Real-space imaging of confined magnetic skyrmion tubes

Abstract

Magnetic skyrmions are topologically nontrivial particles with a potential application as information elements in future spintronic device architectures 1, 2 . While they are commonly portrayed as two dimensional objects, in reality magnetic skyrmions are thought to exist as elongated, tube-like objects extending through the thickness of the sample 3, 4 . The study of this skyrmion tube (SkT) state is highly relevant for investigating skyrmion metastability 5 and for implementation in recently proposed magnonic computing 6 . However, direct experimental imaging of skyrmion tubes has yet to be reported. Here, we demonstrate the first real-space observation of skyrmion tubes in a lamella of FeGe using resonant magnetic x-ray imaging and comparative micromagnetic simulations, confirming their extended structure.The formation of these structures at the edge of the sample highlights the importance of confinement and edge effects in the stabilisation of the SkT state, opening the door to further investigations into this unexplored dimension of the skyrmion spin texture.Skyrmion states are typically stabilised by the interplay of the ferromagnetic exchange and Zeeman energies with the Dzyalohsinskii-Moriya Interaction (DMI) 7 . In ferromagnet/heavy metal multilayer thin films, interfacial DMI is induced by symmetry-breaking spin-orbit coupling at the interface between the layers, leading to the formation of Néel-type skyrmions [8][9][10] . Bulk DMI, arising due to the lack of centrosymmetry in the underlying crystal lattice, is responsible for the formation of Bloch-type skyrmions in a range of chiral ferromagnets [11][12][13][14][15] . In crystals of these bulk materials the skyrmion state is typically only at equilibrium in a limited range of applied magnetic field and temperature just below the Curie temperature, T c , forming a hexagonal skyrmion lattice (SkL) in a plane perpendicular to the applied magnetic field.2 Figure 1 | Visualisation of the skyrmion tube spin texture. Three dimensional visualisation of three magnetic skyrmion tubes from the micromagnetic simulations presented in this paper, illustrating their extended spin structure. The inset highlights the location of the magnetic Bloch point at the end of each skyrmion tube. 3The three dimensional visualisation in Fig. 1 depicts the extended spin structure of three magnetic skyrmion tubes. The dynamics of this skyrmion tube (SkT) state play an important role in the creation and annihilation of skyrmions. For example, metastable skyrmions, which are created beyond the equilibrium thermal range by rapid field cooling 16 , are thought to unwind into topologically trivial magnetic states through the motion of a magnetic Bloch point located at the end of each individual skyrmion tube 3, 5 . Real-space observation of this dimension of the SkT state and its associated dynamics requires an in-plane magnetic field applied perpendicular to the imaging axis. Electron imaging techniques such as Fresnel Lorentz Transmission Electron Microscopy (LTEM) 12, 13 , and elec...

Bulk and surface topological indices for a skyrmion string: current-driven dynamics of skyrmion string in stepped samples

“…In the three dimensional magnets, the skyrmion usually forms rod-like object along the external magnetic field [14][15][16][17][18][19][20][21][22][23][24][25] . When the meandering degree of freedom is introduced, it is better to consider it as skyrmion string.…”

Controlling bimerons as skyrmion analogues by ferroelectric polarization in 2D van der Waals multiferroic heterostructures

“…A magnetic skyrmion, usually a few nanometers in diameter, is a topologically protected magnetic quasi-particle with a whirling spin texture in real space 1 . Stable magnetic skyrmions, emerging in various magnetic materials [1][2][3][4][5][6] as a result of competing Heisenberg exchange and Dzyaloshinskii-Moriya interaction (DMI) 7 , can be driven by low current density [8][9][10][11][12][13] . The creation and annihilation of magnetic skyrmions in thin films have been demonstrated by current 14,15 , marking a significant step toward low-dimensional skyrmion systems.…”

Metastable solitonic states in the strained itinerant helimagnet FeGe

“…Surprisingly, the q-vector length of the skyrmion texture tends to increase with the increment of the magnetic field [ Fig. 6(b)], which is opposite to the tendency for the SkX, isolated skyrmions, or in-plane skyrmion tubes in the conical/ferromagnetic background [44][45][46]. By contrast, application of the in-plane field does not influence the magnitude of the CSL wave vector [ Fig.…”

“…Contributions of more exotic three-dimensional spin textures, such as stacked spirals [52], chiral bobbers [53][54][55], quasimonopoles [56], and in-plane skyrmion tubes [46,57,58], to the scattering patterns are another possible explanation of this result. Further real-space investigations of the possible three-dimensional states using cryogenic coherent [36,59] or focused [46] soft-x-ray methods will help to clarify this question.…”