Direct-Contact Seebeck-Driven Transverse Magneto-Thermoelectric Generation in Magnetic/Thermoelectric Bilayers.

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 Co2 MnGa. The findings highlight the potential of combining magnetic and thermoelectric materials for transverse thermoelectric applications.

Sm-Co-based amorphous alloy films for zero-field operation of transverse thermoelectric generation.

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.

Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission.

We report spin-resolved hard X-ray photoelectron spectroscopy (spin-HAXPES) for a buried Fe thin film in the valence band region. For the spin-HAXPES experiments, we developed an ultracompact built-in Mott-type spin-filter in a sample carrier, which enabled us to use the merit of two-dimensional (2D) multi-channel detector in a recent photoelectron analyser without modifying an apparatus for HAXPES. The effective Sherman function and the single-channel figure of merit (FOM) of the spin-filter were assessed to be -0.07 and 2.0 × 10-4, respectively. By utilizing the 2D detector of the photoelectron analyser, the effective FOM increased by a factor of ~4 × 104 compared to the case when only 1 channel of the 2D detector was used. We have applied spin-HAXPES to MgO(2 nm)/Fe(50 nm)/MgO(001) structures. The spin-HAXPES experiments revealed the majority and minority spin electronic states and the spin polarisation of the buried Fe thin film. Due to the large photoionization cross-section of the 4s orbital of Fe in HAXPES, the spin-resolved spectra mainly reflected the Fe 3d and 4s states. The observed spin-HAXPES and spin polarisation spectral shapes agreed well with the calculated spin-resolved cross-section weighted densities of states and spin polarisation spectra. In contrast, a small discrepancy in the energy scale was recognised due to the electron correlation effects. These results suggest that the electron correlation effects are important in the electronic structure of bulk Fe, and spin-HAXPES is useful for detecting genuine spin-resolved valence band electronic structures of buried magnetic materials.

Prototype fabrication and performance evaluation of a thermoelectric module operating with the Nernst effect.

The Nernst effect generates a voltage transverse to the temperature gradient in the magnetic field. Although the Nernst effect has the potential to realize novel devices in the field of thermoelectric generators and sensors, thermoelectric modules that operate with the Nernst effect have not yet been implemented. Therefore, in this study, a thermoelectric module utilizing the Nernst effect was developed as a prototype, and its performance was evaluated to identify technical issues. The proposed module is fabricated by arranging four rectangular bars of a BiSb-based sintered alloy on an AlN substrate and connecting all the bars in series with Cu plates. As a result of the measurement, when the magnetic field was 5 T, an output power of 0.48 mW was obtained with a temperature difference of 149 K, and a temperature difference of 82 mK occurred as a cooling operation with an applied electrical current of 100 mA.

Seebeck-driven transverse thermoelectric generation.

When a temperature gradient is applied to a closed circuit comprising two different conductors, a charge current is generated via the Seebeck effect1. Here, we utilize the Seebeck-effect-induced charge current to drive 'transverse' thermoelectric generation, which has great potential for energy harvesting and heat sensing applications owing to the orthogonal geometry of the heat-to-charge-current conversion2-9. We found that, in a closed circuit comprising thermoelectric and magnetic materials, artificial hybridization of the Seebeck effect into the anomalous Hall effect10 enables transverse thermoelectric generation with a similar symmetry to the anomalous Nernst effect11-27. Surprisingly, the Seebeck-effect-driven transverse thermopower can be several orders of magnitude larger than the anomalous-Nernst-effect-driven thermopower, which is clearly demonstrated by our experiments using Co2MnGa/Si hybrid materials. The unconventional approach could be a breakthrough in developing applications of transverse thermoelectric generation.

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices.

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co2 Fe(Ge0.5 Ga0.5 ) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.

Structure-property relationship of Co2MnSi thin films in response to He+-irradiation.

We investigated the structure-property relationship of Co2MnSi Heusler thin films upon the irradiation with He+ ions. The variation of the crystal structure with increasing ion fluence has been probed using nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM), and associated with the corresponding changes of the magnetic behavior. A decrease of both the structural order and the moment in saturation is observed. Specifically, we detect a direct transition from a highly L21-ordered to a fully A2-disordered structure type and quantify the evolution of the A2 structural contribution as a function of ion fluence. Complementary TEM analysis reveals a spatially-resolved distribution of the L21 and A2 phases showing that the A2 disorder starts at the upper part of the films. The structural degradation in turn leads to a decreasing magnetic moment in saturation in response to the increasing fluence.

Combinatorial investigation of spin-orbit materials using spin Peltier effect.

Conversion between spin and charge currents is essential in spintronics, since it enables spin-orbit-torque magnetization switching, spin-current-driven thermoelectric generation, and nano-scale thermal energy control. To realize efficient spin-charge conversion, a variety of mechanisms, including spin Hall effects, Rashba-Edelstein effects, and spin-momentum locking in topological insulators, have been investigated and more comprehensive material exploration is necessary. Here we demonstrate high-throughput screening of spin-charge conversion materials by means of the spin Peltier effect (SPE). This is enabled by combining recently-developed SPE-imaging techniques with combinatorial materials science; using a composition-spread alloy film formed on a magnetic insulator, we observe the SPE-induced temperature change due to the spin Hall effect and obtain a continuous mapping of its composition dependence from the single sample. The distribution of the SPE signals reflects local spin-charge conversion capability in the alloy owing to unique heat-generation nature of the SPE. This combinatorial approach will accelerate materials research towards high-performance spintronic devices.

The spin Nernst effect in tungsten.

The spin Hall effect allows the generation of spin current when charge current is passed along materials with large spin-orbit coupling. It has been recently predicted that heat current in a nonmagnetic metal can be converted into spin current via a process referred to as the spin Nernst effect. We report the observation of the spin Nernst effect in W. In W/CoFeB/MgO heterostructures, we find changes in the longitudinal and transverse voltages with magnetic field when temperature gradient is applied across the film. The field dependence of the voltage resembles that of the spin Hall magnetoresistance. A comparison of the temperature gradient-induced voltage and the spin Hall magnetoresistance allows direct estimation of the spin Nernst angle. We find the spin Nernst angle of W to be similar in magnitude but opposite in sign to its spin Hall angle. Under an open-circuit condition, this sign difference results in the spin current generation larger than otherwise. These results highlight the distinct characteristics of the spin Nernst and spin Hall effects, providing pathways to explore materials with unique band structures that may generate large spin current with high efficiency.