Electronic States Microscopy Research Team
Principal Investigator
PI Name  Xiuzhen Yu  

Degree  D.Sci.  
Title  Team Leader  
Brief Resume 


Outline
Our team is working on the realspace observation of electron structures or topological electronspin textures (skyrmion) and their dynamics in strongcorrelation systems by means of atomicresolution electron microscopy. We use various microscopies, such as the insitu imaging technique, differential phase contrast microscopy, electron energyloss 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
Highresolution electron microscopy
Differential phasecontrast microscopy
Results
Direct observation of a single skyrmion and skyrmion cluster dynamics under pulsecurrent excitations
The magnetic skyrmion carrying a topological number −1, as a particlelike topological texture, attracts much attention in fundamental physics as well as in spintronics. The skyrmion Hall motion (a) occurs when the conduction electron passes through skyrmion due to the interaction of the electron and the Berry phase arising from skyrmion. Accordingly, the spin polarized electric current can drive skyrmion motion when its strength is above a critical value J_{C} to overcome the pinning potential for the skyrmion in real materials. To manipulate small magnetic skyrmions, we set out working with a thin helimagnet FeGe with a notch hole, which allowed the spin current to be localized in a specific area near the corner of the notch and hence to generate isolated skyrmions and their cluster. The Hall motion companying a unique rotating motion (counterclockwise in the present experimental setup; Figs. cd) of the threeskyrmions cluster (Fig. b) has been demonstrated for the first time. Lorentz TEM observations (Figs. ef) of pulsecurrent tracking a single skyrmion with 80nm in diameter and their cluster demonstrated that the J_{C} for drives of skyrmions in the thin FeGe is an order of 10^{9} A/m^{2}, three orders smaller than that for drives of magnetic domain walls in ferromagnetic materials.
Directly imaging square lattices of atomicscale skyrmions and nanometric antiskyrmions
The magnetic skyrmion carrying a topological number −1, as a particlelike topological texture, attracts much attention in fundamental physics as well as in spintronics. Skyrmions arising from DzyalosinskiiMoriya interaction have been observed in several magnets with noncentrosymmetric crystal structures. Here we discovered atomic scale skyrmions causing by Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction trough itinerant electrons in a magnet GdRu_{2}Si_{2} 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 Tfield in a (001) thin plate of GdRu_{2}Si_{2}
On the other hand, a square lattice of squareshape antiskyrmions with topological number +1, has been observed in a (001) thin Mn_{1.4}Pt_{0.9}Pd_{0.1}Sn (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. 1b1f). The inplane fieldcontrolled skyrmion helicity has also been revealed (Fig.1c, 1g).
Realspace 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 particlelike 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 inplane anisotropy. Here, the sequential real space observations of spin textures have been performed for a thin helimagnet Co_{8}Zn_{9}Mn_{3} with the inplane 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 meronantimeron lattice (sqML shown in Figs. 1(a)1(b)). Such experimental results agree well with the theoretical predictions of the sqML. By finely increasing the magnetic field, the sqML transforms into a hexagonal skyrmion lattice (hexSkL) (Fig. 1(c)1(d)).
Realspace observations of nanometrictopological spin textures and their dynamics
The nontrivial phenomena, such as highT_{C }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 spinorbit 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 spinpolarized current traverses the skyrmion owing to spin transfer torque. By utilizing microfabrication techniques and in–situ Lorentz TEM observations, we have directly realized the topological spin textures and their dynamics in various materials hosting magnetic skyrmions.
Members
Publications
 X. Z. Yu, D. Morikawa, K. Nakajima, K. Shibata, N. Kanazawa, T. Arima, N. Nagaosa, and Y. Tokura
Motion tracking of 80nmsize skyrmions upon directional current injections
 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
 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 noncentrosymmetric magnet
 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
303, Frontier Research Laboratory
21 Hirosawa, Wako, Saitama 3510198 Japan
TEL：+81(0)484674070
FAX：+81(0)484621687
Email：
yu_x[at]riken.jp
Links
News
 Jun 18, 2020 Manipulating tiny skyrmions with small electric currents
 Jun 06, 2017 51st CEMS Colloquium (June 28, 2017 Dr. Xiuzhen Yu)