Real-Space Observation of Topological Defects in Extended Skyrmion-Strings

Abstract

Exotic topological spin textures such as emergent magnetic monopole/anti-monopoles (hedgehog/anti-hedgehog) in the metastable extended skyrmion-strings attract much attention to the fundamental physics owing to their novel electromagnetic properties. However, the direct imaging of such spin textures is lacking. Here, we report the real-space observation of emergent magnetic monopoles involved in extended skyrmion-strings by Lorentz transmission electron microscopy (TEM) in combination with micromagnetic simulations. The in-plane extended skyrmionstrings are observed directly by Lorentz TEM to accompany the topological hedgehog-like defect, where the skyrmion-string terminates or merges with another skyrmion-string, as well as the surface-related defects where skyrmion-string bends 90°and ends on the surface. We also demonstrate the transformation of a metastabilized lattice of out-of-plane short skyrmion-strings into an in-plane array of extended skyrmion-strings by tuning the magnitude of oblique fields in a room-temperature helimagnet, revealing the stability of such topological spin textures and the possibility to control them.

Tunable Néel–Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure

“…Previous Lorentz TEM observations have shown that in‐plane bias fields align the q ‐vector of Néel‐type helices perpendicular to the field direction in multilayered thin films, [ 18 ] while changing that of the Bloch‐type helices parallel to the field direction in the chiral magnet FeGe. [ 19–21 ] The results suggest that the external in‐plane field can induce a 90°‐rotation difference of q vectors for Néel and Bloch‐type twists in DMI‐stabilized systems. In addition, in‐plane fields align skyrmion strings along the field direction in a thick FeGe thin plate relative to its periodicity.…”

In‐Plane Magnetic Field‐Driven Creation and Annihilation of Magnetic Skyrmion Strings in Nanostructures

“…[ 8,11,15 ] The actualization of these unique topological features in real materials has pushed the investigation towards the less explored dimension along the length of SkS. [ 8–12 ]…”

“…[1][2][3][4] Although magnetic skyrmions are commonly illustrated as two-dimensional spin textures, [5][6][7] they naturally possess a 3D structure resembling elongated strings extending throughout the thickness of the sample up to several millimeters in length which align along the direction of the applied magnetic field. [8][9][10][11][12] These 3D spin textures known as magnetic skyrmion strings (SkS) are analogous to the vortex lines in superfluids [13] and type II superconductors, [14] manifest currentinduced non-linear dynamics and characteristics of magnonic nano-waveguides. [9,10] The broken inversion symmetry in the crystal lattice induces a Dzyaloshinskii-Moriya interaction (DMI) energy term that stabilizes SkS.…”

“…[8,11,15] The actualization of these unique topological features in real materials has pushed the investigation towards the less explored dimension along the length of SkS. [8][9][10][11][12] Crystals of chiral helimagnets with the non-centrosymmetric cubic B20 crystal structure can host SkS and form a periodic hexagonal lattice arrangement in a narrow and restricted window of applied magnetic field and temperature in the magnetic phase diagram (H-T plot) just below their respective Curie temperature, T c . [1,5,11,[15][16][17][18] Nanostructures of skyrmion-hosting materials have been shown to thermodynamically stabilize SkS phase more easily than bulk and thin film systems.…”

Magnetic skyrmion braids

“…These skyrmions are topologically nontrivial textures of the magnetization unit vector field m ( r ), whose filamentary structures resemble vortex-like strings or tubes, with typical diameters of tens of nanometers. The length of such a string is assumed to be limited only by the shape and size of the sample 19 , with direct observations confirming the formation of micrometer-long skyrmion strings 20 , 21 . Previous studies have suggested that magnetic skyrmion strings are (nearly) straight 19 – 25 .…”