Anomalous Nernst Effect Research Articles

Low-Cost Instrument for the Versatile Measurement of Spin Caloritronic Phenomena: Spin Seebeck Effect, Anisotropic Magnetoresistance, Anomalous Hall Effect, and Anomalous Nernst Effect

In this article, we report on a low-cost instrument for the versatile measurement of spin Caloritronics phenomena such as the spin Seebeck effect (SSE), anomalous Nernst effect (ANE) anisotropic magnetoresistance (AMR) and anomalous Hall effect (AHE). Solenoid coils provide a uniform variable magnetic field while the sample was sandwiched between thermal baths and measured in a vacuum chamber. Our results show excellent magnetic field uniformity (± 0.37 mT) within the magnet gap and high stability of the generated temperature difference (± 0.07 K). For verifying the effectiveness of our instrument, YIG/Co structure was used to measure the SSE, AMR, and AHE, while a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Co structure was used for measuring the ANE. Our SSE measurements of the YIG/Co structure were found to be comparable with that of a commercially available instrument. We can therefore conclude that our low-cost and versatile instrument can be used to effectively observe spin Caloritronics phenomena.

Magnon transport and thermoelectric effects in ultrathin Tm3Fe5O12 /Pt nonlocal devices

The possibility of electrically exciting and detecting magnon currents in magnetic insulators has opened exciting perspectives for transporting spin information in electronic devices. However, the role of the magnetic field and the nonlocal thermal gradients on the magnon transport remain unclear. Here, by performing nonlocal harmonic voltage measurements, we investigate magnon transport in perpendicularly magnetized ultrathin ${\mathrm{Tm}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (TmIG) films coupled to Pt electrodes. We show that the first harmonic nonlocal voltage captures spin-driven magnon transport in TmIG, as expected, and the second harmonic is dominated by thermoelectric voltages driven by current-induced thermal gradients at the detector. The magnon diffusion length in TmIG is found to be ${\ensuremath{\lambda}}_{\mathrm{m}}\ensuremath{\sim}0.3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 0.5 T and gradually decays to ${\ensuremath{\lambda}}_{\mathrm{m}}\ensuremath{\sim}0.2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 0.8 T, which we attribute to the suppression of the magnon relaxation time due to the increase of the Gilbert damping with field. By performing current-, magnetic field-, and distance-dependent nonlocal and local measurements we demonstrate that the second harmonic nonlocal voltage exhibits five thermoelectric contributions, which originate from the nonlocal spin Seebeck effect and the ordinary, planar, spin, and anomalous Nernst effects. Our work provides a guide on how to disentangle magnon signals from diverse thermoelectric voltages of spin and magnetic origin in nonlocal magnon devices, and establish the scaling laws of the thermoelectric voltages in metal/insulator bilayers.

Proximity induced longitudinal and transverse thermoelectric response in graphene-ferromagnetic CrBr3 vdW heterostructure

The integration of longitudinal and transverse thermoelectric (TE) fosters various new opportunities in tuning the charge transport behaviour and opens a platform for efficient thermopower devices. The presence of asymmetric electronic structure supposed to accomplish large thermopower and electronic figure of merit. Herein, we investigate magnetic proximity coupled longitudinal and transverse TE behaviour in heterostructure of monolayer semimetal, graphene and a monolayer ferromagnet, CrBr3 under the framework of ab initio-based calculations and employed constant relaxation time approximation (CRTA).The integrated density of states is elevated and asymmetric near Fermi energy region due to seamless proximity integration, depicting mixed character of graphene and CrBr3. The asymmetric nature of electronic structure significantly affects the Seebeck coefficients (S) and electrical conductivity (σ/τ) of heterostructure. The consistent step-like conductance spectrum influences interfacial polarization due to agile proximity integration. The magnitude of Seebeck coefficient (S) is found to be 653 µV K−1 near Fermi level. The heterostructure observes higher electrical conductivity and power factor in n-type region of the order of 106 S m−1 and 1020 cm−3 at room temperature. The dimensionless electronic figure of merit (zT e) advocates the heterostructure system to be an ideal TE material. Alongside longitudinal TE, we also find the heterostructure system is sensitive to anomalous Nernst effect (ANE) (transverse TE) with oscillatory nature. The Seebeck and ANE shows high degree of tunability with applied external electric field. The synergistic existence of Seebeck and ANE due to proximity integration in van der Waals atomic crystal at room temperature will provide realistic approach to experimentally fabricate and develop real-time thermopower devices.

Epitaxial growth of high quality Mn3Sn thin films by pulsed laser deposition

Noncollinear antiferromagnet Weyl semimetal Mn3Sn has recently attracted great research interest. Although large anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and magneto-optical effect have been observed in Mn3Sn, most studies are based on single crystals. So far, it is still challenging to grow high quality epitaxial Mn3Sn thin films with transport and optical properties comparable to their single crystal counterparts. Here, we report the structure and magneto-optical and transport properties of epitaxial Mn3Sn thin films fabricated by pulsed laser deposition (PLD). Highly oriented Mn3+xSn1−x (0001) and (112¯0) epitaxial films are growth on single crystalline Al2O3 and MgO substrates. Large anomalous Hall effect up to ΔρH=3.02 μΩ cm and longitudinal magneto-optical Kerr effect with |θK| = 38.1 mdeg at 633 nm wavelength are measured at 300 K, which are comparable to Mn3Sn single crystals. Our work demonstrates that high quality Mn3Sn epitaxial thin films can be fabricated by PLD, paving the way for future device applications.

Emergence of considerable thermoelectric effect due to the addition of an underlayer in Pt/Co/Pt stack and its application in detecting field free magnetization switching

Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of an adjacent ferromagnetic layer through spin–orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Generally, thin Ta is used as an underlayer to the HM layer for better adhesion and smoothness of the HM layer. Here, we show that Ta addition to the asymmetric stack Pt/Co/Pt gives rise to several compelling effects, viz., thermoelectric effects [particularly, anomalous Nernst effect (ANE)], and enhanced perpendicular magnetic anisotropy which was negligible in a Pt/Co/Pt stack. For this Ta/Pt/Co/Pt stack, the antidamping-SOT values are evaluated after carefully removing the contribution from the ANE and it is found to match the AD-SOT of the Pt/Co/Pt stack. We have observed current-induced field-free magnetization switching Ta/Pt/Co/Pt stack with Co thickness gradient. Furthermore, we have utilized the thermoelectric effects to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method.

Magneto-thermal transport indicating enhanced Nernst response in FeCo/IrMn exchange coupled stacks

We present an analysis of magneto-thermal transport data in IrMn/FeCo bilayers based on the Mott relation and report an enhancement of the Nernst response in the vicinity of the blocking temperature. We measure all four transport coefficients of the longitudinal resistivity, anomalous Hall resistivity, Seebeck effect, and anomalous Nernst effect, and we show a deviation arising around the blocking temperature between the measured Nernst coefficient and the one calculated using the Mott rule. We attribute this discrepancy to spin fluctuations at the antiferromagnet/ferromagnet interface near the blocking temperature. The latter is estimated by magnetometry and magneto-transport measurements.

Role of graphene doped Al[formula omitted]O[formula omitted] flexible sheets as the substrate for Anomalous Nernst Effect study

GnP doped Al2O3 flexible tapes were produced through Tape Casting technique and used as substrates to produce Co2FeAl/GnP doped Al2O3 heterostructures. The Co2FeAl layers were grown using Magnetron Sputtering with a thickness of 30 nm. The integration of Tape Casting and Magnetron sputtering techniques allowed us to explore the thermomagnetic properties of the heterostructure through the Anomalous Nernst Effect. The structural and morphological properties of the substrates and the magnetization curves support the thermomagnetic results. The Co2FeAl ferromagnetic films presented isotropic magnetic behavior, which was reflected in the thermomagnetic curves through the stability of the curve’s shape irrespectively of the thermal gradient and angle of the external magnetic field used during the measurements. Our findings open new possibilities to produce flexible heterostructures with multifunctional properties, where the versatilities of the Tape casting and magnetron sputtering techniques bring new routes to build nanostructured materials.

Tunable anomalous Hall and Nernst effects in MM′X compounds

Based on first-principles calculations, the anomalous Hall conductivity (AHC) and anomalous Nernst conductivities (ANCs) of the XMnP (X = Ti, Zr, Hf) compounds are evaluated, and the possibility to tailor such properties in compounds susceptible to changing the magnetization directions is also investigated. We observe large changes in the calculated AHC and ANC for different magnetization directions that are originating from changes in the band structure all over the whole Brillouin zone. Our study gives a promising clue on engineering magnetic intermetallic compounds for tunable transverse thermoelectric applications.

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.

In-plane magnetotransport phenomena in tilted Weyl semimetals

The nontrivial Berry curvature and chiral anomaly result in plenty of interesting magnetotransport and magneto-thermoelectric transport phenomena in Weyl semimetals, two of which are the planar Hall effect and planar Nernst effect. Based on the semi-classical Boltzmann theory, we theoretically study the in-plane magnetotransport coefficients (including electric conductivity, thermoelectric conductivity and Nernst thermopower) for both type-I and type-II Weyl semimetals using a linearized low-energy Hamiltonian. We find that for tilt-coplanar setup where the electric field (or temperature gradient) is applied along the tilted direction of the Weyl nodes, the planar Hall conductivity (or planar Nernst thermopower) shows a magnetic field linear dependence. And these linear responses do no obey the reported dependence on the angle θ between the magnetic and electric field for planar Hall effect. For tilt-perpendicular setup where the applied field (E or ) and magnetic field are perpendicular to the tilted direction, the planar Hall conductivity (or planar Nernst thermopower) mainly follows a magnetic field quadratic dependence, and it satisfies the characteristic. There are also anomalous Hall effect and anomalous Nernst effect in the plane perpendicular to the tilted direction of the two oppositely tilted Weyl nodes. These findings can be verified experimentally by changing the direction and magnitude of the in-plane magnetic field.

Correlated electronic structure of the kagome metal Mn3Sn

${\mathrm{Mn}}_{3}\mathrm{Sn}$ is a fascinating kagome metal exhibiting anomalous Hall effect, spin Hall effect, and anomalous Nernst effect. Using density functional theory plus dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$), we investigate the electronic structures of ${\mathrm{Mn}}_{3}\mathrm{Sn}$ in the paramagnetic and noncollinear antiferromagnetic (NAFM) states. In the paramagnetic state, ${\mathrm{Mn}}_{3}\mathrm{Sn}$ has intermediate electronic correlation strength and exhibits typical features of kagome metals including flat bands, Dirac points, and saddle points. In the NAFM state, $\mathrm{DFT}+\mathrm{DMFT}$ calculations reproduce well the experimental Mn magnetic moment and angle-resolved photoemission spectroscopy measurements. Electronic correlation shifts the bands around ${E}_{F}$ and moves the Weyl point between $K$ and $M$ points from $\ensuremath{\sim}40\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ above ${E}_{F}$ in DFT calculations to $\ensuremath{\sim}5\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ below ${E}_{F}$, which can strongly enhance the anomalous Hall effect and chiral anomaly observed in ${\mathrm{Mn}}_{3}\mathrm{Sn}$. More importantly, we find that the existence of the Weyl point along the $K\text{\ensuremath{-}}M$ path depends strongly on the ordering pattern of the Mn moments in the NAFM state, which implies that the exotic properties arising from the Weyl points in ${\mathrm{Mn}}_{3}\mathrm{Sn}$ can be manipulated by tuning the ordering pattern of the Mn moments in the NAFM state.

Theoretical and experimental study of anomalous Nernst effect in biphasic magnetic system

We perform a theoretical and experimental investigation of magnetic properties and anomalous Nernst effect in a NiFe/Cr/NiFe trilayer. Our system presents a biphasic magnetic behavior, a response owed to distinct structural properties of the bottom and top magnetic layers. We verify the thermomagnetic results strongly depend on the anisotropies and magnetic response of the biphasic system. Moreover, the thermomagnetic results seem to be much more sensitive to the anisotropies than the magnetometry ones. We show the anomalous Nernst coefficient found in our trilayer is quite comparable with those verified for high-efficient thermomagnetic systems. We also disclose an enhancement if compared with the ones observed for single layers. Beyond, our results highlight the role of each ferromagnetic layer on the thermomagnetic response. Our experimental and theoretical findings show that the top and bottom layers contribute differently to the thermomagnetic signal due to the Cr spacer in the trilayer system.

Emergence of asymmetric skew-scattering dominated anomalous Nernst effect in the spin gapless semiconductors Co1+xFe1−xCrGa

Heusler alloy-based spin gapless semiconductors (SGSs) with very high Curie temperatures $({T}_{C})$ have recently gained enormous attention because of their unconventional electronic structures. They exhibit a nonzero band gap in one of the spin channels and a zero band gap in the other spin channel, making them an important class of materials for tunable spin transport. Here, we report the experimental observation of anomalous Nernst effect (ANE) in ${\mathrm{Co}}_{1+x}{\mathrm{Fe}}_{1\ensuremath{-}x}\mathrm{CrGa}$ ($x=0$, 0.2, 0.4, and 0.5), which are the emerging quaternary Heusler alloy-based SGSs. While the electron-electron elastic scattering and the disorder-mediated weak localization effect play the major roles in electrical transport for all the samples at low temperatures, the magnon-drag effect was found to dominate the longitudinal thermoelectric transport. The ANE coefficient at room temperature increases from $\ensuremath{\approx}0.018\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $x=0$ to $\ensuremath{\approx}0.063\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for $x=0.5$, which is higher than that for ${\mathrm{Ni}}_{81}\phantom{\rule{0.16em}{0ex}}{\mathrm{Fe}}_{19}$ and compressively strained $\mathrm{SrRu}{\mathrm{O}}_{3}$ films. Our analysis indicates that the observed ANE in these samples originates from asymmetric skew scattering of charge carriers.

Enhancement of the anomalous Nernst effect in epitaxial Fe4N films grown on SrTiO3(001) substrates with oxygen deficient layers

Anomalous Nernst effect of epitaxial Fe4N films on MgO(001), MgAl2O4(MAO)(001), and SrTiO3(STO)(001) substrates grown by molecular beam epitaxy was investigated. Moderately large anomalous Nernst coefficients (SANE) of 1.4 and 1.7 μV/K were obtained in the Fe4N films on the MgO(001) and MAO(001) substrates, respectively, and large anomalous Hall angles (∼0.06) and transverse thermoelectric conductivities [∼1.3 A/(m K)] were derived from the experimental results. On the other hand, a large effective SANE of 2.8 μV/K was obtained in the Fe4N film on the STO(001) substrate. The origin of the enhanced effective SANE is the negatively large Seebeck coefficient (SSE) in an oxygen deficient STO layer near the surface of the STO substrate. This indicates that it is possible to enhance the effective SANE of ferromagnetic materials by utilizing adjacent materials with large |SSE| such as the oxygen deficient STO layer.

Giant and Robust Anomalous Nernst Effect in a Polycrystalline Topological Ferromagnet at Room Temperature

AbstractRecent discoveries of the topological magnets have opened a new path for developing a much simpler thermoelectric conversion module using the anomalous Nernst effect (ANE). To accelerate such innovation, it is essential to design materials suitable for industrial processes, and thus a high‐ANE polycrystalline material has been highly desired. Recently, the giant room‐temperature ANE has been reported in single crystals of the topological ferromagnet Fe3Ga. Owning to its cubic structure, the anomalous Hall effect and ANE are isotropic. These properties potentially allow to employ a polycrystalline form of the material to design an ANE‐based thermopile. Here, a giant and robust room‐temperature ANE in the polycrystalline FexGa4−x (2.96 &lt; x &lt; 3.15) is reported, which can be enhanced up to 5.4 µV K−1; this value hits the highest room‐temperature record for polycrystalline magnets. Comparison of the experimental results with the theoretical study of the Fe‐doping effect on the transport properties of FexGa4−x reveals that the Fermi energy tuning near the topological nodal‐web structure is the key to enhancing the ANE. Moreover, the large value of more than 5.1 µV K−1 is observed for an extended region of composition, confirming the robust characteristics of the topological electronic structure.

Thermoelectrics: From longitudinal to transverse

In this article, we show fundamentals and recent advances on the transverse thermoelectric generation, in which a thermopower is generated in the direction perpendicular to an applied temperature gradient. Transverse thermoelectric generation is one of the central topics in condensed matter physics, and can be a breakthrough approach to solve long-standing technological problems with contact resistances in thermoelectric generators. We review here the six currently known driving mechanisms: the ordinary Nernst effect, the anomalous Nernst effect, goniopolar materials, the spin Seebeck effect, Seebeck-driven transverse thermoelectric generation, and ($p \times n$)-type multilayers. This article summarizes the principles and functionalities of these transverse thermoelectric effects and discusses their potential as "Future energy".

Anomalous Nernst effect in the nonmagnetic nodal-line semimetal PbTaSe2

PbTaSe$_2$ is a unique topological material, in which the number of nodal lines is expected to change at the structural transition between the lower temperature/pressure "L" phase and the higher temperature/pressure "H" phase. We report the anomalous Nernst effect attributed to the Berry curvature of nodal lines and its change with the structural transition. In the L phase, the Nernst coefficient ($S_{yx}$) shows the step-like magnetic field dependence reminiscent of the anomalous Nernst effect of nonmagnetic Dirac/Weyl semimetals. By applying hydrostatic pressure, we discovered that the amplitude of the anomalous component significantly decreases at the transition to the H phase, which might correspond to the partial annihilation of nodal line structures.

High-Power-Density Energy-Harvesting Devices Based on the Anomalous Nernst Effect of Co/Pt Magnetic Multilayers.

The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here, we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is 3 orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a system characterized by a high ANE thermopower (∼1 μV/K), low electrical resistivity, and perpendicular magnetic anisotropy. These innovations allow us to obtain power densities of around 13 ± 2 W/cm3. We believe that this design may find uses in harvesting wasted energy, e.g., in electronic devices.

Anomalous Nernst thermopile made of a single element iron

Anomalous Nernst thermopiles that utilize spin–orbit coupling in magnetic materials are beneficial for energy harvesting. Under an out-of-plane temperature gradient, the thermopiles can generate sizable and scalable thermoelectric voltages due to the anomalous Nernst effect (ANE). In this work, we show that a thermopile made of only one element, Iron (Fe), can generate a significant spin-dependent thermoelectric signal. We find that by varying the thickness of Fe, the anomalous Nernst signal changes sign and becomes exceptionally large in the thinner-thickness region. As a result, the anomalous Nernst thermopile consisting of only Fe exhibits a sizable thermoelectric signal. Moreover, the plateau behavior in the thermopile owing to the sequential flipping of magnetic moments demonstrates that the ANE could serve as a sensitive tool to detect magnetic switching. The multi-level ANE signals further indicate the anomalous Nernst thermopiles can be utilized as the ANE valve for logic gates. These results provide an essential guide for energy harvesting and spin-dependent electrothermal sensor using magnetic materials.

Proximity-magnetized quantum spin Hall insulator: monolayer 1 T’ WTe2/Cr2Ge2Te6

Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe2, a two-dimensional quantum spin Hall insulator, and few-layer Cr2Ge2Te6, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe2 manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.