Weinan Zhou

We report the observation of a spatial allocation of Peltier-effect-induced temperature modulation through the anomalous Hall effect (AHE) in a hybrid system comprising a magnetic metal and a thermoelectric semiconductor. AHE in the magnetic metal generates a transverse electric field perpendicular to the applied bias voltage, inducing charge injection into the thermoelectric layer via the closed circuit and resultant heating and cooling generation in a transverse geometry. Using an active thermal imaging technique, lock-in thermography, we visually extract the AHE-originating thermal contribution from other magneto-thermoelectric effects. Such heat allocation enables flexible design of transverse thermoelectric conversion, providing a pathway toward on-chip thermal management and selective temperature control at designated regions.

Heat-flux sensors (HFSs) based on the transverse thermoelectric phenomenon of the anomalous Nernst effect have attracted increasing interest in recent years due to the advantages stemming from simple and planar sensor structures. However, the difference in Seebeck coefficients of the constituent materials makes the sensors also sensitive to an in-plane temperature gradient along the wire direction and can give rise to an undesirable offset in the sensor output. In this study, to mitigate this offset, improve structural efficiency, and simplify device fabrication, we propose a single-material anomalous Nernst HFS with antiparallel magnetization alignment in neighboring wires. Using heat-assisted magnetization reversal through electric-current-driven Joule heating, we were able to locally control the magnetization and realize such an antiparallel alignment in devices microfabricated from an L10-FePt thin film with an in-plane magnetic easy axis. Systematic measurements on a single FePt wire showed the conditions of electric current and magnetic field required for heat-assisted magnetization reversal; these conditions were then applied to a Π-shaped FePt element to selectively reverse one half of its magnetization and achieve antiparallel alignment. As a result, the Π-shaped element with antiparallel magnetization exhibits nearly twice the heat-flux sensitivity of a single wire. These results establish heat-assisted magnetization reversal as an effective way to locally control magnetization for constructing offset-free, single-material anomalous Nernst HFSs.

The anomalous Nernst effect (ANE) garners considerable attention owing to its capacity to generate a transverse electric field, which facilitates the development of advanced thermoelectric devices with simplified structures, such as heat flux sensors. Nevertheless, despite the advantages of ANE‐based thermoelectric technologies, the need for external magnetic fields in most ANE materials hinders their practical applications. The advancement of permanent magnets showing excellent ANE performance is a crucial solution. In this study, we introduce Pt ₂ CoNi‐based thermoelectric permanent magnet thin films that demonstrate substantial in‐plane and out‐of‐plane coercivities (exceeding 1 T) and exhibit impressive ANE performance with a thermopower exceeding 1.8 µV K ⁻¹ . A comprehensive evaluation of the electrical/thermoelectric transport reveals that these films possess a giant transverse thermoelectric conductivity of 4.1 Am ⁻¹ K ⁻¹ at room temperature, which is among the highest values for permanent magnet films. Furthermore, through systematic and high‐throughput investigations, we identify a broad range of compositions for Pt _{2‐ q ‐ p} Co _{1+ q} Ni _{1+ p} films with tunable coercivity and comparable ANE performance, and propose a conceptual design for a single‐material‐based thermopile utilizing these newly developed materials. This study offers a pathway for the development of efficient and sustainable ANE‐based thermoelectric technologies that do not require an external magnetic field.

This letter proposes and demonstrates a flexible architecture for an anomalous-Nernst-effect -based heat flux sensor inspired by kirigami, a traditional Japanese paper-cutting art. A prototype device comprises an amorphous Sm19Co81 film and Ta/Au electrode on a flexible substrate pre-patterned with a periodic-cutting kirigami structure. The estimated heat flux sensitivity of the kirigami-structured sensor was comparable to that of a conventional sensor without kirigami structure and remained unchanged even when it was stretched by 10%. Our approach opens a pathway toward versatile and mechanically compliant heat flux sensing that is applicable to various surfaces.

We propose a simple and reliable measurement method for the thermal Hall effect by the use of thermocouples fabricated directly on a target material. The thermal Hall angle has been typically characterized by measuring diagonal and off diagonal terms of the thermal conductivity tensor with manually attached bulk thermometers such as thermocouples and resistance temperature sensors. However, the presence of contact thermal resistance between the material and thermometers and the ambiguity of actual temperature information under a magnetic field hinder the quantitative evaluation of the thermal Hall angle. In this work, to characterize the temperature gradient with low contact thermal resistance, we deposit a thermoelectric NiCu thin film directly on the target materials, pattern it into a crossbar shape, and attach two Au wires at the hot and cold junctions to form a closed electrical circuit, i.e., an on-slab thermocouple. Owing to the ideal fourfold symmetry of the structure, we can determine the thermal Hall angle simply from the ratio of the measured thermoelectric voltages in the longitudinal (input) and transverse (output) directions for the on-slab thermocouples under the out-of-plane magnetic field. The measurements for the Co2MnGa Heusler alloy and Ni slabs by the proposed method find that the thermal Hall effect was clearly detected in a wide temperature range from 60 to 330 K, which is quantitatively consistent with the literature. This approach offers a simple and reliable route to investigate the thermal Hall effect and reveal the wide variety of carrier transport and thermal conversion phenomena.

Co₂FeSi is considered a half-metallic ferromagnet and a Weyl semimetal, however, its predicted properties have been shown to be heavily influenced by how the on-site Coulomb interaction is incorporated, which remains controversial. In this study, we measure the anomalous Nernst conductivity (α_xy) and anisotropic magnetoresistance (AMR) effect of a Co₂FeSi thin film from low temperature to room temperature, and compare the results with those of first-principles calculations using different values for on-site Coulomb interaction at the Co site (U_Co). Our measurements reveal that α_xy is less than 0.1 A m⁻¹ K⁻¹ at room temperature and decreases slightly with the temperature. The observed values of α_xy are more than one order of magnitude smaller than the predictions unless a small but finite U_Co is incorporated. The AMR effect exhibits a notable sign change from negative to positive with increasing temperature, which is inconsistent with the predicted band structure calculated using a large U_Co value. By combining these experimental observations with first-principles calculations, we estimate that the appropriate U_Co value is approximately 1–2 eV. Our findings provide valuable insight into the correlation effect in Co₂FeSi, emphasizing the critical role of the on-site Coulomb interaction in accurately describing the transport properties of Co-based Heusler alloys.

Thermal imaging technology has significantly advanced the fields of thermal engineering and fundamental physics by enabling precise and non-contact temperature measurements. In the field of spin caloritronics, an active infrared emission microscopy approach based on lock-in thermography (LIT) has significantly advanced the study of thermoelectric and thermal transport phenomena in magnetic materials and their hybrid structures; however, its measurements have been largely limited to room temperature and above. Here, we report on the measurements of transverse thermoelectric conversion in a magnetic material, the anomalous Ettingshausen effect (AEE), below room temperature using the LIT technique. Although the infrared emission decreases with decreasing temperature, the high temperature resolution of LIT allows to observe the AEE-induced temperature modulation in a ferromagnetic Heusler alloy Co2MnGa slab down to 200 K and reliably quantify the anomalous Nernst coefficient down to around 240 K. The methodology demonstrated in this work will pave the way for further developments in both fundamental research and practical applications in spin caloritronics.

We demonstrate a high-throughput experimental characterization of anomalous Nernst conductivity ( α x y A ) of L1₀-ordered CoPt using Co_1–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 α x y A 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 α x y A 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 α x y A of L1₀-Co₅₀Pt₅₀ agrees with the experimental α x y A 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 α x y A 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 α x y A originating from the intrinsic mechanism for future thermoelectric applications.

Transverse thermoelectric generation converts temperature gradient in one direction into an electric field perpendicular to that direction and is expected to be a promising alternative in creating simple‐structured thermoelectric modules that can avoid the challenging problems facing traditional Seebeck‐effect‐based modules. Recently, large transverse thermopower has been observed in closed circuits consisting of magnetic and thermoelectric materials, called the Seebeck‐driven transverse magneto‐thermoelectric generation (STTG). However, the closed‐circuit structure complicates its broad applications. Here, STTG is realized in the simplest way to combine magnetic and thermoelectric materials, namely, by stacking a magnetic layer and a thermoelectric layer together to form a bilayer. The transverse thermopower is predicted to vary with changing layer thicknesses and peaks at a much larger value under an optimal thickness ratio. This behavior is verified in the experiment, through a series of samples prepared by depositing Fe–Ga alloy thin films of various thicknesses onto n‐type Si substrates. The measured transverse thermopower reaches 15.2 ± 0.4 µV K⁻¹, which is a fivefold increase from that of Fe–Ga alloy and much larger than the current room temperature record observed in Weyl semimetal Co₂MnGa. The findings highlight the potential of combining magnetic and thermoelectric materials for transverse thermoelectric applications.