Chi-Feng Pai

  • Title: Associate Professor
  • Telephone: +886-2-33662585
  • E-Mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
  • Office: Room 472, College of Engineering
  • Education:

    Ph.D., Applied Physics, Cornell University, 2014

  • Research Summary:

    My research interests include the growth and the characterization of magnetic materials, as well as the potential applications of novel materials in the field of spintronics. My research group will be focusing on discovering novel spin-charge transport properties, for instance, spin Hall magnetoresistance (SMR) and spin Hall effect (SHE), in various material systems.

  • Research Topics:

    I. Materials

    Magnetic heterostructure growth and characterization

    Thin-film ferromagnetic materials deposited in adjacent to other transition metals and/or oxide layers can induce interfacial magnetic anisotropy. For instance, as shown in this dark-field STEM image, the ~1nm FeCoB alloy that is sandwiched between a Hf buffer layer and a MgO capping layer demonstrates perpendicular magnetic anisotropy (PMA). This PMA can be further tuned by post-annealing process, applied strain/stress, as well as applied voltage across the heterostructure.

    Image courtesy of David A. Muller group at Cornell University

    II. Devices

    Spin Hall effect induced spin transfer torque and its applications: memory and logic

    The spin Hall effect (SHE) in heavy transition metals has been experimentally proved to be strong enough to induce magnetization switching and magnetic oscillations in the adjacent ferromagnetic layer via spin transfer torque (STT) mechanism. The spin Hall angle, which roughly describes the sign and the magnitude of the charge-to-spin conversion efficiency, is about 0.06, -0.15, and -0.30 for Pt, Ta, and W, respectively.

    Other emergent materials systems such as topological insulators have also been proved to possess giant spin Hall effect, with spin Hall angle possibly greater than unity (>100%). Our research aims for materials systems with high spin Hall efficiency and their potential applications, for instance in the magnetic random access memory (MRAM) industry.

    Illustration of a 3-terminal device utilizing the SHE-STT

    III. Advanced Probing

    Chiral domain wall motion and dynamics of exotic spin textures

    The domain wall in magnetic nanowires can serve as an information carrier between magnetic memory elements. The efficiency of the domain wall propagation, however, is strongly related to the type of the ferromagnetic material, the magnetic anisotropy therein, and the strength of the spin transfer torque stemming from the magnetic heterostructure. The dynamics of domain walls and other spin textures (for instance, like skyrmions) in magnetic heterostructures with perpendicular magnetic anisotropy (PMA) has been shown to be controlled by the spin Hall effect (SHE) from the bulk and the Dzyaloshinskii-Moriya Interaction (DMI) at the transition metal-ferromagnetic material interface. We are interested in both the physics and the applications of these dynamical properties of spin textures.

  • Main experience:

    May 2016 - present, Consulting Research Fellow, ITRI
    Aug. 2014 - Nov. 2014, Postdoctoral Research Associate, Cornell University
    Nov. 2014 - Dec. 2015, Postdoctoral Research Associate, Massachusetts Institute of Technology

  • Awards:

    2016                Asian Union of Magnetics Societies Young Researcher Award

    2018                Industrial Technology Research Institute Outstanding Research Award

    2019                Ministry of Science and Technology Young Scholar Fellowship

    2020                Taiwan Semiconductor Industry Association Award for Young Researcher

    2020                Ministry of Science and Technology Ta-You Wu Memorial Award

  • Selected Publication:

    [1] Chi-Feng Pai*, "Switching by Topological Insulators," Nature Materials 17, 755 (2018).

    [2] T.-Y. Tsai, T.-Y. Chen, C.-T. Wu, H.-I Chan, C.-F. Pai*, "Spin-orbit torque magnetometry by wide-field magneto-optical Kerr effect," Scientific Reports 8, 5613 (2018).

    [3] T.-Y. Chen, T.-C. Chuang, S.-Y. Huang, H.-W. Yen, and C.-F. Pai*, "Spin-orbit torque from a magnetic heterostructure of high-entropy alloy," Physical Review Applied 8,044005 (2017).

    [4] C. O. Avci, A. Quindeau, C.-F. Pai, M. Mann, L. Caretta, A. S. Tang, M. C. Onbasli, C. A. Ross, and G. S. D. Beach, "Current-Induced Switching in a Magnetic Insulator," Nature Materials 16, 309 (2016).

    [5] C.-F. Pai*, M. Mann, A. J. Tan, and G. S. D. Beach, “Determination of Spin Torque Efficiencies in Heterostructures with Perpendicular Magnetic Anisotropy,” Physical Review B 93, 144409 (2016). (Editor's Suggestion)