Magnetic stripes and skyrmions with helicity reversals

Abstract

It was recently realized that topological spin textures do not merely have mathematical beauty but can also give rise to unique functionalities of magnetic materials. An example is the skyrmion—a nano-sized bundle of noncoplanar spins—that by virtue of its nontrivial topology acts as a flux of magnetic field on spin-polarized electrons. Lorentz transmission electron microscopy recently emerged as a powerful tool for direct visualization of skyrmions in noncentrosymmetric helimagnets. Topologically, skyrmions are equivalent to magnetic bubbles (cylindrical domains) in ferromagnetic thin films, which were extensively explored in the 1970s for data storage applications. In this study we use Lorentz microscopy to image magnetic domain patterns in the prototypical magnetic oxide–M-type hexaferrite with a hint of scandium. Surprisingly, we find that the magnetic bubbles and stripes in the hexaferrite have a much more complex structure than the skyrmions and spirals in helimagnets, which we associate with the new degree of freedom—helicity (or vector spin chirality) describing the direction of spin rotation across the domain walls. We observe numerous random reversals of helicity in the stripe domain state. Random helicity of cylindrical domain walls coexists with the positional order of magnetic bubbles in a triangular lattice. Most unexpectedly, we observe regular helicity reversals inside skyrmions with an unusual multiple-ring structure.

A Centrosymmetric Hexagonal Magnet with Superstable Biskyrmion Magnetic Nanodomains in a Wide Temperature Range of 100–340 K

“…The formation of biskyrmions stems from the helicity degree of freedom (+ or -) of each skyrmion, which can be realized only in centrosymmetric magnets and not in chiral-lattice magnets with DM interactions. However, in contrast with the random polarity of skyrmions observed in another centrosymmetric magnet [21] , the polar direction of the biskyrmion described here is fixed in MnNiGa, which agrees well with previous observations in bilayered magnetites [22] .…”

“…The green and red regions shown in Figure 2b correspond to in-plane anti-parallel magnetic domains, while the black regions represent the out-of plane magnetic domains. The revolving behavior of the biskyrmions under a vertical magnetic field is similar to that reported previously [5,7,8,21,22] . The dynamics of formation and disappearance of the biskyrmions with increasing applied magnetic field were successfully recorded by using insitu Lorentz TEM (see Supplementary Movie, SI).…”

“…In addition to the observation of magnetic skyrmions in chiral magnets characterized by DM interactions, magnetic skyrmions have also been recently identified in centrosymmetric magnets with uniaxial magnetic anisotropy [21] . In particular, magnetic biskyrmions (or skyrmion molecules), a new type of skyrmion formed by two skyrmions of opposite spin helicities, have been found in a centrosymmetric bilayered magnetite [22] .…”

Creation of skyrmions and antiskyrmions by local heating

“…In chiral magnets, the sign of the DM interaction determines the helicity, for example, either f 0 ¼ p/2 or À p/2, whereas these two structures have the same energy in dipolar magnets. This fact enables richer skyrmion structures in the latter case compared with the former case 11 . The equilibrium properties of skyrmions as well as the current-driven dynamics of skyrmions have been recently studied intensively [12][13][14][15][16][17][18] .…”

Controlled nucleation of topological defects in the stripe domain patterns of lateral multilayers with perpendicular magnetic anisotropy

“…1,2 The actual domain pattern in a given PMA film can be very complex depending on material parameters, sample geometry, and magnetic and thermal history, [3][4][5] but a simple description of disordered stripe patterns can be achieved if the concept of topological defects within the two-dimensional (2D) periodic stripe magnetic structure is used. 6,7 Defects such as dislocations, 8 disclinations, 7 grain boundaries, 9 or even skyrmions 10,11 have been observed. These topological defects play an important role in magnetization reversal processes and magnetization dynamics of PMA materials [12][13][14] and, also, in the physics of phase transitions in 2D modulated phases.…”