The Fabrication and Characterization of Spintronic Terahertz Emitter Using Magnetic/Non-Magnetic Bilayer with Nanometer Thickness

The response time of electrons below the fermi energy in solids to electromagnetic waves is known to be on the order of femtoseconds. As a result, the energy transfer between electromagnetic waves and spins is ultrafast, at the terahertz frequency [1]. Similarly, spin currents generated by spin polarization are described according to the LLG equation, i.e., the time dynamics of magnetization, and spin currents are expected to have a response time equivalent to that of electrons in the solid to electromagnetic waves. Furthermore, the effect of those spin currents being converted into electric currents by spin-orbit interactions is known as the inverse spin Hall effect, and the response time of those phenomena is also on the order of sub-picoseconds.<br/>In 2013, Kampfrath et al. reported the emission of terahertz waves by femtosecond laser (photon energy of 1.55eV) irradiation of magnetic/nonmagnetic bilayers composed of nanometer-thick films [2]. Subsequently, those devices were named STE (Spintronic Terahertz emitters (STEs), which have been actively studied in basic research as well as in application fields. We have been studying STE using ultrathin (several nanometer-thick) bilayers (Pt/Fe, W/Fe, etc.) of nonmagnetic/magnetic materials on inexpensive, large-aperture (large area) substrates such as glass substrates, and the intensity of terahertz radiation by femtosecond laser irradiation is comparable to that of conventionally used nonlinear optical crystal ZnTe, and further advances are being made [3,4]. Specifically, we have been studying 1) how to efficiently generate spin currents by femtosecond laser irradiation and 2) how to maximize the inverse spin Hall effect at the interface between magnetic and nonmagnetic materials.<br/>In this report, we quantitatively evaluate the spin Hall angle, which is the figure of merit of the effect shown in 2) above mentioned, and investigate its relation to the experimental terahertz wave radiation intensity. The spin Hall angle was obtained by analyzing the magnetic field dependence of the spin Hall magnetoresistance [5], which is used to evaluate spintronic devices, and the terahertz radiation intensity was obtained using our TDS system. The obtained results show a good linear correspondence between the magnitude of the change in spin Hall magnetoresistance and the intensity of terahertz radiation. We plan to reflect these results in the fabrication conditions and in the progress of the technique to further improve the performance of this system.<br/><br/>References<br/>[1] K. Yamaguchi, M. Nakajima, and T. Suemoto, Phys. Rev. Lett. 105, 237201 (2010).<br/>[2] T. Kampfrath, M. Battiato, P. Maldonado, et al.,Nat. Nanotechnol., 8, 256 (2013).<br/>[3] Y. Koike, S. Tetsukawa, M. Nishitani, H. Kitahara, V. K. P. Mag-usara, M. Asakawa, M. Yoshimura, M. Tani, M. Nakajima, 45th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) (2020).<br/>[4] T. Matsunaga, V. K. Mag-Usara, K. Ejiri, S. Tetsukawa, S. Liu, V. C. Agulto, S. Nishitani, M. Nishitani, M. Yoshimura, M. Nakajima, 47th International Conference on Infrared, Millimeter, and Terahertz Waves, (IRMMW-THz) (2022).<br/>[5] M.Kawaguchi, D.Towa, Yong-Chang Lau, S. Takahashi, and M. Hayashi, Appl. Phys. Lett., 112, 202405 (2018).