of states near the Fermi energy. It shows technical advantages over the conventional longitudinal Seebeck effect in converting waste heat to electricity due to its unique transverse geometry. However, materials showing giant ANE remain to be explored. Herein,a large ANE thermopower of Syx ≈ 2 µV K-1 at room temperature in ferromagnetic Fe3 Pt epitaxial films is reported, which also show a giant transverse thermoelectric conductivity of αyx ≈ 4 A K-1 m-1 and a remarkable coercive field of 1300Oe. The theoretical analysis reveals that the strong spin-orbit interaction in addition to the hybridization between Pt 5d and Fe 3d electrons leads to a series of distinct energy gaps and large Berry curvature in the Brillouin zone, which is the key for the large ANE. These results highlight the important roles of both Berry curvature and spin-orbit coupling in achieving large ANE at zero magnetic field, providing pathways to explore materials with giant transverse thermoelectric effect without an external magnetic field.

VTaNbAl: A class of spin gapless semiconductors with topological nontrivial features

We report a class of spin gapless semiconductors (SGSs) hosting topological nontrivial features along with a fully compensated ferrimagnetic behavior in VTaNbAl, a quaternary Heusler alloy. Unlike conventional SGSs, this class of compounds acquires a unique band structure with opposite spin characters in the valence and conduction band edges, making them potential candidates for spin valves and a large anomalous Nernst effect. Interestingly, despite compensated ferrimagnetic (CF) behavior, VTaNbAl shows a reasonably large anomalous Hall effect possibly arising from the intrinsic nonvanishing Berry curvature. The CF state breaks the time-reversal symmetry and hence opens the possibility of Weyl nodes. We found four pairs of Weyl nodes located near the Fermi level leading to the nonvanishing Berry curvature, and hence a large anomalous Hall conductivity ($\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$). Our experimental synthesis confirms VTaNbAl to crystallize in a cubic Heusler structure with $A2$-type disorder. A magnetization measurement shows a very small saturation moment ($\ensuremath{\sim}{10}^{\ensuremath{-}3}{\textmu{}}_{B}/\text{f.u.}$), which agrees fairly well with our theoretical findings of fully compensated ferrimagnetism in the alloy. Resistivity shows a metalliclike nature, with the signature of disorder-induced weak localization in the low-temperature limit. The overall variation with varying temperature, however, is extremely weak.

Enhanced Anomalous Nernst Effect by Tuning the Chemical Potential in the Topological Kagome Ferromagnet Fe3Sn2

Large anomalous Nernst effect (ANE) in magnetic topological materials (MTMs) has attracted various attentions because of its potential thermoelectric applications. Recently, MTMs with the kagome lattice have become a focus of research to investigate enhanced ANE. In this work, the ANE (${S}_{yx}^{A}$) at room temperature in a kagome ferromagnet ${\mathrm{Fe}}_{3}{\mathrm{Sn}}_{2}$ with massive Dirac bands is greatly enhanced to $5.44\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}{\mathrm{VK}}^{\ensuremath{-}1}$ when its Fermi level is gradually tuned to Dirac nodal points. Then the intrinsic contributions to the enhanced ${S}_{yx}^{A}$ is further investigated. The anomalous transverse Peltier conductivity ${\ensuremath{\alpha}}_{yx}^{A}$ shows the linear temperature-dependent behavior, which is consistent with Berry-curvature-contributed ${\ensuremath{\alpha}}_{yx}^{A}$, and this behavior is different from temperature-dependent variation of magnetization. Combined with theoretic calculations, the ${\ensuremath{\alpha}}_{yx}^{A}$ and $|{\ensuremath{\alpha}}_{yx}^{A}/{\ensuremath{\sigma}}_{xy}^{A}|$ ratio increases significantly when approaching the massive Dirac cone, indicating that the massive Dirac dispersion and intrinsic contributions play a role in enhanced ANE instead of the side-jump mechanism. Our work demonstrates an effective universal approach to enhance ANE in such MTMs.

Anomalous Nernst effect in compensated ferrimagnetic CoxGd1-x films

The anomalous Nernst effect (ANE) is one of the most intriguing thermoelectric phenomena, which has attracted growing interest both for its underlying physics and potential applications. Typically, a large ANE response is observed in magnets with pronounced magnetizations or nontrivial Berry curvature. Here, we report a significant ANE signal in compensated ferrimagnetic CoxGd1-x amorphous films, which exhibit vanishingly small magnetization. In particular, we found that the polarity of ANE signal is dominated by the magnetization orientation of the transition metal Co sublattices, rather than the net magnetization of CoxGd1-x films. This observation is not expected from the conventional understandings of ANE but is analogous to the anomalous Hall effect in compensated ferrimagnets. The joint contribution from the Berry curvature and the scattering mechanisms near the Co-dominant Fermi surface could interpret the Co-dominant property of ANE. Our work may trigger a more comprehensive understanding of ANE and may be useful for building energy-harvesting devices by employing ANE in compensated ferrimagnets.

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation

ABSTRACT Transverse thermoelectric generation using magnetic materials is essential to develop active thermal engineering technologies, for which the improvements of not only the thermoelectric output but also applicability and versatility are required. In this study, using combinatorial material science and lock-in thermography technique, we have systematically investigated the transverse thermoelectric performance of Sm-Co-based alloy films. The high-throughput material investigation revealed the best Sm-Co-based alloys with the large anomalous Nernst effect (ANE) as well as the anomalous Ettingshausen effect (AEE). In addition to ANE/AEE, we discovered unique and superior material properties in these alloys: the amorphous structure, low thermal conductivity, and large in-plane coercivity and remanent magnetization. These properties make it advantageous over conventional materials to realize heat flux sensing applications based on ANE, as our Sm-Co-based films can generate thermoelectric output without an external magnetic field. Importantly, the amorphous nature enables the fabrication of these films on various substrates including flexible sheets, making the large-scale and low-cost manufacturing easier. Our demonstration will provide a pathway to develop flexible transverse thermoelectric devices for smart thermal management.

Large Room Temperature Anomalous Transverse Thermoelectric Effect in Kagome Antiferromagnet YMn6 Sn6.

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K-1 at room temperature in a kagome antiferromagnet YMn6 Sn6 single crystal is obtained. The obtained value is larger than that of state-of-the-art canted antiferromagnetic (AFM) materials and comparable with ferromagnetic systems. The large anomalous Nernst effect (ANE) can be attributed to the net Berry curvature near the Fermi level. Furthermore, the ANE of the AFM YMn6 Sn6 exceeds the magnetization scaling relationship of conventional ferromagnets. The results clearly illustrate that AFM material YMn6 Sn6 is an ideal topological material for room-temperature transverse thermoelectric applications.

Efficient room-temperature magnetization direction detection by means of the enhanced anomalous Nernst effect in a Weyl ferromagnet

The topological nature of Heusler-alloy-based Weyl ferromagnet, Co${}_{2}$MnGa, results in an abundance of intriguing physical phenomena. In this work, a colossal enhancement of unidirectional magnetoresistance induced by the large anomalous Nernst effect is realized in nano-sized wires, which enables the operation of a prototypical non-volatile all-electric two-terminal memory device at room temperature. A size scaling investigation indicates that there is further room for improving the magnetoresistance ratio. These findings can pave a new avenue for creating efficient thermo-spintronic devices using novel topological materials.

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.

Strain‐Induced Large Anomalous Nernst Effect in Polycrystalline Co2MnGa/AlN Multilayers

AbstractThe anomalous Nernst effect (ANE), one of the thermoelectric effects in a magnet, is recently attracting growing interest with its potential for the next generation high‐efficiency energy‐harvesting applications. However, the reported thermoelectric conversion efficiencies with ANE are small compared to those with Seebeck effect, which hampers its practical application. Unlike the intensive effort of seeking novel materials with large intrinsic ANE, here a new pathway is proposed to enhance the ANE by exploiting the strain induced in a multilayer structure. A large anomalous Nernst coefficient of 4.9 µV K−1 is achieved for a polycrystalline Co2MnGa/AlN multilayer film deposited on an amorphous substrate, which is larger than 3.8 µV K−1 for a polycrystalline Co2MnGa single layer film. The enhanced ANE is attributed to the effect of interfacial strain on the Seebeck coefficient, which is barely discussed yet. Since the AlN layer is available on any substrate materials, even on a flexible polyimide substrate large ANE is successfully achieved for the Co2MnGa/AlN stack. The findings and demonstration have opened a way to develop the ANE‐based potential applications.

Anomalous transport properties of the antiferromagnetic Weyl semimetals Mn3 X (X = Sn, Ge)

Noncollinear antiferromagnets Mn3 X (X = Sn, Ge) are characterized by a large anomalous Hall effect originating from a large Berry curvature despite a vanishingly small magnetization. From recent first-principle theories, the large Berry curvature is predicted to be induced by a existence of Weyl nodes broken time-reversal symmetry. The large anomalous Nernst effect is also contributed by the magnetic Weyl state around the Fermi level EF, and likely shares its origin with the anomalous Hall effect. The thermoelectric transport S(T) and thermomagnetic transport Sji(T) are thus investigated in single crystals of Mn3 X. Here, Mn3 X exhibits a large anomalous Nernst effect; in particular, the signal magnitude of Mn3Ge exceeds 1μV/K, which is 1.5 times that of Mn3Sn. The Weyl properties are discussed by analyzing the thermal conductivity, specific heat, and Seebeck and Nernst effects. We also evaluate the zero-field Nernst-driven thermoelectric figure of merit for device applications in the antiferromagnets Mn3 X.

Exchange-biased topological transverse thermoelectric effects in a Kagome ferrimagnet

Kagome metal TbMn6Sn6 was recently discovered to be a ferrimagnetic topological Dirac material by scanning tunneling microscopy/spectroscopy measurements. Here, we report the observation of large anomalous Nernst effect and anomalous thermal Hall effect in this compound. The anomalous transverse transport is consistent with the Berry curvature contribution from the massive Dirac gaps in the 3D momentum space as demonstrated by our first-principles calculations. Furthermore, the transverse thermoelectric transport exhibits asymmetry with respect to the applied magnetic field, i.e., an exchange-bias behavior. Together, these features place TbMn6Sn6 as a promising system for the outstanding thermoelectric performance based on anomalous Nernst effect.

Large anomalous Nernst effect and nodal plane in an iron-based kagome ferromagnet.

Anomalous Nernst effect (ANE), converting a heat flow to transverse electric voltage, originates from the Berry phase of electronic wave function near the Fermi energy EF. Thus, the ANE provides a sensitive probe to detect a topological state that produces large Berry curvature. In addition, a magnet that exhibits a large ANE using low-cost and safe elements will be useful to develop a novel energy harvesting technology. Here, we report our observation of a high ANE exceeding 3 microvolts per kelvin above room temperature in the kagome ferromagnet Fe3Sn with the Curie temperature of 760 kelvin. Our theoretical analysis clarifies that a “nodal plane” produces a flat hexagonal frame with strongly enhanced Berry curvature, resulting in the large ANE. Our discovery of the large ANE in Fe3Sn opens the path for the previously unexplored functionality of flat degenerate electronic states and for developing flexible film thermopile and heat current sensors.