The departing idea of MAGNEPIC is to use rare-earth iron garnets as a tunable magnetic material platform and create novel spintronic device concepts based on these materials. To this end, we created two new laboratories, one dedicated to thin film fabrication based on the magnetron sputtering technique and the other for high-precision optical and electrical device characterization. We optimized the growth of TbIG, TmIG, and YIG ultrathin films. These materials possess high perpendicular anisotropy, low perpendicular anisotropy, and in-plane anisotropy, respectively, which are beneficial to test different device ideas. Out of these three materials, we specifically focused on TbIG. We characterized its structural, chemical, magnetic, and interfacial spin transport properties in a broad range of temperature and preparation conditions, which we reported in a paper. We can now grow TbIG down to 3 nm, which is hugely beneficial to test some device ideas we propose in MAGNEPIC, where interfacial effects are dominant.
The electrical characterization setup is our primary tool to test the spintronic devices. In this setup, we have characterized the current-induced spin-orbit torques in an archetypical spintronic system, Pt/YIG. To quantify the torques more precisely, we improved the existing methodology by performing the measurements in an unconventional geometry and combining our findings with numerical simulations. Aside from the precise determination of the torque values, to our surprise, we found that the damping-like torque loses efficiency at high current injection due to the rise in device temperature. We reported the improved methodology and the above findings in a paper. In a parallel effort, we have realized a novel spin valve device where one magnetic layer is TbIG instead of a typical conducting magnet. Here, we used TbIG|Cu|TbCo trilayers, where TbCo serves as the reference layer, and Cu is a nonmagnetic spacer. For a fixed polarization of TbCo, the resistance of Cu|TbCo displayed two distinct levels for the “up” and “down” polarization of TbIG, respectively. This leads to a simple and powerful two-terminal device concept where binary data could be stored and electrically accessed in a magnetic insulator. These exciting findings represent a significant milestone and breakthrough for the MAGNEPIC project and the entire spintronics research community.
We developed two optical characterization setups. The first one is a wide-field magneto-optic Kerr effect microscopy. This setup uses an LED light shone onto the substrate surface, and depending on the magnetization state (up/down) of the film, the reflected light changes its polarization. The analysis of the reflected light provides magnetic imaging of the surface. This setup is compelling for measuring nonuniform magnetic textures (domain walls, skyrmions) with a lateral resolution of <1 micrometer. In this setup, we have been performing a comprehensive study to understand how the domain walls respond to the current injection in a metal/TbIG bilayer where we use different types of metals that can produce spin-orbit torques. The second optical setup is still in progress. Here, we use a laser instead of an LED, and the final goal is to focus the beam down to 1 micrometer spot and get information on the local magnetization state of the films. This setup acquires data at a much faster rate, enabling time-resolved dynamical characterization of, e.g. domain walls. However, the focusing unit has not yet been implemented, and the spot size is currently in the sub-mm range, which is still helpful for inspecting the basic properties of continuous films.
