Dynamical Defects in Rotating Magnetic Skyrmion Lattices

Abstract

The chiral magnet Cu2OSeO3 hosts a skyrmion lattice, that may be equivalently described as a superposition of plane waves or lattice of particle-like topological objects. A thermal gradient may break up the skyrmion lattice and induce rotating domains raising the question which of these scenarios better describes the violent dynamics at the domain boundaries. Here we show that in an inhomogeneous temperature gradient caused by illumination in a Lorentz Transmission Electron Microscope different parts of the skyrmion lattice can be set into motion with different angular velocities. Tracking the time dependence we show that the constant rearrangement of domain walls is governed by dynamic 5-7 defects arranging into lines. An analysis of the associated defect density is described by Frank's equation and agrees well with classical 2D-Monte Carlo simulations. Fluctuations of boundaries show surge-like rearrangement of skyrmion clusters driven by defect rearrangement consistent with simulations treating skyrmions as point particles. Our findings underline the particle character of the skyrmion.In the last decade a non collinear topological spin texture, the skyrmion, has attracted great attention representing a new type of topological soliton in magnetic materials. Skyrmion lattices are periodic arrangements of a kind of magnetic whirls that may be found in a great variety of chiral magnets [1][2][3][4][5][6][7][8][9][10][11][12], as well as thin magnetic (multi-) layers [13][14][15][16]. The topology of skyrmions is encoded in a quantized winding number of the spin orientation. Emergent magnetic and electric fields describe the efficient coupling of the topological spin texture to electrons and magnons [17][18][19][20]. Skyrmion lattices may be described by two distinct approaches. In a wave-like picture as suggested e.g. by Small-Angle-Neutron Scattering (SANS) measurements, the skyrmionic crystal may be accounted for by a superposition of three spin helices with their propagation vector rotated by 120• with respect to each other. From this point of view, the individual (solitonic) character of single skyrmions vanishes in the collective. Note that in most materials in the skyrmion lattice phase, higher order scattering is basically absent; for MnSi it is of the order of 10 −4 suggesting a rather smooth spin texture [21]. In contrast, in a particle-like picture skyrmions are viewed as individual solitonic particles. Indeed, individual skyrmions have been observed early on [6, 13] but the non-linear character as well as the degree of the particlecharacter in the skyrmion phase remained unresolved. In fact, recent studies reveal strong deformation of the precise shape of skyrmions under large strain [22].The validity of either approach may be tested critically in studies of imperfect skyrmion lattices, alluding to similarities with well-known atomic lattices which also allows to verify particle conservation. In fact, the existence of defects and domains in skyrmion lattices has been reported [23][24][25][26], however,...

Reorientations, relaxations, metastabilities, and multidomains of skyrmion lattices

“…These processes possibly reflect the movement of (topological) domain walls and/or their pinning to defects of the crystallographic lattice [36]. Slow relaxations that originate from multiple processes have also been observed around the A phase by ac magnetic susceptibility measurements in Fe 1−x Co x Si [44].…”

Shear banding, intermittency, jamming, and dynamic phases for skyrmions in inhomogeneous pinning arrays

“…Another interesting effect is that shear banding could arise for driving either parallel or perpendicular to the inhomogeneous pinning regions due to the Magnus force. There have been some studies of skyrmions under inhomogeneous drives in the absence of pinning which produced evidence for rigid flow, disordered flow, and shear banding effects [76][77][78] .…”

Stabilization of the skyrmion crystal phase and transport in thin-film antiferromagnets

“…Introduction.-Skyrmions are the quintessence of spatially localized solitons in quasi-two-dimensional (quasi-2D) spin systems [1]. These magnetic textures exhibit a particlelike behavior [2,3], carry topological charge and are protected against structural distortions and moderate external perturbations [4]. Skyrmions arise in magnetic systems with broken inversion symmetry and spin-orbit coupling [5,6], and have been observed in a plethora of ferromagnetic materials [7][8][9][10].…”