Anomalous Nernst Effect Research Articles

Thermally Driven Spin Transport of Epitaxial FeRh Films with a Non-magnetic Pt Layer via the Longitudinal Spin Seebeck Effect.

FeRh has been demonstrated to be an important material for the observation of magnetic phase transitions, such as the first-order transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state, in response to changes in the temperature. This is because of the magnetic moment induced in Rh atoms above the magnetic phase transition temperature. In the present study, we focus on the longitudinal spin Seebeck effect (LSSE), which involves the generation of spin voltage as a result of a temperature gradient in FM materials or FM insulators, and experimentally assess the effect of the crystalline quality of FeRh films and the properties of the substrate on the LSSE thermopower during the FM-AFM phase transition. The measured LSSE thermopower of an epitaxial (110)-oriented FeRh film grown on an Al2O3 substrate is approximately 60 times higher than that of a polycrystalline FeRh film on a SiO2/Si substrate. This can be explained by the high magnetic sensitivity and superior FM properties of (110)-oriented epitaxial FeRh films. Furthermore, by comparing the transverse thermoelectric voltage for in-plane magnetized (IM) and perpendicularly magnetized (PM) configurations, we quantitively evaluate the contribution of the exclusive anomalous Nernst effect (ANE) to the LSSE signals in the FeRh/Al2O3 structure, finding it to be approximately 15-30% over a temperature range of 75-300 K. LSSE measurements in Pt/FeRh films are thus demonstrated to provide a versatile pathway for the development of thermoelectric power generation applications and other practical spintronics and neuromorphic computing devices.

Abnormal Magnetic Phase Transition in Mixed-Phase (110)-Oriented FeRh Films on Al2O3 Substrates via the Anomalous Nernst Effect.

Iron rhodium (FeRh) undergoes a first-orderanti-ferromagnetic to ferromagnetic phase transition above its Curie temperature. By measuring the anomalous Nernst effect (ANE) in (110)-oriented FeRh films on Al2O3 substrates, the ANE thermopower over a temperature range of 100-350 K is observed, with similar magnetic transport behaviors observed for in-plane magnetization (IM) and out-of-plane magnetization (PM) configurations. The temperature-dependent magnetization-magnetic field strength (M-H) curves revealed that the ANE voltage is proportional to the magnetization of the material, but additional features magnetic textures not shown in the M-H curves remained intractable. In particular, a sign reversal occurred for the ANE thermopower signal near zero field in the mixed-magnetic-phase films at low temperatures, which is attributed to the diamagnetic properties of the Al2O3 substrate. Finite element method simulations associated with the Heisenberg spin model and Landau-Lifshitz-Gilbert equation strongly supported the abnormal heat transport behavior from the Al2O3 substrate during the experimentally observed magnetic phase transition for the IM and PM configurations. The results demonstrate that FeRh films on an Al2O3 substrate exhibit unusual behavior compared to other ferromagnetic materials, indicating their potential for use in novel applications associated with practical spintronics device design, neuromorphic computing, and magnetic memory.

Enhancement of Transverse Thermoelectric Conversion by Interface‐Induced Spin Current in Ferromagnetic Metal/Nonmagnetic Insulator Hybrid‐Structure

AbstractTransverse thermoelectric conversion phenomena including the anomalous Ettingshausen effect (AEE) and anomalous Nernst effect (ANE) in magnetic materials are actively investigated to realize versatile cooling and energy harvesting technologies. However, further improvement of the thermoelectric performance of AEE and ANE is still required, and most research efforts have focused on material exploration. Here, a new approach to improve the transverse thermoelectric conversion performance through interface engineering is reported by focusing on the transverse thermoelectric phenomena that output heat currents. A longitudinal charge current in a ferromagnetic metal Ni/nonmagnetic insulator Bi2WO6 hybrid‐structure induces a larger transverse heat current than that of AEE in a Ni single‐layer. It is indicated that the enhancement of the transverse thermoelectric conversion is due to the generation of a heat current concomitant with a spin current induced at the Ni/Bi2WO6 interface. This finding demonstrates that the interface of a magnetic metal/nonmagnetic insulator has a potential to improve the transverse thermoelectric performance, extending the applicability of nonmagnetic insulators to the field of spin caloritronics and thermoelectrics.

Light-induced anomalous Hall, Nernst, and thermal Hall effects in black phosphorus thin films

Light-induced anomalous Hall, Nernst, and thermal Hall effects in black phosphorus thin films

Anomalous Nernst effect in the noncollinear antiferromagnet Mn5Si3

Investigating the off-diagonal components of the conductivity and thermoelectric tensor of materials hosting complex antiferromagnetic structures has become a viable method to reveal the effects of topology and chirality on the electronic transport in these systems. In this respect, Mn5Si3 is an interesting metallic compound that exhibits several antiferromagnetic phases below 100 K with different collinear and noncollinear arrangements of Mn magnetic moments determined from neutron scattering. Previous electronic transport measurements have shown that the transitions between the various phases give rise to large changes of the anomalous Hall effect. Here, we report measurements of the anomalous Nernst effect of Mn5Si3 single crystals that also show clear transitions between the different magnetic phases. In the noncollinear phase, we observe an unusual sign change of the zero-field Nernst signal with a concomitant decrease of the Hall signal and a gradual reduction of the remanent magnetization. Furthermore, a symmetry analysis of the proposed magnetic structures shows that both effects should actually vanish. These results indicate a symmetry-breaking modification of the magnetic state with a rearrangement of the magnetic moments at low temperatures, thus questioning the previously reported models for the noncollinear magnetic structure obtained from neutron scattering.

Optical control of berry curvature in gated WSe2 bilayers

Abstract Unlike single layers of 2H transition metal dichalcogenides (TMDCs), bilayers of 2H TMDCs maintain inversion and time reversal (TR) symmetries, resulting in a vanishing Berry curvature ( Ω ( k ) ∼ 0 ) that inhibits various potential transport phenomena. A nonzero Berry curvature ( Ω ( k ) ≠ 0 ) is imperative for the occurrence of several unconventional transport phenomena, including the anomalous Hall effect and the anomalous Nernst effect. To overcome this limitation, we break these symmetries in bilayer TMDCs using electrostatic gating and circularly polarized light as external means. For non-gated WSe2 bilayers, circularly polarized light breaks TR symmetry, creating a finite Berry curvature signal in both conduction and valence bands, controllable by light intensity and its polarity. In gated WSe2 bilayers, where inversion symmetry is also broken, we observe a sign reversal in Berry curvature within the conduction bands, the extent of which depends on the relative strengths of the electric gating and light intensity. Overall, under finite bias and light intensity, the 2H bilayers of WSe2 exhibits finite spin Hall, valley Hall, and anomalous Hall conductivities, which depend on the strengths of the applied perturbations.

Enhancing thermoelectric effect with BaTiO3-doped ZrO2 tapes and ferromagnetic nanostructures

The Anomalous Nernst Effect (ANE) emerges as a promising candidate for converting heat into electricity through magnetic nanostructures. However, the substrate plays a central role in optimizing this phenomenon for future technological applications. Tapes of ZrO2 and BaTiO3-doped ZrO2, manufactured by tape casting, possess characteristic flexibility in the green state, which makes them suitable for use as coverings on virtually any curved surface. Moreover, BaTiO3 ceramic materials present piezoelectric properties, which, when combined with thermoelectric properties, can generate interesting heterostructures with simultaneous thermal and vibrational power conversion capabilities. This study explores the combination of tape casting and magnetron sputtering to improve the functionalization of ceramic tapes with ferromagnetic nanostructures. Specifically, it investigates the quasi-static and thermoelectric properties of heterostructures created by depositing NiFe thin films onto sintered ZrO2 and BaTiO3-doped ZrO2 tapes using magnetron sputtering. The results indicate a 71 % increase in the effective thermoelectric coefficient as the amount of BaTiO3 in the ceramic substrate increases. These findings suggest a new approach to integrating these techniques for the production of multifunctional materials for thermoelectric energy conversion.

Bipolar transverse thermopower and low thermal conductivity for an anomalous Nernst-type heat flux sensor in GdCo alloys

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

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

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

Large anomalous Nernst conductivity of L10-ordered CoPt in CoPt composition-spread thin films

We demonstrate a high-throughput experimental characterization of anomalous Nernst conductivity ( αxyA ) of L10-ordered CoPt using Co1–x Pt x composition-spread thin films on MgO(100) substrates. The compositional dependence of the anomalous Nernst effect, anomalous Hall effect (AHE) and Seebeck effect is systematically measured. As increasing the Pt concentration, the crystal structure in the composition-spread film grown at 500 °C changes from face-centered cubic (fcc) Co, A1-disordered CoPt, L10-ordered CoPt, A1-CoPt to fcc Pt. The largest αxyA of 2.52 A m–1 K–1 is obtained in L10-CoPt for Pt-rich composition of x = 70%, which is larger than that for an additionally fabricated nearly stoichiometric L10-Co48Pt52 reference uniform film. The contribution from direct conversion of a temperature gradient to a transverse charge current through αxyA is dominant to the total anomalous Nernst coefficient compared to the AHE-related contribution. From a scaling analysis of the AHE, the intrinsic contribution is found to be dominant for x = 70%. A theoretical calculation for αxyA of L10-Co50Pt50 agrees with the experimental αxyA value for the nearly stoichiometric reference film by considering on-site Coulomb interaction for Co atoms. We also point out the possible electron doping effect by the addition of Pt in L10-CoPt, which could explain the larger αxyA for the off-stoichiometric Pt-rich composition than that for the nearly stoichiometric one. Our experimental and theoretical results suggest the potential of L10-CoPt with a large αxyA originating from the intrinsic mechanism for future thermoelectric applications.

Giant Room-Temperature Topological Hall Effect in a Square-Net Ferromagnet LaMn2Ge2.

A topological magnetic material showcases a multitude of intriguing properties resulting from the compelling interplay between topology and magnetism. These include notable phenomena such as a large anomalous Nernst effect (ANE), an anomalous Hall effect (AHE), and a topological Hall effect (THE). In most cases, topological transport phenomena are prevalent at temperatures considerably lower than room temperature, presenting a challenge for practical applications. However, the noncollinear ferromagnetic (FM) LaMn2Ge2, characterized by a Mn square-net lattice and a notably high Curie temperature (TC) of approximately 325 K, defies this trend as a topological semimetal. This work observes a giant topological Hall resistivity, , of ≈4.5 µΩ cm at room temperature when the angle between the applied field and the c-axis is 75°, which is significantly higher than state-of-the-art materials with noncoplanar spin structures. The single crystal neutron diffraction measurements agree with an incommensurate conical magnetic structure as the ground state. This observation suggests the enhanced spin chirality resulting from the noncoplanar spin configuration when the applied field is away from the magnetic easy axis as the origin of a large contribution to the observed THE. The findings unequivocally demonstrate that the FM LaMn2Ge2 holds great promise as a potential topological semimetal for spintronic applications even at room temperature.

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.

Prediction of giant anomalous Nernst effect in Sm(Co,Ni)5

Sm-Co bulk alloys are well-known permanent magnets having large remanent magnetizations and coercive forces and are widely used in many industrial products. Recently, a large transverse thermoelectric conversion was observed for SmCo5 over a wide temperature range in the absence of magnetic fields. The large thermoelectric conductivity tensors (αxy) was also confirmed by the first-principles density functional theory (DFT) calculations. In this study, we predicted further enhancement of the αxy by including Ni in Co site of SmCo5. We showed that the αxy of Sm(Co1−xNix)5 increases with increasing the Ni ratio and takes the maximum value αxy = 11.3 A K−1 m−1 around x = 0.08 at 300 K, which is about 77% enhancement of αxy = 6.4 A K−1 m−1 in SmCo5. We clarified that the band proximity points near the nodal line of Sm(Co0.92Ni0.08)5 are the main contributing factor to the large Berry curvature, providing the steep slope of the energy dependence in the anomalous Hall conductivity around the Fermi energy.

Constructing anisotropic bulk Ni/Pt nanocomposites to enhance transverse thermoelectric efficiency

Anisotropic bulk Ni/Pt nanocomposites were constructed by aligning Ni/Pt nanowires, combining the advantages of nanotechnology, spin Seebeck effect (SSE), and anomalous Nernst effect (ANE) to enhance thermoelectric performance. The thermoelectric power can be improved by increasing the ANE and SSE coefficient, enhancing the thermal resistance in the direction of heat flow, and improving the utilization efficiency of the Ni/Pt interface. The results demonstrated that the ANE and SSE volage (VANE+SSE) of Ni/Pt nanocomposites are larger than the ANE volage (VANE) in bulk Ni nanocomposite. When the external magnetic field is along the nanowire, the VANE+SSE is increased by a factor of 1.52 compared to its perpendicular orientation. This work provides a new idea to enhance SSE and ANE voltage by constructing anisotropic bulk nanocomposites, providing an opportunity for their application in thermoelectric devices.

Extraordinary Thermoelectric Properties of Topological Surface States in Quantum-Confined Cd3As2 Thin Films.

Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd3As2 thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd3As2 layer is thin. In particular, a peak Seebeck coefficient of nearly 500µVK-1 and a corresponding thermoelectric power factor over 30mWK-2m-1 are observed at 5K in a 25-nm-thick sample. Combining angle-dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first-principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum-confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenictemperatures.

Large transverse thermoelectric effect induced by the mixed-dimensionality of Fermi surfaces

Transverse thermoelectric effect, the conversion of longitudinal heat current into transverse electric current, or vice versa, offers a promising energy harvesting technology. Materials with axis-dependent conduction polarity, known as p × n-type conductors or goniopolar materials, are potential candidate, because the non-zero transverse elements of thermopower tensor appear under rotational operation, though the availability is highly limited. Here, we report that a ternary metal LaPt2B with unique crystal structure exhibits axis-dependent thermopower polarity, which is driven by mixed-dimensional Fermi surfaces consisting of quasi-one-dimensional hole sheet with out-of-plane velocity and quasi-two-dimensional electron sheets with in-plane velocity. The ideal mixed-dimensional conductor LaPt2B exhibits an extremely large transverse Peltier conductivity up to ∣αyx∣ = 130 A K−1 m−1, and its transverse thermoelectric performance surpasses those of topological magnets utilizing the anomalous Nernst effect. These results thus manifest the mixed-dimensionality as a key property for efficient transverse thermoelectric conversion.

Large thermoelectric transport in magnetically coupled CrI3/1T-MoS2 vdW heterostructure via spin–charge interconversion

Low-dimensional materials with prominent thermoelectric (TE) effect play a pivotal role in realizing state-of-the-art nanoscale TE devices. The fusion of TE effect with the magnetism through seamless integration of TE and magnetic materials in the 2D limit offers access to control longitudinal as well as transverse TE properties via magnetic proximity effect. Herein, we design a van der Waals (vdW) heterostructure of metallic 1T-MoS2 with promising TE properties and a layer-dependent magnetic CrI3 material. The result highlights exotic electronic and magnetic configurations of the designed monolayer-CrI3/1T-MoS2 vdW heterostructure, which show magnetically-coupled TE characteristics. The observed remarkable magnetic proximity stems from large magnetic anisotropy energy and spin polarization, which are found to be 2.21 meV Cr−1 and 12.30%, respectively. To this end, the semiconducting CrI3 layer with intrinsic magnetism leads to efficient control and tunability of the observed spin-correlated anomalous Nernst effect. Moreover, a large dimensionless figure of merit of ∼6 and a power factor of ∼3.8×1011/τ∘ Wm−1K−2s−1 near the Fermi level at 300 K endorse the rejuvenated TE effect. The strong relativistic spin–orbit coupling validates the significant correlation of TE properties with intrinsic magnetic configuration. The present study underscores the significance of the magnetic proximity-governed TE effect in vdW heterostructures to engineer low-dimensional TE devices.

Spin Seebeck effect and large spin conversion in amorphous Fe2TiSb/polycrystalline Y3Fe5O12 thin films

This study investigates spin current generation in a Fe2TiSb/Y3Fe5O12 multi-layer thin film as prepared via the magnetron sputtering method. Comprehensive characterization techniques are employed to assess film properties, including X-ray diffraction, energy-dispersive X-ray spectroscopy, Scanning electron microscopy, and Vibrating sample magnetometer. The Y3Fe5O12 material exhibits a polycrystalline ferromagnetic insulator behavior, while the 20 nm-thick Fe2TiSb film displays small ferromagnetic metal properties with an amorphous structure. Spin current analysis utilizes the longitudinal spin Seebeck effect configuration, considering magnetic field and temperature dependencies and the results show that spin conversion within the Fe2TiSb/Y3Fe5O12 structure is influenced by both the spin Seebeck effect and the anomalous Nernst effect, resulting in an overall spin signal enhancement. The spin Seebeck coefficient of Fe2TiSb/Y3Fe5O12 was approximately 0.103 μV/K within a magnetic field of 300 mT.

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.

Measurement setup for Nernst and Seebeck effect at high temperatures and magnetic fields tested on elemental bismuth and full-Heusler compounds.

In this work, a measurement setup to study the Seebeck and Nernst effect at high temperatures and high magnetic fields is introduced and discussed. The measurement system allows for simultaneous measurements of both thermoelectric effects up to 700K and magnetic fields up to 12T. Based on theoretical concepts, measurement equations are derived that counteract constant spurious offset voltages and, therefore, inhibit systematic errors in the measurement setup. The functionality is demonstrated on polycrystalline samples of elemental bismuth as well as various full-Heusler materials, exhibiting an anomalous Nernst effect. In all samples, the measured Seebeck and Nernst coefficients align excellently with the reported values. This allows future research to substantially extend the measured temperature and field intervals, commonly limited to temperatures below room temperature. For the first time, the thermoelectric and thermomagnetic properties of these materials are reported up to temperatures of 560K.