Electronic States Microscopy Research Team

Real-Space Observations of Three-Dimensional Antiskyrmions and Skyrmion Strings

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 Néel-type 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 real-space 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 top-view (upper panel) and cross-section view (down panel). The dual-tilt 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. 1e-f). Our exploits will lead to an advanced understanding of 3D spin textures in a broad class of magnets.

(a) Schematic of a deformed skyrmion string. (b) 3D scalar field map of some skyrmions strings observed in the top-view (upper panel) and the cross-section view (down panel) in a CoZnMn alloy. (c) The schematic of an antiskyrmion. (d-f) The 3D phase image (d) and corresponding vector field maps of antiskyrmions (e-f).

Direct observation of a single skyrmion and skyrmion cluster dynamics under pulse-current excitations

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. 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 (counter-clockwise in the present experimental setup; Figs. c-d) of the three-skyrmions cluster (Fig. b) has been demonstrated for the first time. Lorentz TEM  observations (Figs. e-f) of pulse-current tracking a single skyrmion with 80-nm in diameter and their cluster demonstrated that the J_C for drives of skyrmions in the thin FeGe is an order of 10⁹ A/m², three orders smaller than that for drives of magnetic domain walls in ferromagnetic materials.

(a) Skyrmion (colored vortices) Hall motion driven by electric currents. Longer arrows show current flow directions. (b-d) Lorentz TEM images of skyrmion Hall motion companying a rotation of skyrmion cluster with current-pulse excitations. (e-g) Lorentz TEM images of tracking a single skyrmion and a skyrmion cluster after several pulse-current excitations.

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 Mn_1.4Pt_0.9Pd_0.1Sn (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 Mn_1.4Pt_0.9Pd_0.1Sn. (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-T_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 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 in–situ 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.