Direct Demonstration of Topological Stability of Magnetic Skyrmions via Topology Manipulation

Abstract

Topological protection precludes a continuous deformation between topologically inequivalent configurations in a continuum. Motivated by this concept, magnetic skyrmions, topologically nontrivial spin textures, are expected to exhibit the topological stability, thereby offering a prospect as a nanometer-scale non-volatile information carrier. In real materials, however, atomic spins are configured as not continuous but discrete distribution, which raises a fundamental question if the topological stability is indeed preserved for real magnetic skyrmions. Answering this question necessitates a direct comparison between topologically nontrivial and trivial spin textures, but the direct comparison in one sample under the same magnetic fields has been challenging. Here we report how to selectively achieve either a skyrmion state or a topologically trivial bubble state in a single specimen and thereby show how robust the skyrmion structure is in comparison with the bubbles for the first time. We demonstrate that topologically nontrivial magnetic skyrmions show longer lifetimes than trivial bubble structures, evidencing the topological stability in a real discrete system. Our work corroborates the physical importance of the topology in the magnetic materials, which has hitherto been suggested by mathematical arguments, providing an important step towards everdense and more-stable magnetic devices.

Current‐Controlled Topological Magnetic Transformations in a Nanostructured Kagome Magnet

“…Owing to different formation mechanisms of skyrmions and bubbles, skyrmions and bubbles are the only stable phases at normal field and large tilted field, respectively. [ 26–32 ] Therefore, for the Fe 3 Sn 2 thin plate, the controlled current‐induced skyrmion−bubble transformations can be realized in an intermediate tilted angular range ( Figure a), such as from ≈0.8° to 3.0° at B ≈ 500 mT. Although skyrmions can be metastable phases even at zero field, bubbles merge together to form a long stripe domain in the low field region at B ≤ 400 mT.…”

“…[ 12 ] The switching of topological magnetic charge can be realized by transformations between skyrmions with an integer charge Q and topologically trivial magnetic states with zero Q , such as ferromagnet, [ 13–17 ] helix or stripes, [ 18–25 ] and bubbles. [ 26–33 ] Previous studies have realized the current‐induced skyrmion‐ferromagnet transformations. [ 34,35 ] However, thus far, current‐induced skyrmion−stripe (or helix) and skyrmion−bubble transformations which occur along with the manipulation of topological charges in a reproducible and reversible way have not been experimentally demonstrated.…”

“…The uniaxial magnetic crystal Fe 3 Sn 2 possesses two types of localized magnetic objects, i.e., topologically nontrivial skyrmion bubbles with an integer charge Q and topologically trivial bubbles with zero‐ Q . [ 26–28 ] Because skyrmion bubbles are topologically equivalent to chiral skyrmions, [ 26–32 ] the two classes of magnetic textures share the same electromagnetic properties, which are determined by topology Q . Thus, skyrmion bubbles are also called skyrmions in achiral uniaxial ferromagnets.…”

“…Similar multi‐ Q magnetic objects are also observed in antiskyrmion‐hosting Mn 1.4 Pt 0.9 Pd 0.1 Sn and Fe 1.9 Ni 0.9 Pd 0.2 P alloys. [ 36,37 ] However, previous magnetic‐field‐controlled topological magnetic transformations among these multi‐ Q objects are not compatible for future electrically controlled devices, [ 31,32,36,37 ] therefore, current‐controlled transformations remain elusive.…”

“…Thus, skyrmion bubbles are also called skyrmions in achiral uniaxial ferromagnets. [ 30–33 ] Here, for simplicity, skyrmion bubbles and topologically trivial bubbles in Fe 3 Sn 2 are named skyrmions and bubbles, respectively. Similar multi‐ Q magnetic objects are also observed in antiskyrmion‐hosting Mn 1.4 Pt 0.9 Pd 0.1 Sn and Fe 1.9 Ni 0.9 Pd 0.2 P alloys.…”