Anomalous Ettingshausen Effect Research Articles

Infrared imaging of magnetic octupole domains in non-collinear antiferromagnets

ABSTRACT Magnetic structure plays a pivotal role in the functionality of antiferromagnets (AFMs), which not only can be employed to encode digital data but also yields novel phenomena. Despite its growing significance, visualizing the antiferromagnetic domain structure remains a challenge, particularly for non-collinear AFMs. Currently, the observation of magnetic domains in non-collinear antiferromagnetic materials is feasible only in Mn3Sn, underscoring the limitations of existing techniques that necessitate distinct methods for in-plane and out-of-plane magnetic domain imaging. In this study, we present a versatile method for imaging the antiferromagnetic domain structure in a series of non-collinear antiferromagnetic materials by utilizing the anomalous Ettingshausen effect (AEE), which resolves both the magnetic octupole moments parallel and perpendicular to the sample surface. Temperature modulation due to AEE originating from different magnetic domains is measured by lock-in thermography, revealing distinct behaviors of octupole domains in different antiferromagnets. This work delivers an efficient technique for the visualization of magnetic domains in non-collinear AFMs, which enables comprehensive study of the magnetization process at the microscopic level and paves the way for potential advancements in 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.

Anomalous Ettingshausen effect in iron–carbon alloys

We have investigated the anomalous Ettingshausen effect (AEE) in iron–carbon alloys, i.e., cast irons and steel, using the lock-in thermography. All the alloys exhibit the clear AEE-induced temperature modulation, and their anomalous Ettingshausen coefficient is an order of magnitude greater than that of the pure iron at room temperature. The dimensionless figure of merit for AEE in the ductile cast iron is 55 times greater than that in the pure iron owing to the significant increase in the anomalous Ettingshausen coefficient. Our result reveals a potential of iron–carbon alloys as transverse thermoelectric materials, although the composition and microstructures optimizations are necessary.

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

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

Seebeck effect in nanomagnets

We present a theory of the Seebeck effect in nanomagnets with dimensions smaller than the spin diffusion length, showing that the spin accumulation generated by a temperature gradient strongly affects the thermopower. We also identify a correction arising from the transverse temperature gradient induced by the anomalous Ettingshausen effect and an induced spin-heat accumulation gradient. The relevance of these effects for nanoscale magnets is illustrated by ab initio calculations on dilute magnetic alloys.

Lock-in thermoreflectance as a tool for investigating spin caloritronics

We report lock-in thermoreflectance (LITR) measurements of spin-caloritronic phenomena in magnetic hybrid structures. In the LITR measurements, the temperature modulation signals due to the spin Peltier effect (SPE) and anomalous Ettingshausen effect (AEE) were detected through the temperature dependence of the optical reflectivity of the sample surface. By using the lock-in technique, the reflectivity modulation in response to an alternating charge current applied to the samples can be detected sensitively, which allows us to clarify the transient responses of the SPE and AEE and gives a clue to separate these phenomena. We applied this method to a junction system comprising a ferromagnetic metal film formed on a magnetic insulator substrate and found that the transient response unique to the SPE can be observed even when the SPE and AEE coexist. We also checked that the lock-in signals of the reflected light intensity exhibit no light polarization rotation; the observed temperature modulation is free from parasitic magneto-optical effects. The SPE and AEE can thus be measured not only by the conventional laser-based LITR but also by light-emitting-diode-based LITR, which enables versatile and cost-effective measurements.

Possible Nonlinear Anomalous Thermoelectric Effect in Organic Massive Dirac Fermion System

We propose a novel current-induced thermoelectric phenomenon, the nonlinear anomalous Ettingshausen effect (AEE), at zero magnetic field in inversion-asymmetric conductors. As an example, we discuss the weak charge ordering state in a layered organic conductor alpha-(BEDT-TTF)2I3, which is a two-dimensional massive Dirac fermion system with a pair of tilted Dirac cones. The nonlinear AEE is a thermoelectric analogue of the nonlinear anomalous Hall effect, which is recently observed in alpha-(BEDT-TTF)2I3, and these two effects generally appear simultaneously. The nonlinear AEE generates a transverse heat current, which exhibits rectifying characteristics, namely unidirectionality even under an AC current.

Combinatorial tuning of electronic structure and thermoelectric properties in Co2MnAl1−xSix Weyl semimetals

Tuning of the Fermi level (EF) near Weyl points is one of the promising approaches to realize the large anomalous Nernst effect (ANE). In this work, we introduce an efficient approach to tune EF for the Co2MnAl Weyl semimetal through a layer-by-layer combinatorial deposition of the Co2MnAl1−xSix (CMAS) thin film. A single-crystalline composition-spread film with x varied from 0 to 1 was fabricated. The structural characterization reveals the formation of a single-phase CMAS alloy throughout the composition range with a gradual improvement of L21 order with x similar to the co-sputtered single layered film, which validates the present fabrication technique. Hard x-ray photoemission spectroscopy for the CMAS composition-spread film directly confirmed the rigid band-like EF shift of ∼0.40 eV toward the composition gradient direction from x = 0 to 1. The anomalous Ettingshausen effect (AEE), the reciprocal of the ANE, has been measured for the whole x range using a single strip along the composition gradient using the lock-in thermography technique. The similarity of the x dependence of observed AEE and ANE signals clearly demonstrates that AEE measurement on the composition-spread film is an effective approach to investigate the composition dependence of the ANE of Weyl semimetal thin films and realize the highest performance without fabricating several films, which will accelerate the research on ANE-based energy harvesting.

A Novel Method to Observe Topological Nature Using Nonlinear Thermoelectricity

Researchers at the Institute for Solid State Physics, University of Tokyo, recently proposed a current-induced thermoelectric phenomenon called the “nonlinear anomalous Ettingshausen effect” in organic massive Dirac electron systems. This phenomenon allowed for a novel method of observing the topological nature of Dirac electron systems by inducing the time-reversal-symmetry breaking using an electric current, which can then be detected via simple thermoelectric measurements.

Nonlinear Thermoelectric Transport Will Open a New and Exotic Way to Detect Topological Nature of Dirac Electrons in Solids

Recently, it was proposed that a novel nonlinear anomalous Ettingshausen effect enabled a new and exotic approach to observe topological nature of Dirac electrons in solids using thermoelectric properties.

How a Potentially Novel Effect Converts AC Current into DC Heat Flow

A novel current-induced thermoelectric phenomenon, the nonlinear anomalous Ettingshausen effect, was discussed. As an example, the weak charge ordering state in an organic conductor α-(BEDT-TTF)2I3 was focused on. This effect generates a transverse heat current with rectifying characteristics, namely unidirectionality even under AC fields.

High-temperature dependence of anomalous Ettingshausen effect in SmCo5-type permanent magnets

The anomalous Ettingshausen effect (AEE) in SmCo5-type permanent magnets has been investigated in the high-temperature range from room temperature to around 600 K. The anomalous Ettingshausen coefficient of the SmCo5 and (SmGd)Co5 magnets monotonically increases with increasing the temperature and shows a similar temperature dependence, while the coefficient of SmCo5 is slightly larger than that of (SmGd)Co5 at high temperatures. The dimensionless figure of merit for the AEE in SmCo5 at high temperatures is much greater than the previous record obtained for the anomalous Nernst effect. The observed high-temperature behavior of the AEE is discussed based on the first-principles calculations of transverse transport coefficients.

Spin-mediated charge-to-heat current conversion phenomena in ferromagnetic binary alloys

Spin-mediated charge-to-heat current conversion phenomena, i.e., the anomalous Ettingshausen effect (AEE) and the anisotropic magneto-Peltier effect (AMPE), have been investigated in various ferromagnetic Ni-Fe, Ni-Pt, Ni-Pd, and Fe-Pt binary alloys at room temperature. When a charge current is applied to a ferromagnetic conductor, the AMPE modulates the Peltier coefficient depending on the angle between the directions of the charge current and magnetization, while the AEE generates a heat current in the direction perpendicular to both the charge current and magnetization. We observed the strong material dependence of the thermoelectric conversion coefficients and figures of merit of these phenomena. Among the ferromagnetic alloys used in this study, Ni$_{95}$Pt$_{5}$ exhibits the largest AMPE of which the anisotropy of the Peltier coefficient is $\sim 12\%$. In contrast, the magnitude of the AEE signals is moderate in Ni$_{95}$Pt$_{5}$ but largest in Ni$_{75}$Pt$_{25}$ and Ni$_{50}$Fe$_{50}$. We discuss these behaviors by exploring the relations between these charge-to-heat current conversion phenomena and other transport as well as magnetic properties. This systematic study will provide a clue for clarifying the mechanisms of the AMPE and AEE and for enhancing the thermoelectric conversion efficiency of these phenomena.

Enhancement of temperature change induced by anomalous Ettingshausen effect in thin Ni films on suspended membrane substrates

We demonstrate the effect of substrates on the temperature change induced by the anomalous Ettingshausen effect in ferromagnetic thin films. The experimental results show that, in comparison with conventional experiments using bulk substrates, the temperature modulation due to the anomalous Ettingshausen effect in Ni films can be enhanced by more than an order of magnitude using suspended membrane substrates, which reduce thermal dissipation from the films to the substrates. The temperature modulation can be further enhanced with the help of a simple suspended thermopile configuration consisting of two Ni strips separated by a small distance. These demonstrations will provide a guideline for the thermal design of thin-film-based spin-caloritronic devices.

Magneto-optical painting of heat current

Active control of heat flow is crucial for the thermal management of increasingly complex electronic and spintronic devices. In addition to conventional heat transport engineering, spin caloritronics has received extensive attention as a heat control principle owing to its high controllability and unique thermal energy conversion symmetry. Here we demonstrate that the direction of heat currents generated by spin-caloritronic phenomena can be changed simply by illuminating magnetic materials with visible light. The optical control of heat currents is realized through a combination of the spin-driven thermoelectric conversion called an anomalous Ettingshausen effect and all-optical helicity-dependent switching of magnetization. This approach enables not only pinpoint manipulation and flexible design of the heat current distribution by patterning the illuminating light but also on/off control of the resulting temperature modulation by tuning the light polarization. These versatile heat control functionalities will open up a pathway for nanoscale thermal energy engineering.

Observation of anomalous Ettingshausen effect and large transverse thermoelectric conductivity in permanent magnets

This study focuses on the potential of permanent magnets as thermoelectric converters. It is found that a SmCo5-type magnet exhibits a large anomalous Ettingshausen effect (AEE) at room temperature and that its charge-to-heat current conversion coefficient is more than one order of magnitude greater than that of typical ferromagnetic metals. The large AEE is an exclusive feature of the SmCo5-type magnet among various permanent magnets in practical use, which is independent of the conventional performance of magnets based on static magnetic properties. The experimental results show that the large AEE originates from the intrinsic transverse thermoelectric conductivity of SmCo5. This finding makes a connection between permanent magnets and thermal energy engineering, providing the basis for creating “thermoelectric permanent magnets.”

Electric-field-induced on–off switching of anomalous Ettingshausen effect in ultrathin Co films

We have investigated the electric field effect on magneto-thermoelectric conversion in ultrathin Co films with a naturally-oxidized surface and a solid-state capacitor structure. By means of the thermoelectric imaging technique based on the lock-in thermography, we demonstrate the reversible on–off switching of heat currents generated by the anomalous Ettingshausen effect in response to the electric-field-induced magnetic phase transition in the Co films. The electric-field-induced switching property is found to be tuned by changing the Co thickness. Our finding will provide a method for reconfigurable and pin-point switching of thermoelectric conversion properties in spin-caloritronic devices.

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

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

Systematic Investigation of Anisotropic Magneto–Peltier Effect and Anomalous Ettingshausen Effect in Ni Thin Films

The anisotropic magneto-Peltier effect (AMPE) and anomalous Ettingshausen effect (AEE) have been investigated in U-shaped Ni thin films of varying thickness and substrate by means of the lock-in thermography (LIT) method. We have established a procedure to extract pure AMPE and AEE contributions, separated from other thermoelectric effects, for ferromagnetic thin films. The measurements of the magnetic-field-angle $\theta_{\rm H}$ dependence of the LIT images clearly show that the temperature modulation induced by the AMPE (AEE) in the Ni films varies with the $\cos 2\theta_{\rm H}$ ($\cos \theta_{\rm H}$) pattern, confirming the symmetry of the AMPE (AEE). The systematic LIT measurements using various substrates show that the AMPE-induced temperature modulation decreases with the increase in thermal conductivity of the substrates, whereas the AEE-induced temperature modulation is almost independent of the thermal conductivity, indicating that the heat loss into the substrates plays an important role in determining the magnitude of the AMPE-induced temperature modulation in thin films. Our experimental results were reproduced by numerical calculations based on a two-dimensional finite element method. These findings provide a platform for investigating the AMPE and AEE in thin film devices.

Anomalous Ettingshausen effect in ferrimagnetic Co–Gd

We report the observation of anomalous Ettingshausen effect (AEE) in ferrimagnetic Co100−xGdx amorphous alloy thin films, which was visualized using lock-in thermography. We found that the sign of AEE is determined by the magnetic moments of Co sublattice, as well as those of the magneto-optical Kerr effect and the anomalous Hall effect, and the magnitude of AEE does not decrease even when x is close to the magnetization compensation composition. Our experimental results suggest that the magnetically-compensated ferrimagnets become candidates for high-performance thermomagnetic materials with tiny magnetization.