ICYS Annual Report 2023

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ICYS Annual Report 2023Yuta SASAKI1. Outline of Research2. Research ActivitiesFig. 1. (a) Schematic and (b)photo image of newly constructed all-optical switching setup. Fig. 2. (a) Microstructure and (b) magnetic hysteresis for the sample. Schematic image of ultrafast spin current assist and MOKE contrast of all-optical switching. Fig. 3. Normalized line profile of MOKE contrast. References1) A. V. Kimel, and M. Li, Nat. Rev. Mater. 4, 189-200 (2019).2) Y. Xu, and S. Mangin, J. Magn. Magn. Mater. 564, 170169 (2022).Society 5.0 is a future social model, which is highly integrated cyberspace and physical space, proposed by the Cabinet Office, Government of Japan. In this social model, we can obtain technical knowledges and solutions unconsciously by using AI computing and big data analysis. In order to realize this society model, a data center will be connected to a lot of sensing devices for collecting the big data such as climate, temperature, human health, location information, transport information, etc. Since data traffic and amount of data processing in the data center will drastically increase in coming Society 5.0, huge energy will be consumed in the current network system. In order to realize the low energy sustainable society consumption and ultrafast processing system is required.For low energy and ultrafast processing, optical data center with all-optical digital network system is proposed as a future data center. Optical switching-based interconnect architectures have been proved effective in reducing or eliminating the electronic components for high-energy efficiency. When we consider the optical data center with the all-optical digital networking system, the data will be recorded by an all-optical way. Main storage in current data center is Hard-disk-drive and L10-FePt nano-granular film is used as a magnetic recording media material. Considering the compatibility with existing data storage technology, the all-optical recording technology in L10-FePt nano-granular film is crucial issue.All-optical switching measurement has been performed in GdFe(Co), TbFe, and Co/Pt multilayer films using magneto-optical Kerr effect (MOKE) imaging technique to detect magnetization contrast in previous reports1), 2). Here, I would like to report the progress of the improvement of the MOKE imaging set up for all-optical switching (AOS) measurement, optimization of stacking structure for AOS in FePt thin films, and investigation of the AOS in FePt nano-granular thin films. After the improvement of measurement set up, now we can discuss AOS quantitatively with high accuracy and reproducibility.toward zero emission, Construction of AOS set up for quantitative measurementFor the AOS measurement, spot radius and laser fluence largely effect on the switching property. Therefore, additional lens and iris were inserted into the optical set up to stabilize the laser spot at the focus position on the film surface as shown in figures 1(a) and 1(b). AOS measurement for the FePt nano-granular filmsFilm samples with a stacking structure from substrate side, FePt-30%C (4 or 2)/Ru(0.3-3.5) /CoFe(2)/Pt(2), was fabricated on the MgO(001) single crystal substrate. The MgO(001) substrate was thermally annealed prior to the deposition, and FePt-30%C layer was grown at a substrate heating temperature of Ts=500℃ using the co-sputtering form C and FePt targets. The Ru spacer, CoFe and Pt layers were deposited at room temperature.Figure 2(a) and 2(b) show the microstructure and magnetic property for the sample. As indicated in figure 2(c), ultrafast spin current mediated by laser pulse irradiation from CoFe insertion layer assisted the partial magnetization switching for this nano-granular film as seen in figures 2(d) and 2(e). for quantitative discussion, the line profile of MOKE contrast was evaluated in figure 3. Although the partial magnetization switching assisted by the ultrafast spin current have been confirmed, deterministic switching depending on the optical helicity was not observed. Further material development for the helicity dependent control of magnetism in L10-FePt is required. Research Digest 21All-optical Control of THz Magnetization Dynamics in L10-FePt Nano-granular film