Articles

 Nov 22, 2021 Heat flow controls the movement of skyrmions in an insulating magnet
 Magnetic vortices could be manipulated by waste heat to realize lowpower computing applications
PI Name  Xiuzhen Yu  

Degree  D.Sci.  
Title  Team Leader  
Brief Resume 


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 phasecontrast microscopy, 3D magnetic imaging, 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.
Physics, Engineering, Materials Science
Electronic states
Lorentz microscopy
Analytical electron microscopy
Highresolution electron microscopy
Differential phasecontrast microscopy
3D magnetic imaging
The spin whirl, magnetic skyrmion carrying a topological number −1, attracts much attention in fundamental physics and spintronics. As an antiparticle of skyrmion, the antiskyrmion exhibits opposite topological charge and unique spin texture composed of the alternating Bloch and Néeltype spirals. The present research aims to understand nontrivial 3D forms of topological objects and explore emergent electromagnetic properties in magnetic materials. We developed tomographic Lorentz TEM with a high spatial resolution (5 nm), and built an algorithm for 3D vector field reconstruction, allowing realspace observation of 3D topological spin textures, such as the deformed skyrmion string (Fig. 1a). Figure 1b shows the scalar field maps of some skyrmion strings in the topview (upper panel) and crosssection view (down panel). The dualtilt series of 2D Lorentz TEM images were employed to demonstrate a 3D vector field map of the antiskyrmion (Fig. 1c). Systematic analyses of the tilt series of Lorentz TEM images allow us to build 3D phase images (Fig. 1d) and the corresponding vector field maps of a single antiskyrmion and its cluster (Figs. 1ef). Our exploits will lead to an advanced understanding of 3D spin textures in a broad class of magnets.
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.
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).
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)).
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.
Xiuzhen Yu 
Team Leader  yu_x[at]riken.jp  

Yi Ling Chiew 
Technical Scientist  
Fehmi Sami Yasin 
Special Postdoctoral Researcher  
Yao Guang 
Postdoctoral Researcher  
Shunsuke Mori 
Postdoctoral Researcher  
Kiyomi Nakajima 
Technical Staff I  
Kiyou Shibata 
Visiting Scientist  
Daisuke Morikawa 
Visiting Scientist  
Nobuto Nakanishi 
Visiting Scientist 
RealSpace Observations of ThreeDimensional Antiskyrmions and Skyrmion Strings
Realspace observations of 60nm skyrmion dynamics in an insulating magnet under low heat flow
Nanometric square skyrmion lattice in a centrosymmetric tetragonal magnet
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)5035023493
FAX：+81(0)484621687
Email：
yu_x[at]riken.jp