Femtosecond Laser-Induced Terahertz Emission in Spintronic Multilayers and the Role of Hot Phonons

Abstract

Spintronic devices have emerged as a promising next-generation technology, as they can utilize spin current―defined as spin-polarized electron flow without a net electric current―thereby avoiding Joule heating. This technology is aimed at energy-efficient and high-speed signal processing, where low power consumption and sub-picosecond response times are essential. In this context, terahertz (THz) spintronics has recently attracted significant attention. In this study, we investigated spintronic THz emission from ferromagnetic (FM) and nonmagnetic (NM) multilayer structures, focusing on FM/NM bilayers and NM/FM/NM trilayers, which were excited by femtosecond laser pulses. The samples were fabricated with optimized thicknesses for each structure, and the emission efficiency was examined as a function of laser fluence, with slight variations observed between the structures. To gain deeper insight into the emission mechanism, THz time-domain spectroscopy was used to extract the charge currents that govern the THz emission process. Through this apporach, we observed a laser-fluence-dependent temporal delay in the THz emission process, which was attributed to hot phonon effects. Our findings highlight the importance of mulilayer design and phonon engineering in spintronic THz emission and are expected to provide useful insights for future research and design of ultrafast spintronic THz devices