Mid-Infrared Ellipsometry Spectroscopy of Electronic and Spin Phenomena in Spintronic Devices

• Jorge Puebla (Dept Electronic Science and Engineering, Kyoto University / Center for Spintronics Research Network (CSRN), Kyoto University)

Room: 5F 1st Meeting Room Spectroscopic ellipsometry (SE) is a non-destructive optical technique widely used to characterize thin-film heterostructures. While it is best known for accurately determining film thickness and surface roughness, broadband SE performed over a wide spectral range and multiple angles of incidence can also provide detailed information on the optical properties of individual layers and buried interfaces, particularly in the far-infrared (1–24 meV) and mid-infrared (25–500 meV) regions. Since 2019, we have collaborated on applying mid-infrared SE to investigate the two-dimensional electron gas (2DEG) formed at the interface between polycrystalline Cu and Bi$_{2}$O$_3$. From these measurements, we extracted the interfacial conductivity, carrier relaxation time, and spin-orbit coupling strength [1]. Although similar information can be obtained by angle-resolved photoemission spectroscopy (ARPES), ARPES requires high-quality crystalline samples, making it unsuitable for many polycrystalline or amorphous heterostructures. Notably, mid-infrared SE also offers more opportunities for spintronics and magnetism. In the late 1960s, Professor Toru Moriya highlighted the importance of studying the frequency-dependent optical conductivity of magnetic materials, including deviations from the Drude model, alongside light scattering phenomena [2, 3]. Motivated by this work, we applied mid-infrared SE to the archetypal heavy metal/CoFeB/MgO heterostructure [4]. Our study yielded (i) the mid-infrared dielectric function of Co$_{20}$Fe$_{60}$B$_{20}$, previously unavailable in the literature [5]; (ii) the dielectric tensor of the interfacial 2DEG; and (iii) the identification of a discrete optical feature associated with spin-orbit coupling. An intriguing next step is to uncover spectroscopic ellipsometry (SE) signatures of the Dzyaloshinskii-Moriya interaction (DMI), a key antisymmetric exchange interaction responsible for stabilizing non-collinear magnetic textures, including chiral spin structures. If time permits, I will also present our latest results on a unified anisotropic TO-LO dielectric function model for LiNbO$_{3}$ in the mid-infrared [6]. **References** [1] J. M. Flores-Camacho, J. Puebla, F. Auvray, A. Lastras-Martinez, Y. Otani, R. E. Balderas-Navarro, Phys. Rev. B 100 235449 (2019). [2] T. Moriya, Theory of Absorption and Scattering of Light by Magnetic Crystals, J. Appl. Phys. 39, 2 (1968). [3] T. Moriya, M. Inoue, Frequency-dependent electrical conductivity of dilute magnetic alloys, J. Phys. Soc. Jap., 27, 2 (1969). [4] J. M. Flores-Camacho, B. Rana, R. E. Balderas-Navarro, A. Lastras-Martinez, Y. Otani, J. Puebla, J. Phys. D: Appl. Phys. 56, 315301 (2023). [5] A. Santos-Amador, J. Puebla, O. Del Pozo-Zamudio, S. Vazquez-Miranda, R.E. Balderas-Navarro, Thin Solid Films, 839, 140903 (2026). [6] A. Santos-Amador, J. Puebla, O. Del Pozo-Zamudio, and R.E. Balderas-Navarro, under preparation (2026).

Selectivity of Electron Spins and Phonon Angular Momenta in Driven Chiral Media

• Yoshihiko Togawa (Osaka Metropolitan University / QuaRC)

Room: 5F 1st Meeting Room It has been clarified that the spin polarization phenomena initially found in chiral organic molecules occur even in chiral inorganic crystals. The spin signals appear in a variety of chiral materials from molecules to crystals irrespective of electrical conductivity ranged from metals to insulators. As a manifestation of chirality, which corresponds to time-even pseudo scalar, works as a microscopic chiral term in driven chiral media and hybridizes translational and rotational degrees of freedom, a polarization of angular momenta appears under charge or thermal flow. In the talk, recent experimental progress on the key findings made with various kinds of chiral inorganic materials will be presented. A comprehensive understanding of these nontrivial features will clarify the mystery of chirality-induced dynamical responses such as chirality-induced spin selectivity (CISS).