Enhanced Spin-Thermoelectric Generation in Generators Incorporating Liquid Phase Epitaxial Bi-Substituted Yttrium Iron Garnet Films for Constructing an Energy Storage System Combined with Supercapacitors
Authors: Matsuda Ryota , Imamura Masaaki , Tashima Daisuke 
Published: December 2024

Recently, energy harvesting, which converts waste heat into energy, has been attracting attention as a highly beneficial future technology. We have focused our attention on the spin Seebeck effect, based on spintronics technology, which enables thermoelectric power generation. The spin Seebeck effect is a phenomenon that enables the conversion of thermal energy into spin-thermoelectric (STE) voltage. An insulator-based STE generator is composed of a thin paramagnetic metal (PM) layer for generating a voltage V
STE via the inverse spin Hall effect (ISHE) and a ferrimagnetic insulator (FMI) film used for producing spin-wave spin currents under a temperature gradient ∇T.(1) We studied thick yttrium iron garnet (Y3Fe5O12, YIG) films of ~ 10 μm grown by using liquid phase epitaxy (LPE), and confirmed that the films were of acceptable crystallinity.(2) Pt, a paramagnetic heavy metal, and YIG, an FMI, are typical substances used for the thin PM layer and the FMI film, respectively. We previously proposed a role of spin–orbit coupling with local 3d-electron spins in Fe3+ ions for thermal energy transfer from the phonon system to the spin system in a STE generation,(3) and also proposed that the exchange interaction acting between neighboring spins creates the role of driving force for generating spin-wave spin currents in YIG.(4) Spin-wave spin currents produced in the YIG from the effects of ∇T, which carry spin-angular-momenta between neighboring spins, are pumped up at the Pt/YIG interface into the Pt layer, and the spin currents produced in the Pt layer are converted into charge currents by means of the ISHE. The charge currents generate the electric voltage V
STE (or V
ISHE) in the Pt layer. A figure included in this abstract shows the dependence of the spin-thermoelectric voltage V
STE on ∇T and that of the V
STE on the temperature difference in the range of a maximum ΔT of 150 °C between the top cold Cu block and the bottom hot block as shown in the inset of the figure. The probe distance l
d was set to 5 mm, and the Pt layer width w
Pt was 1 mm. A high V
STE value of 90 µV was observed at Δ
T = 150 °C (∇T = 300 × 10−3 °C/µm) for the generator incorporating a Y2.35Bi0.65Fe5O12 film of 10 µm thickness. An issue to be addressed on the STE generator is whether the generator can be utilized for the generation of electric power. A semiconductor Seebeck device is a practical choice in the generation of electrical energy. However, the stable operation of the semiconductor Seebeck device is limited to 100 °C or less. The STE generators using Bi-substituted YIG films that have a Curie temperature of 280 °C are expected to operate in a higher temperature range of 100-250 °C. From the measurement of load characteristics, we have confirmed that the STE generators can function as a voltage source. By connecting the STE generator to a Supercapacitor(5) that exhibits a large capacitance and a shorter charging time than a rechargeable battery, it may be possible to construct an innovative energy storage system. The system proposed in our presentation is considered to have a capability of recovering the waste heat of relatively high temperatures exhausted in factories and power plants. We are aiming for observing the V
STE value of 250–500 mV at ΔT = 150–250 °C (∇T ≈ 400×10−3 °C/mm) by incorporating the Bi:YIG films of high Bi content into STE generators.

(1) M. Imamura et al., Electr. Eng. Jpn.
208, 10‒20 (2019). (2) M. Imamura et al, AIP Advances
11, 035143-1- 035143-5. (3) M. Imamura et al., IEEE Trans. Magn.
58, 4500111-1−4500111-11 (2022). (4) M. Imamura et al., AIP Advances
13, 025001-1−025001-5 (2023). (5) T. Ryu et al., Journal of Physics: Conference Series
2368, 012002-1−012002-8 (2022).

Figure 1