Anomalous Nernst Effect Research Articles

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.

Giant, Linearly Increasing Spin-Orbit Torque Efficiency in Symmetry-Broken Spin-Orbit Torque Superlattices.

Magnetic heterostructures with high spin-orbit torque efficiency and low impedance have great promise for low-power spintronic technologies. We report a symmetry-broken spin-orbit superlattice [Pt0.75Cu0.25/Co/Ta]n, in which the dampinglike spin-orbit torque efficiency accumulates linearly with the repeat number n and achieves a giant value of >200% when n = 16, which is 100 times stronger than that of a conventional magnetic heterostructure with a clean Pt (e.g., 2% at a resistivity of 7 μΩ cm). The giant spin-orbit torque effect arises predominantly from the spin Hall effect of Pt0.75Cu0.25. The anomalous Nernst effect increases remarkably as the repeat number n increases, implying a critical need to include the thermal effect in the analysis of magnetic superlattices and multilayers. The giant spin-orbit torque, low resistivity, and strong anomalous Nernst effect suggest the great potential of the superlattice [Pt0.75Cu0.25/Co/Ta]n for low-power memory and logic technologies as well as high-performance thermoelectric battery and sensor applications.

Exploring the interfacial structure and crystallinity for direct growth of Mn3Sn(0001) on sapphire (0001) by molecular beam epitaxy

The Kagome antiferromagnet Mn3Sn has garnered a lot of attention due to the presence of interesting properties such as anomalous Hall effects and Nernst effects. Until now, few papers have been reported to grow using molecular beam epitaxy with a buffer layer or on other substrates. In this paper, we discuss the synthesis of crystalline Mn3Sn layers, prepared on Al2O3 (0001) without a buffer layer using molecular beam epitaxy. The growth is monitored in-situ using reflection high energy electron diffraction and measured ex-situ using X-ray diffraction, Rutherford back-scattering, and cross-sectional scanning transmission electron microscopy. The samples were deposited at 524 ± 5 °C, with an Mn: Sn atomic flux ratio of 3.2:1 for 90 min. Orientation relationships between the Mn3Sn films and the sapphire substrates are determined from in-plane and out-of-plane measurements. Our analysis indicates that the resulting film is predominantly c−plane oriented. Lastly, the samples prepared in this way were found to be discontiguous, showing a 3-dimensional morphology. According to first-principles calculations, the Mn3Sn exhibits a displaced Kagome structure in the very first stages of growth, for 2 ML and 4 ML growth on Al2O3 (0001). This result is corroborated by calculating the surface formation energies and explains the observed RHEED patterns.

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.

Longitudinal Spin Seebeck Effect Thermopiles Based on Flexible Co-Rich Amorphous Ribbons/Pt Thin-Film Heterostructures.

Thermoelectric phenomena, such as the Anomalous Nernst and Longitudinal Spin Seebeck Effects, are promising for sensor applications in the area of renewable energy. In the case of flexible electronic materials, the request is even larger because they can be integrated into devices having complex shape surfaces. Here, we reveal that Pt promotes an enhancement of the thermoelectric response in Co-rich ribbon/Pt heterostructures due to the spin-to-charge conversion. Moreover, we demonstrated that the employment of the thermopiles configuration in this system increases the induced thermoelectric current, a fact related to the considerable decrease in the electric resistance of the system. By comparing present findings with the literature, we were able to design a flexible thermopile based on LSSE without the lithography process. Additionally, the thermoelectric voltage found in the studied flexible heterostructures is comparable to the ones verified for rigid systems.

Anomalous Nernst thermoelectric generation in multilayer-laminated coiled magnetic wires

Thermoelectric generation using the anomalous Nernst effect (ANE) is demonstrated in multilayer-laminated coils comprising ferromagnetic metal wires stacked in the radial direction. By applying a temperature gradient to the coils in the radial direction, the thermoelectric voltage and power induced by the ANE appear and increase in proportion to the number of stacked layers. This device structure is useful for improving the thermoelectric performance and versatility of energy-harvesting devices based on the ANE.

Anomalous Nernst Effect Induced Terahertz Emission in a Single Ferromagnetic Film.

Despite its important role in understanding ultrafast spin dynamics and revealing novel spin/orbit effects, the mechanism of the terahertz (THz) emission from a single ferromagnetic nanofilm upon a femtosecond laser pump still remains elusive. Recent experiments have shown exotic symmetry, which is not expected from the routinely adopted mechanism of ultrafast demagnetization. Here, by developing a bidirectional pump-THz emission spectroscopy and associated symmetry analysis method, we set a benchmark for the experimental distinction of the THz emission induced by various mechanisms. Our results unambiguously unveil a new mechanism─anomalous Nernst effect (ANE) induced THz emission due to the ultrafast temperature gradient created by a femtosecond laser. Quantitative analysis shows that the THz emission exhibits interesting thickness dependence where different mechanisms dominate at different thickness ranges. Our work not only clarifies the origin of the ferromagnetic-based THz emission but also offers a fertile platform for investigating the ultrafast optomagnetism and THz spintronics.

Investigation of the effect of neighboring materials on the Anomalous Nernst effect

The Anomalous Nernst effect (ANE) is an important mechanism that enables magnetic metals to be used for heat-to-electrical energy conversion. This phenomenon has been thought to be independent of neighboring layers. Contrary to this assumption, this paper shows theoretical and experimental evidence of how the neighboring layer affects the ANE voltage signal. First, we investigated the voltage generated in a Py/Cu bilayer, with Ta, Pt, and SiO2 as capping layer materials. Next, we performed a Py thickness dependence study of the thermal voltage in the same sample sets. Both simulated and experimental results suggest that the spin current calculated/measured within the ferromagnetic materials can be altered by the boundary conditions, and the effect of the neighboring materials is more significant for thin Py. Second, we compared the thermal voltages generated from the sample 3Py/6Cu/3Py/4SiO2 and 3Py/6Pt/3Py/4SiO2. Then, the thermal voltage generated from the same sample set was examined after insertion of a silicon dioxide layer, as in 3Py/3Cu/2SiO2/3Cu/3Py/4SiO2 and 3Py/3Pt/2SiO2/3Pt/3Py/4SiO2. We found that the insertion of the SiO2 layer between two Cu layers lowers the size of the thermal voltage remarkably comparing to when the SiO2 layer is inserted between two Pt layers.

Giant anomalous Hall and Nernst conductivities in cubic Fe3Pt and Ni3In

The topological anomalous Hall effect and the anomalous Nernst effect are of great interest for the development of spintronic devices for energy-efficient data storage and energy harvesting. Using first-principles calculations, we have studied the anomalous Hall conductivity (AHC) and anomalous Nernst conductivity (ANC) of six ferromagnetic compounds Co3Al, Fe3Ge, Fe3Pt, Ni3Ga, Ni3In, and Rh3Cr belonging to the cubic Cu3Au structure. A large AHC of 1973 S cm−1 is obtained for Fe3Pt. Parallel to the large AHC, a giant ANC of −5.41 A mK−1 and −4.18 A mK−1 is observed for Ni3In and Co3Al, respectively. Finally, we found that AHC and ANC can be adjusted by switching the magnetization direction.

Iron nitrides substituted with transition metals: DFT study of promising systems for anomalous Nernst effect

Iron nitrides substituted with transition metals: DFT study of promising systems for anomalous Nernst effect

Nodal-line symmetry breaking induced colossal anomalous Hall and Nernst effects in Cu2CoSn Heusler compound

The presence of topological band crossings near the Fermi energy is essential for the realization of large anomalous transport properties in the materials. The topological semimetals host such properties owing to their unique topological band structure, such as Weyl points or nodal lines (NLs), those are protected by certain symmetries of the crystal. When the NLs break out in the system, a large Berry curvature arises in the surrounding area of the gapped NL. In the present work, we studied anomalous transport properties of Cu2CoSn compound, which has a cubic Heusler crystal structure (space group: Fm3¯m). Cu2CoSn full Heusler compound possesses NLs at the Fermi level that is protected by mirror reflection symmetries of the lattice. Upon introducing the spin–orbit coupling in the Hamiltonian and by setting the magnetization axis along the [001] direction, we found that the NLs are gapped out and large Berry curvature arises in the system. The integral of Berry curvature gives the intrinsic anomalous Hall conductivity (AHC) about 1003 S/cm and the anomalous Nernst conductivity (ANC) about 3.98 A/m K at the Fermi level. These values of AHC and ANC are comparable to the largest reported values for the Co2MnGa Heusler compound. Therefore, Cu2CoSn becomes a newborn member of the family of full Heusler compounds, which possesses giant AHC and ANC that can be useful for the spintronics application.

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.

Roll-to-Roll Printing of Anomalous Nernst Thermopile for Direct Sensing of Perpendicular Heat Flux.

The anomalous Nernst effect (ANE) converts heat flux perpendicular to the plane into electricity, in sharp contrast with the Seebeck effect (SE), enabling mass production, large area, and flexibility of their devices through ordinary thin-film fabrication techniques. Heat flux sensors, one of the most promising applications of ANE, are powerful devices for evaluating heat flow and could lead to energy savings through efficient thermal management. In reality, however, SE caused by the in-plane heat flux is always superimposed on the measurement signal, making it difficult to evaluate the perpendicular heat flux. Here we fabricate ANE-type heat flux sensors that selectively detect the perpendicular heat flux by adjusting the net Seebeck coefficient in their thermopile circuit with mass-producible Roll-to-Roll sputtering methods. The direct sensing of perpendicular heat flux using ANE-based flexible thermopiles, as well as their simple fabrication process, paves the way for the practical application of thin-film thermoelectric devices. This article is protected by copyright. All rights reserved.

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.

Tuning the anomalous Nernst and Hall effects with shifting the chemical potential in Fe-doped and Ni-doped Co3Sn2S2

Co3Sn2S2 is believed to be a magnetic Weyl semimetal. It displays large anomalous Hall, Nernst and thermal Hall effects with a remarkably large anomalous Hall angle. Here, we present a comprehensive study of how substituting Co by Fe or Ni affects the electrical and thermoelectric transport. We find that doping alters the amplitude of the anomalous transverse coefficients. The maximum decrease in the amplitude of the low-temperature anomalous Hall conductivity is twofold. Comparing our results with theoretical calculations of the Berry spectrum assuming a rigid shift of the Fermi level, we find that given the modest shift in the position of the chemical potential induced by doping, the experimentally observed variation occurs five times faster than expected. Doping affects the amplitude and the sign of the anomalous Nernst coefficient. Despite these drastic changes, the amplitude of the ratio at the Curie temperature remains close to , in agreement with the scaling relationship observed across many topological magnets.

Intrinsic Berry curvature driven anomalous Nernst thermopower in the semimetallic Heusler alloy CoFeVSb

Understanding of spin-heat coupling mechanisms and magnetothermoelectric phenomena, including the anomalous Nernst effect (ANE), in emergent quaternary Heusler alloys is of practical importance for applications in thermal management and energy harvesting. Here, we demonstrate an intrinsic Berry curvature mediated anomalous Nernst thermopower in CoFeVSb, which orders magnetically at high temperature (${T}_{C}\ensuremath{\approx}850\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) with a large saturation magnetization of $\ensuremath{\approx}2.2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.$ at room temperature. We show that the electron-electron elastic and electron-magnon inelastic scattering dominate longitudinal electrical transport at low temperatures ($T\ensuremath{\le}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$), whereas the electron-phonon and electron-magnon scatterings govern it at higher $\phantom{\rule{0.28em}{0ex}}T$. The longitudinal thermopower is resulted mainly from the diffusive contribution with a very large longitudinal Seebeck coefficient ($42\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 395 K). The value of the anomalous Nernst coefficient (${S}_{\mathrm{ANE}}$) for CoFeVSb at room temperature is $0.039\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ which is higher than the compressively strained $\mathrm{SrRu}{\mathrm{O}}_{3}$ film ($0.03\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) as well as the spin gapless semiconductor CoFeCrGa ($0.018\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.28em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$). On lowering $T$, both the ordinary Nernst coefficient and carrier mobility increase but an opposite trend is found for ${S}_{\mathrm{ANE}}$. Our ab initio simulations reveal the topological semimetallic nature of CoFeVSb with a pair of Weyl points. These Weyl crossings result in a significant contribution to the Berry curvature, leading to an intrinsic anomalous Hall conductivity (${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$) of $\ensuremath{\approx}85$ S/cm, which matches well with experiment (77 S/cm at 2 K). Our experimental findings and ab initio calculations support the dominance of the intrinsic Berry curvature in the observed ANE. The ratio of ${\ensuremath{\sigma}}_{xy}^{\mathrm{AHE}}$ to the transverse anomalous thermoelectric conductivity (${\ensuremath{\alpha}}_{xy}^{\mathrm{ANE}}$) shows an increasing trend with $T$ attaining a sizable fraction of $\phantom{\rule{0.28em}{0ex}}\frac{{k}_{B}}{e}\phantom{\rule{0.28em}{0ex}}(\ensuremath{\approx}0.35\frac{{k}_{B}}{e})$ at room temperature.

Molecular beam epitaxy and crystal structure of majority a-plane-oriented and substrate-strained Mn3Sn thin films grown directly on sapphire (0001)

The Kagome antiferromagnet Mn3Sn has garnered much attention due to the presence of exciting properties such as anomalous Hall and Nernst effects. This paper discusses the synthesis of crystalline Mn3Sn thin films, prepared on Al2O3 (0001) substrates at 453±5°C using molecular beam epitaxy. The growth is monitored in situ using reflection high energy electron diffraction and measured ex situ using x-ray diffraction, Rutherford back-scattering, and cross-sectional scanning transmission electron microscopy. Our analysis shows the in-plane lattice constants of a1,M=4.117±0.027 Å and a2,M=4.943±0.033 Å, which is a very unexpected result when compared to the bulk a-plane Mn3Sn. This indicates a strain in the film and makes it challenging to provide a straightforward explanation. In an effort to explain our results, we discuss two possible orientation relationships between the Mn3Sn films and the sapphire substrates. Samples prepared under these conditions appear to have smooth surfaces locally, but overall the film has a 3D island morphology. First-principles calculations provide atomic models of the Mn3Sn (112¯0) lattice on Al2O3 (0001) high symmetry sites, indicating that the L3-R90° is the most stable configuration. A detailed discussion of the experimental data and theoretical results, as well as strain effects, is provided.

Berry curvature contributions of kagome-lattice fragments in amorphous Fe–Sn thin films

Amorphous semiconductors are widely applied to electronic and energy-conversion devices owing to their high performance and simple fabrication processes. The topological concept of the Berry curvature is generally ill-defined in amorphous solids, due to the absence of long-range crystalline order. Here, we demonstrate that the Berry curvature in the short-range crystalline order of kagome-lattice fragments effectively contributes to the anomalous electrical and magneto-thermoelectric properties in Fe–Sn amorphous films. The Fe–Sn films on glass substrates exhibit large anomalous Hall and Nernst effects comparable to those of the single crystals of topological semimetals Fe3Sn2 and Fe3Sn. With modelling, we reveal that the Berry curvature contribution in the amorphous state likely originates from randomly distributed kagome-lattice fragments. This microscopic interpretation sheds light on the topology of amorphous materials, which may lead to the realization of functional topological amorphous electronic devices.

Intrinsic and extrinsic anomalous transport properties of the Heusler ferromagnets Fe2CoAl and Fe2NiAl from first principles

Recently, Heusler ferromagnets have been found to exhibit unconventional anomalous electric, thermal, and thermoelectric transport properties. In this study, we employed first-principles density functional theory calculations to systematically investigate both intrinsic and extrinsic contributions to the anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and anomalous thermal Hall effect (ATHE) in two Heusler ferromagnets: Fe$_2$CoAl and Fe$_2$NiAl. Our analysis reveals that the extrinsic mechanism originating from disorder dominates the AHE and ATHE in Fe$_2$CoAl , primarily due to the steep band dispersions across the Fermi energy and corresponding high longitudinal electronic conductivity. Conversely, the intrinsic Berry phase mechanism, physically linked to nearly flat bands around the Fermi energy and gapped by spin-orbit interaction band crossings, governs the AHE and ATHE in Fe$_2$NiAl. With respect to ANE, both intrinsic and extrinsic mechanisms are competing in Fe$_2$CoAl as well as in Fe$_2$NiAl. Furthermore, Fe$_2$CoAl and Fe$_2$NiAl exhibit tunable and remarkably pronounced anomalous transport properties. For instance, the anomalous Nernst and anomalous thermal Hall conductivities in Fe$_2$NiAl attain giant values of 8.29 A/Km and 1.19 W/Km, respectively, at room temperature. To provide a useful comparison, we also thoroughly investigated the anomalous transport properties of Co$_2$MnGa. Our findings suggest that Heusler ferromagnets Fe$_2$CoAl and Fe$_2$NiAl are promising candidates for spintronics and spin-caloritronics applications.