Anatomy of the Anomalous Nernst and Spin Seebeck Effects in Fe-Based Metallic Multilayers

The relationship between heat, spin, and charge transport in magnetic materials is a central topic in materials physics [1]. In the past decade, the spin Seebeck effect (SSE) has been in the spotlight of research efforts as it allowed the generation of spin currents in magnetic materials without an accompanying charge flow [2]. In the SSE, the spin current flows parallel to the temperature gradient with polarization along the magnetization axis. The SSE can be detected by injecting the generated spin current into a metal layer with large spin-orbit coupling (e.g., Pt) and measuring the inverse spin hall effect voltage [3]. The SSE has been predominantly studied in magnetic insulator/heavy metal systems because, in all-metallic systems such as Co/Pt and Fe/Pt, the inverse SHE signals in Pt become indistinguishable by symmetry from the anomalous Nernst effect (ANE) signal in the magnetic layer [4]. Despite its difficulty, the possibility of simultaneous and additive generation of SSE and ANE and rich fundamental physics different from insulating systems make ferromagnetic metal (FM) heterostructures appealing from both applied and fundamental perspectives [4].<br/>Among 3d FMs, the case of Fe is especially intriguing. The ANE coefficient of Fe exhibits an atypical thickness dependence. It is positive in ultrathin films (typically &lt; 5 nm) and becomes negative for thicker films and the bulk [5]. This starkly contrasts with all other 3d FMs where the ANE coefficient is positive independently of the thickness. As a result, while in, e.g., Co/Pt, the ANE and SSE voltages are expected to be always additive, in Fe/Pt, they can be additive or competing depending on the Fe thickness and other interface-related parameters. Consequently, Fe-based heterostructures provide a suitable platform for engineering thermoelectric devices once underlying physical mechanisms are properly understood.<br/>In this work [6], we study the ANE and SSE in a wide variety of Fe-based multilayer films under the influence of ΔT applied perpendicular to the film plane. Single Fe layers show thermoelectric signals of purely ANE origin, controlled by the power (P_h) dissipated through the heater. In contrast, in Fe/Pt, we find a large SSE contribution comparable to the ANE characterized in Fe alone. We find that both ANE and SSE consist of two components, one temperature-independent (linear in P_h) and another temperature-dependent (quadratic in P_h), the latter typically overlooked in such measurements. We identify a combination of Fe and Pt thicknesses where the quadratic term dominates the thermoelectric signals due to the linear terms of ANE and SSE nearly canceling each other. At this regime, the thermoelectric signals in the same device undergo a sign reversal at a critical P_h, which was not reported or observed before. Collective data suggest that the quadratic ANE term is due to the reduction in the saturation magnetization of Fe at higher temperatures, whereas the quadratic SSE term pinpoints bulk magnon contributions. We furthermore show that both the linear and quadratic terms can be precisely controlled by changing the multilayer composition and stacking order, Fe and Pt thicknesses, and nonmagnetic metal doping of Fe. These results provide a solid ground for understanding the thermoelectric effects in similar structures and developing engineering strategies for micro-energy harvesting devices or heat flow sensors.<br/>References:<br/>[1] Back et al. J. Phys. D: Appl. Phys. 52, 230301 (2019)<br/>[2] Uchida et al. Nature 455, 778 (2008)<br/>[3] Uchida et al. Appl. Phys. Lett. 97, 172505 (2010)<br/>[4] Yi et al. Adv. Funct. Mater. 30, 2004024 (2020).<br/>[5] Chuang et al. Phys. Rev. B, 96, 174406 (2017)<br/>[6] De Sousa et al., in prep. (2024)