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Yuta Ishii, Yusuke Kozuka, Yuichi Yamasaki, Hironori Nakao

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[Development of Multi-scale Soft X-ray Diffraction Microscope for Observing Spin Textures](https://mdr.nims.go.jp/datasets/02d5b78e-8428-465e-b84a-dac4433eee80)

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Development of Multi-scale Soft X-ray DiffractionMicroscope for Observing Spin TexturesYuta Ishii1,2, Yusuke Kozuka3, Yuichi Yamasaki1,3, and Hironori Nakao11Photon Factory, Institute of Materials Structure Science,High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan2Department of Physics, Tohoku University, Sendai 980-8578, Japan3National Institute for Materials Science (NIMS), Tsukuba 305-0047, JapanE-mail: hironori.nakao@kek.jp(Received July 14, 2022)We developed a multiscale soft X-ray diffraction microscope for observing spin textures and mea-sured the magnetic domain on a [Pt/Co/Ta] multilayer film. Using this microscope, we obtainednearly real-space images of the magnetic domain without any analysis, in addition to zooming capa-bility.KEYWORDS: Fresnel diffraction, coherent diffraction imaging, magnetic imaging, resonantx-ray scattering1. IntroductionSpin textures, such as magnetic skyrmions, exhibit various fascinating physical properties andhave been extensively investigated from the viewpoints of both fundamental science and their ap-plications in next-generation spintronic devices [1]. Resonant X-ray scattering (RXS) is a powerfultechnique for observing the ordering of electronic degrees of freedom (i.e., charge, orbital, spin, andmultipole degrees of freedom) [2, 3]. Moreover, in the soft X-ray region, the RXS signal at the L2,3edge (2p→ 3d transition) can directly probe the 3d electronic state, which is critical to understandingthe physical properties in 3d transition-metal compounds. Especially, the enhancement of magneticscattering is enormous at the L2,3 edge, and a strong magnetic signal similar to that of magnetic neu-tron scattering can be observed. For example, Yamasaki et al. detected a magnetic skyrmion crystalin a thin-plate sample (< 200 nm) of FeGe using transmitted small-angle RXS [4]. The results alsoindicated that the coherence of soft X-ray can be utilized even in Photon Factory, which is an oldsynchrotron radiation facility operated from 1982 [5]. Hence, we conducted magnetic X-ray imag-ing studies at the Photon Factory, which led to real-space imaging of the spin texture by coherentdiffraction [6–8] and holography [7].We also investigated the spin texture by scanning transmission X-ray microscopy (STXM) [9],where the sample was placed at the focal point of a Fresnel zone plate (FZP). During this experi-ment, we investigated the images recorded when the sample was positioned between the focal pointand the X-ray detector (Fig. 1). To demonstrate this idea experimentally, we recorded images using acharge-coupled device (CCD) camera temporarily installed in the STXM instrument and obtained ahigh-quality magnetic domain image without any analysis. After these measurements, we noticed thatthis kind of microscope was reported as Gabor-type in-line holography [10]. In this microscope, re-construction is needed to obtain the real-space image because diffraction from the sample is recordedby the CCD camera. However, we noted that the raw data were almost a real-space image [10], con-sistent with our results. Moreover, the field of view can be freely changed by changing the sampleposition, although the observable region was limited by the pinhole in our magnetic X-ray imaging1JPS Conf. Proc. , 011190 (2023)©2023 The Author(s)https://doi.org/10.7566/JPSCP.38.01119038must maintain attribution to the author(s) and the title of the article, journal citation, and DOI.Proc. 29th Int. Conf. Low Temperature Physics (LT29)011190-1This article is published by the Physical Society of Japan under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this workProceedings of the 29th International Conference on Low Temperature Physics (LT29)Downloaded from journals.jps.jp by （研）物質・材料研究機構 on 03/11/24http://creativecommons.org/licenses/by/4.0/　Fig. 1. Schematic of the multiscale soft X-ray diffraction microscope with a Fresnel zone plate (FZP), order-sorting aperture (OSA), sample, and CCD camera. The first-order diffraction from the FZP is selected by theOSA. The direct beam is stopped by the central stop on the FZP and the OSA.studies [6–8]. Zooming ability is an especially important characteristic of this microscope. In addi-tion, high spatial resolution was achieved via reconstruction analysis [11]. Consequently, this tech-nique is suitable for revealing unexpected responses of spin textures under external fields becausethe important region that needs to be observed for understanding the physical properties is unknownbefore the measurement. In this report, we present details of the microscope we developed to measurespin textures and demonstrate its zooming ability.2. ExperimentsA magnetic multilayer film, [Pt(3 nm)/Co(1 nm)/Ta(2 nm)]5, which is known to be a room-temperature magnetic skyrmion compound [12], was fabricated on a Si3N4 membrane (thickness:100 nm) with a capping layer of SiO2 (20 nm) and an underlayer of Ta (5 nm) using a sputter depo-sition technique. The perpendicular magnetic anisotropy was measured by magnetic susceptibility ofthe single layer film [Pt(3 nm)/Co(1 nm)/Ta(2 nm)].The imaging experiment was performed at the APPLE-II type undulator beamline, BL-13A, inthe Photon Factory [13]. Right circularly polarized (RCP) and left circularly polarized (LCP) X-rayswere used at the Co L3 edge (779 eV) [14]. A schematic of the multiscale soft X-ray diffractionmicroscope is shown in Fig. 1. The incident X-rays were focused by the FZP (diameter R = 120 µm;central beam stop diameter 60 µm). The focal length was f = 1.51 mm at 779 eV, and the focalposition was defined as z = 0. The diffraction pattern was recorded using an in-vacuum CCD camera(Teledyne Princeton PMI2048, 2048 × 2048 pixels, pixel size 13.5 µm) installed at zd = 145 mmfrom the focal position. The sample was placed between the focal point and the detector. Depending2011190-2JPS Conf. Proc. , 011190 (2023)38Proceedings of the 29th International Conference on Low Temperature Physics (LT29)Downloaded from journals.jps.jp by （研）物質・材料研究機構 on 03/11/24on the sample position, zs, the sample image was magnified by the geometrical factor, g = zd/zs,although the magnification was limited by the focused beam size near the focal point. Therefore,this microscope is useful for multiscale observation. At zs2 shown in Fig. 1, the whole of sampleexisted within the divergent incident beam from the FZP. Hence, the signals scattered by the sampleinterfered with the incident beam, corresponding to the reference wave in holography. The size of thefield of view, zsR/ f , could be changed by varying the sample position, zs. This microscope has beenpreviously reported as the Gabor-type in-line holography, which has been mainly used to observebiological specimens such as cells [10,15]. The sample shape was properly reconstructed because theedge scattering was detected clearly. On the other hand, the sample was larger than the field of viewwhen the sample was placed at zs1. Therefore, only the signals from the sample were measured, andno reference wave was present. This experimental configuration was applied for magnetic domainimaging in the present study.3. Results and discussionMagnetic domain imaging using the experimental setup detailed in the preceding section has notbeen previously reported. Hence, we simulated the Fresnel diffraction from a magnetic domain, asshown in Fig. 2. Figure 2(a) is the magnetic domain used for the simulation; only the region irradiatedwith X-rays is shown. The magnetic domain therefore exhibits a doughnut shape, reflecting the shapeof the FZP. The Fresnel diffraction from a magnetic domain was calculated with zs = 500 µm, asshown in Fig. 2(c). The calculated image is similar to the real-space image of the magnetic domain　Fig. 2. (a) Magnetic domain used for the simulation and (b) the magnified image. (c) Calculated Fresneldiffraction from the magnetic domain and (d) the magnified image. (e) Differential image between Fresneldiffraction patterns calculated for RCP and LCP X-rays.3011190-3JPS Conf. Proc. , 011190 (2023)38Proceedings of the 29th International Conference on Low Temperature Physics (LT29)Downloaded from journals.jps.jp by （研）物質・材料研究機構 on 03/11/24　Fig. 3. Differential images of Fresnel diffraction from the [Pt/Co/Ta] multilayer film measured at room tem-perature with the sample positioned at (a) zs = 1000 µm, (b) zs = 500 µm, and (c) zs = 250 µm.(Fig. 2(a)), although edge scattering and a moiré pattern from the FZP are observed in the magnifiedimage (Fig. 2(d)). Figure 2(e) is the differential image between Fresnel diffraction patterns calculatedfor RCP and LCP X-rays. The contrast of the magnetic domain is enhanced, and the artifacts fromthe FZP are reduced.To test our microscope, we measured the Fresnel diffraction from the magnetic multilayer film atthe Co L3 edge (779 eV) [14]. The images were recorded at zd = 145 mm with an exposure time ofapproximately 10 s [16]. The differential images between RCP and LCP X-rays are shown in Fig. 3.The differential intensity is normalized by the spatial distribution of the incident X-ray intensity. Theleft region of all the images shows large noise because the incident X-ray intensity was weak inthis region. At zs = 1000 µm, the magnetic domain in a wide region is observed. By changing thesample position zs from 1000 µm to 250 µm, we can zoom in on a part of the magnetic domain. Thegeometrical factor g is 145, 290, and 580 at zs = 1000 µm, 500 µm, and 250 µm, respectively. Hence,we demonstrated almost real-space imaging of spin texture, which corresponds to X-ray magneticcircular dichroism (XMCD) imaging. Moreover, the image can be acquired in a single shot, as shownin Fig. 2(c). These results indicate that this technique is more suitable for time-resolved imaging thanscanning-type techniques such as STXM.We have developed a multiscale soft X-ray diffraction microscope for observing spin textures.Using this microscope, we successfully measured nearly real-space images of the magnetic domainon a magnetic multilayer film without any analysis. The zooming ability of this microscope was alsodemonstrated. In this work, we noted that the real-space image can be recorded without any analysis.Using the reciprocal-space information provided by this microscope, we recently detected a spiralphase distribution of a vortex beam. Moreover, we propose that this technique would be an effec-tive probe for topological defects in spin textures and have indicated its potential to characterize thetopological numbers of the defects [17, 18]. These results indicate that our soft X-ray diffraction mi-croscope still has unexplored capabilities to measure spin textures and that the observation techniqueshould be selected depending on the target object. To clarify the capabilities of our soft X-ray diffrac-tion microscope, we are conducting magnetic imaging studies to detect various spin-textures. Finally,we consider that the dynamics and kinetics of spin-textures will become a future important subject inour synchrotron radiation facility [19]; hence, further investigations using our soft X-ray diffractionmicroscope are strongly desired.4011190-4JPS Conf. Proc. , 011190 (2023)38Proceedings of the 29th International Conference on Low Temperature Physics (LT29)Downloaded from journals.jps.jp by （研）物質・材料研究機構 on 03/11/24The authors are grateful to Y. Suzuki, M. Mizumaki, and T. Arima for fruitful discussions. Wealso acknowledge Y. Takeichi, K. Ono, K. Mase, and K. Amemiya for technical support in thesoft X-ray measurements. This research was partially supported by JSPS KAKENHI with projectNos. JP17K05130, JP19H04399, JP19K23590, JP20K20107, JP20H04458, and JP22K18271, byMEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118068681,by the Research Foundation for Opto-Science and Technology, and by PRESTO (JPMJPR177A),CREST(JPMJCR1861), Japan Science and Technology Agency (JST). This study was performed un-der the approval of the Photon Factory Program Advisory Committee (Proposal Nos. 2018S2-006,2019G553, 2021S2-004, and 2021PF-S003).References[1] Y. Tokura and N. Kanazawa, Chem. Rev. 121, 2857 (2021).[2] T. A. W. Beale, G. Beutier, S. R. Bland, A. Bombardi, L. Bouchenoire, O. Bunău, S. Di Matteo,J. Fernández-Rodrı́guez, J. E. Hamann-Borrero, J. Herrero-Martı́n, V. L. R. Jacques, R. D. Johnson,A. Juhin, T. Matsumura, C. Mazzoli, A. M. Mulders, H. Nakao, J. Okamoto, S. Partzsch, A. J. Princep,V. Scagnoli, J. Strempfer, C. Vecchini, Y. Wakabayashi, H. C. Walker, D. Wermeille and Y. Yamasaki,Eur. Phys. J. Special Topics 208, 89 (2012).[3] T. Matsumura, H. 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Rosenhahn, Ultramicroscopy 111, 1131 (2011).[16] The shutter system was not work well in this experiment. Hence the exposure time was almost 10 second.[17] Y. Ishii, K. Yamamoto, Y. Yokoyama, M. Mizumaki, H. Nakao, T. Arima, and Y. Yamasaki, Phys. Rev.Applied 14, 064069 (2020).[18] Y. Ishii, H. Nakao, M. Mizumaki, Y. Wakabayashi, T. Arima, and Y. Yamasaki, Sci. Rep. 12,1044 (2022).[19] K. Harada, N. Funamori, N. Yamamoto, Y. Shimosaki, M. Shimada, T. Miyajima, K. Umemori, H. Sakai,N. Nakamura, S. Sakanaka, Y. Kobayashi, T. Honda, S. Nozawa, H. Nakao, Y. Niwa, D. Wakabayashi,K. Amemiya, and N. Igarashi, J. Synchrotron Rad. 29, 118 (2022).5011190-5JPS Conf. Proc. , 011190 (2023)38Proceedings of the 29th International Conference on Low Temperature Physics (LT29)Downloaded from journals.jps.jp by （研）物質・材料研究機構 on 03/11/24