Electronic States Microscopy Research Team

Principal Investigator

PI Name Xiuzhen Yu
Degree D.Sci.
Title Team Leader
Brief Resume
1990Master, Department of Semiconductor, Faculty of Electronic Science, Jilin University
2002Technician, Tokura Spin Superstructure Project, ERATO, Japan Science and Technology Agency
2006Engineer, Advanced Electron Microscopy Group, Advanced Nano Characterization Center, NIMS
2008Doctor of Science, Department of Physics, Graduate School of Science and Faculty of Science, Tohoku University
2008Researcher, Advanced Electron Microscopy Group, Advanced Nano Characterization Center, NIMS
2010Researcher, Tokura Multiferroic Project, ERATO, Japan Science and Technology Agency
2011Postdoctoral Researcher, Quantum Science on Strong Correlation Group, Advanced Science Institute, RIKEN
2013Senior Research Scientist, Strong Correlation Physics Research Group, Strong Correlation Physics Division, RIKEN Center for Emergent Matter Science (CEMS)
2017Team Leader, Electronic States Microscopy Research Team, RIKEN CEMS (-present)

Outline

Our team is working on the real-space observation of electron structures or topological electron-spin textures (skyrmion) and their dynamics in strong-correlation systems by means of atomic-resolution electron microscopy. We use various microscopies, such as the in-situ imaging technique, differential phase contrast microscopy, electron energy-loss spectroscopy, and energy dispersive spectroscopy, etc., to explore the electronic structures and their dynamical phase transitions with external stimuli. We also use these powerful tools to quantitatively characterize the nanometric magnetic and electric fields in topological matters to exploit emergent phenomena and hence their possible applications in the spintronics.

Research Fields

Physics, Engineering, Materials Science

Keywords

Electronic states
Lorentz microscopy
Analytical electron microscopy
High-resolution electron microscopy
Differential phase-contrast microscopy

Results

Directly imaging square lattices of atomic-scale skyrmions and nanometric antiskyrmions

The magnetic skyrmion carrying a topological number −1, as a particle-like topological texture, attracts much attention in fundamental physics as well as in spintronics. Skyrmions arising from Dzyalosinskii-Moriya interaction have been observed in several magnets with non-centrosymmetric crystal structures. Here we discovered atomic scale skyrmions causing by Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction trough itinerant electrons in a magnet GdRu2Si2 with the centrosymmetric crystal structure. Figure 1a shows a square lattice of skyrmions (surrounded by dashed lines) observed at 8 K under a 1.95 T-field in a (001) thin plate of GdRu2Si2

On the other hand, a square lattice of square-shape antiskyrmions with topological number +1, has been observed in a (001) thin Mn1.4Pt0.9Pd0.1Sn (Fig. 1b). By tuning the external magnetic field and the temperature, the controlled transformations between antiskyrmios and elliptic skyrmions as well as their lattice forms have been demonstrated (Figs. 1b-1f). The in-plane field-controlled skyrmion helicity has also been revealed (Fig.1c, 1g).

(a) An underfocused TEM image observed at 8K under a 1.95 T-field in a (001) GdRu2Si2 thin plate. Circular domains surrounded by dashed lines show skyrmions, whereas small doted contrasts indicate crystalline elements. (b, c) Underfocused Lorentz TEM images for a square lattice of antiskyrmions (b) and for a triangular lattice of elliptic skyrmions (c) observed in a thin Mn1.4Pt0.9Pd0.1Sn. (d, e-f)Magnetization textures for a antiskyrmion (d), a skyrmion with counterclockwise helicity (e)and a skyrmion with clockwise helicity (f). (g) A triangular skyrmion lattice composed of elliptic skyrmions with unique helicity (counterclockwise).

 

Real-space observations of a square lattice of merons and antimerons

The topological spin texture, magnetic skyrmion indexed by an integer topological number, is of increasing interest in physical science and spintronics owing to its particle-like topological nature. On the other hand, meron (antimeron) carrying a topological number of ±1/2 is theoretically predicted but has not been experimentally confirmed yet in the helimagnets with the in-plane anisotropy. Here, the sequential real space observations of spin textures have been performed for a thin helimagnet Co8Zn9Mn3 with the in-plane anisotropy.

With application of a weak field (20 mT) at a room temperature (295 K), the real space images observed in the thin helimagnet directly demonstrate the formation of a square meron-antimeron lattice (sq-ML shown in Figs. 1(a)-1(b)). Such experimental results agree well with the theoretical predictions of the sq-ML. By finely increasing the magnetic field, the sq-ML transforms into a hexagonal skyrmion lattice (hex-SkL) (Fig. 1(c)-1(d)).

(a) A Lorentz TEM image and (b) the corresponding magnetization textures for a square meron-antimeron lattice realized in a chiral-lattice magnet Co8Zn9Mn3.
(c) A Lorentz TEM image and (d) the corresponding magnetization textures for a hexagonal skyrmion lattice realized in the same sample.

 

Real-space observations of nanometric-topological spin textures and their dynamics

The nontrivial phenomena, such as high-TC superconductivity and colossal magnetoresistance (CMR), are caused by electronic phase transitions in strongly correlated electron systems with weak external stimuli. Among them, skyrmion, i.e., nanometric topological spin texture arising from strong spin-orbit interaction is attracting much attention since it is considered to bear potential for future functional devices. In skyrmion, several hundreds of spins swirl with a unique direction and wrap a unit sphere. Particularly, skyrmion carrying a topological number can be driven by an extremely small current which is six orders of magnitudes lower than that for a drive of the domain wall in ferromagnets.

The emergent field induced by this nontrivial topological spin texture should deflect conducting electrons and hence cause novel magnetic transport phenomena such as the topological Hall effect. As a counteraction, the skyrmion Hall motion appears when the spin-polarized current traverses the skyrmion owing to spin transfer torque. By utilizing micro-fabrication techniques and insitu Lorentz TEM observations, we have directly realized the topological spin textures and their dynamics in various materials hosting magnetic skyrmions.

 

Isolated skyrmions and recrystallization of isolated skyrmions in a helimagnet FeGe with a decrease of the magnetic bias-field. The left panel shows independent “isolated” skyrmions (white dots) at a higher bias-field, 425 mT. The right panel represents a mixed state of skyrmion crystal (SkX) and conical domains (C) with a reduction of the bias field from 425 mT to 150 mT.

Members

Xiuzhen Yu

Team Leader yu_x[at]riken.jp R

Konstantin Vitalievitch Iakoubovskii

Technical Scientist

Licong Peng

Special Postdoctoral Researcher

Fehmi Sami Yasin

Postdoctoral Researcher

Sebastian Gunther Schneider

Postdoctoral Researcher

Kiyomi Nakajima

Technical Staff I

Yoshio Matsui

Visiting Scientist

Kiyou Shibata

Visiting Scientist

Daisuke Morikawa

Visiting Scientist

Publications

  1. N. D. Khanh, T. Nakajima, X. Z. Yu, S. Gao, K. Shibata, M. Hirschberger, Y. Yamasaki, H. Sagayama, H. Nakao, L. C. Peng, K. Nakajima, R. Takagi, T. H. Arima, Y. Tokura, and S. Seki

    Nanometric square skyrmion lattice in a centrosymmetric tetragonal magnet

    Nat. Nanotechnol. 15, 444 (2020)
  2. L. Peng, R. Takagi, W. Koshibae, K. Shibata, K. Nakajima, T. Arima, N. Nagaosa, S. Seki, X. Yu, and Y. Tokura

    Controlled transformation of skyrmions and antiskyrmions in a non-centrosymmetric magnet

    Nat. Nanotechnol. 15, 181 (2020)
  3. X. Z. Yu, W. Koshibae, Y. Tokunaga, K. Shibata, Y. Taguchi, N. Nagaosa, and Y. Tokura

    Transformation between meron and skyrmion topological spin textures in a chiral magnet

    Nature 564, 95 (2018)
  4. M. Mochizuki, X. Z. Yu, S. Seki, N. Kanazawa, W. Koshibae, J. Zang, M. Mostovoy, Y. Tokura, and N. Nagaosa

    Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect

    Nat. Mater. 13, 241 (2014)

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