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COHERENT SCATTERING IMAGING OF SKYRMIONS UNDER HIGH APPLIED MAGNETIC FIELDS AND LOW TEMPERATURES

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Profiting from the coherence of synchrotron light, scientists have performed both reciprocal and real-space observations of magnetic skyrmion lattice deformation in a chiral magnet Co8Zn8Mn4. The study of these materials is key for developing futures spintronic applications such as racetrack memory and logic devices.

Cerdanyola del Vallès, 10th May 2019. The interplay between exchange interaction, antisymmetric Dzyaloshinskii-Moriya interaction, and magnetocrystalline anisotropy may cause incommensurate spin phases such as helical, conical, and Bloch-type skyrmion lattice states. The typical size of a magnetic skyrmion varies in a range from a few to a few hundred nanometers which makes them promising candidates for future spintronic applications such as skyrmion racetrack memory - with storage density higher than solid-state memory devices- and logic devices.

Coherent soft X-ray scattering and imaging are powerful tools to study the spin ordering in multicomponent magnetic compounds with element selectivity

In this experiment, a skyrmion-hosting compound Co8Zn8Mn4 was investigated at cryogenic temperatures and applied high magnetic fields by a group of researchers from the Japanese RIKEN Center of Emergent Matter Science, National Institute for Materials Science, the Science and Technology Agency, University of Tokyo, the Institute of Materials Structure Science and Photon Factory, as well as from the ALBA Synchrotron.

Researchers were able to perfom lensless soft X-ray imaging of the magnetic texture at cryogenic temperatures down to 20K under high applied magnetic fields up to 2 TeslaHERALDO imaging (holography with extended reference by autocorrelation linerar differential operation) was used with the circularly polarized soft X-rays, while coherent diffraction imaging was performed with a linearly polarized beam. The coherence of the synchrotron light enabled to successfully combine small-angle scattering in transmission geometry with coherent diffraction imaging and employed the small-angle scattering patterns for the real-space reconstruction of the local magnetization distribution via iterative phase retrieval algorithm, giving the complementary information to the HERALDO data. Advanced material synthesis and nanofabrication of Co8Zn8Mnthin plates by focused ion-beam milling was done at RIKEN Center for Emergent Matter Science. X-ray experiments were performed at beamlines BL-16A at Photon Factory and BL29 BOREAS at ALBA. In particular, the measurements at ALBA have been performed using the MaReS scattering endstation that enables coherent scattering imaging under high applied magnetic fields at low temperatures.

Both lensless imaging methods provided a real-space resolution of 30 nm and allowed researchers to record a magnetic texture in the temperature range between 20 K and 120 K, demonstrating the elongation of the skyrmions along the principal crystallographic axes at low temperatures. Micromagnetic simulations showed that such deformation is driven by a decreasing ratio of symmetric exchange interaction to antisymmetric Dzyaloshinskii-Moriya interaction in the system and effect of the cubic anisotropy. This mimics low-temperature antiferromagnetic frustration of the Mn sublattice in Co8Zn8Mn8. At lower temperature, antiferromagnetic correlations of Mn atoms are superimposed onto the long-range helical (skyrmion) modulation, resulting in shortening of the helical pitch and deformation of skyrmions. However, this effect is reversible and the hexagonal skyrmion lattice from elongated skyrmion state can be restored by increasing magnetic field even when the effective exchange interaction is reduced.

IM_SkyrmionsBOREAS_2

Fig.: Sketch of the (a) XMCD, (b) RSXS and (c) HERALDO experiments. (d) Scanning electron microscopy (SEM) image of the RSXS sample aperture. (e) Thin plate of the Co8Zn8Mn8 fixed onto the membrane. (f) SEM image of the sample aperture and the reference.

IM_SkyrmionsBOREAS_3

Fig.: Top panel (left to right): Scanning electron microscopy (SEM) image of the sample mask defined by Focused ion beam milling. Flow of the phase retrieval algorithm for the real-space inversion of the coherent x-ray speckle patterns. Coherent RSXS speckle pattern measured from Co8Zn8Mn4 at 25 K and magnetic field B = 70 mT at MaReS endstation (ALBA). Bottom panel (left to right): Real-space reconstruction of magnetization pattern obtained by phase-retrieval routine and its magnification. Micromagnetic simulation of the elongated skyrmion structure under the influence of decreased effective exchange interaction and cubic anisotropy.

IM_SkyrmionsBOREAS_1

Fig.: Coherent soft x-ray speckle patterns measured for Co8Zn8Mn4 sample at L3 absorption edge of Co at different temperatures 150 K, 120 K, 25 K (top panel, left to right) and applied field of 70 mT. White scale bar corresponds to 0.05 nm−1. Bottom panel shows micromagnetic simulations of the corresponding skyrmionic spin textures.

Reference: Element-specific soft x-ray spectroscopy, scattering, and imaging studies of the skyrmion-hosting compound Co8Zn8MnV. Ukleev, Y. Yamasaki, D. Morikawa, K. Karube, K. Shibata, Y. Tokunaga, Y. Okamura, K. Amemiya, M. Valvidares, H. Nakao, Y. Taguchi, Y. Tokura, and T. Arima Physical Review B 99, 144408

Acknowledgements
This work has been supported in part by PRESTO Grant Number JPMJPR177A from Japan Science and Technology Agency (JST), "Materials research by Information Integration" Initiative (MI2) project of the Support Program for Starting Up Innovation Hub from JST, the Japan Society for the Promotion of Science through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), and JSPS KAKENHI Grant Number 16H05990. V. U. acknowledges funding from the SNF Sinergia CRSII5-171003 NanoSkyrmionics. M. V. acknowledges additional funding to the MARES endstation by grants MICINN ICTS-2009-02, FIS2013-45469-C4-3-R and FIS2016- 78591-C3-2-R (AEI/FEDER, UE).

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