Large Transverse Thermoelectric Effect in Iron-Based Binary Ferromagnets
Nakatsuji and Miwa Groups
Thermoelectricity provides vital technology for versatile energy harvesting and heat current sensors. So far, thermoelectric technologies are focused on the longitudinal Seebeck effect. Its transverse counterpart, the anomalous Nernst effect (ANE), has recently gained significant attention, owing to several potential benefits [1,2]. Namely, the transverse geometry of the Nernst effect enables simplified structures of thermoelectric generators with enhanced conversion efficiency once the suitable materials become readily available. Moreover, the transverse geometry is hypothetically better suited for thermoelectric conversion, as the Ettingshausen heat current should support the Nernst voltage while the Peltier heat current may suppress the Seebeck voltage . A server obstacle here is that the anomalous Nernst effect is too small compared to the Seebeck effect for real-life thermoelectric applications. Thus, it is essential to design a new class of materials that exhibit a large ANE without an external magnetic field.
To find candidate compounds efficiently, we carried out a high-throughput calculation to screen materials and found two candidate iron-based cubic compounds, Fe3X (X = Ga, Al) (Fig. 1a). We discovered record high spontaneous transverse thermoelectric effects at room temperature in these materials, reaching about 6 μVK-1 in Fe3Ga and 4 μVK-1 in Fe3Al (Fig. 1b). We then succeeded in fabricating Fe3Ga and Fe3Al thin films that display an ANE of about 4 μVK-1 and 2 μVK-1 at room temperature with an applied in-plane temperature gradient. For an out-of-plane temperature gradient, these thin films exhibit a zero-field ANE, with a coercivity BC of the in-plane magnetization being about 40 Oe in Fe3Ga and 20 Oe in Fe3Al, respectively. These features are suitable for designing low-cost, flexible thermoelectric generators.
The comparison between experiment and theory indicates that the Fermi energy tuning to the nodal web is the key to the substantial enhancement in the transverse thermoelectric coefficient. Figure 1(c) shows a schematic of the nodal web, a flat band structure made of interconnected nodal lines. The Berry curvature is particularly enhanced at the momenta connecting the edge of the nodal web around the L point at the Brillouin zone boundary, extending over a quasi-2D area spanned by the web. The strongly enhanced Berry curvature occurs near the momenta originally belonging to the flat nodal web, such as the one around the L point.
Our innovative iron-based thermoelectric material represents a significant step toward commercializing thermoelectric generators that can power small devices such as remote sensors or wearable devices. It is vital to enhance the coercivity further to achieve stable performance in daily use. Finally, it would be an interesting future direction to look for an enhanced output by combining the ANE with the spin Seebeck effect, both of which occur in the same transverse geometry. 
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