Thermally driven spin transport and spin-charge conversion in functional magnetic thin-film devices

Abstract

Conventional thermoelectric (TE) technologies, which utilize charge carriers to convert heat into electrical energy, are fundamentally limited by the coupled nature of electrical and thermal conductivities. These limitations fundamentally constrain their efficiency and scalability in practical applications. In contrast, spin thermoelectric (STE) conversion has emerged as a next-generation approach by decoupling heat and charge transport path through the use of spin currents. An STE device typically combines a magnetic insulator, which generates pure spin currents via thermally excited magnons, with a heavy metal layer that converts the injected spin current into a transverse voltage through the inverse spin Hall effect. This orthogonal geometry between heat flow and voltage generation reduces parasitic losses and enables novel device architectures. Realizing efficient STE conversion requires magnetic materials with low thermal conductivity, high magnon generation efficiency, and compatibility with scalable fabrication methods—motivating the exploration of new functional thin films for spin caloritronic applications. This dissertation explores two major directions: scalable device fabrication using low-temperature and solution-based processes, and fundamental understanding of spin–charge conversion mechanisms in symmetry-sensitive magnetic systems. These efforts are aimed at improving practical integration and revealing new physics for next-generation spin caloritronic applications. In chapter 2, the development of transparent and scalable spin thermoelectric (STE) devices using solution-processed Y3Fe5O12 (YIG) thin films. Fabricated via spin-coating techniques, the YIG films exhibit excellent crystallinity and extremely smooth surfaces, both critical for minimizing interfacial spin scattering and enabling efficient magnon transport. Combined with ultrathin Pt and Cr spin detector layers, these devices achieve high optical transmittance while generating clear and robust longitudinal spin Seebeck effect (LSSE) signals under controlled thermal gradients. These transparent spin thermoelectric devices exhibit simultaneously high optical transmittance and enhanced LSSE performance, highlighting their potential for next-generation energy conversion systems requiring both efficiency and visibility. Chapter 3 focuses on molecular magnetic systems. V–Cr Prussian Blue Analogue (PBA) thin films were synthesized via room-temperature electrochemical deposition, enabling direct integration on spin Hall-active Cr electrodes. The resulting films exhibited long-range ferrimagnetic ordering near room temperature (TC ≈ 324 K), along with low magnetic damping and stable magnetization. Ferromagnetic resonance (FMR) measurements confirmed effecitve magnon excitation capability, while LSSE-based evaluations demonstrated thermal spin injection potential. The use of molecule-based magnets in spintronic devices provides a novel pathway for achieving tunable magnetic and spin transport properties through synthetic chemistry and ambient processing. Chapter 4 investigates spin–charge conversion in epitaxial RuO2 thin films, where altermagnetic spin splitting emerges despite the absence of net magnetization. By reversing the Néel vector orientation through field-cooling and performing LSSE measurements along orthogonal crystallographic axes, the study reveals distinct symmetry-dependent variations in spin–charge conversion behavior. This study identifies and separates the inverse altermagnetic spin-splitting effect (IASSE) from conventional inverse spin Hall effects. These results demonstrate a symmetry-governed, nonrelativistic mechanism for spin–charge conversion and highlight the potential of RuO2 for low-dissipation spintronic applications.