Collective antiskyrmion-mediated phase transition and defect-induced melting in chiral magnetic films

Abstract

Magnetic phase transitions are a manifestation of competing interactions whose behavior is critically modified by defects and becomes even more complex when topological constraints are involved. In particular, the investigation of skyrmions and skyrmion lattices offers insight into fundamental processes of topological-charge creation and annihilation upon changing the magnetic state. Nonetheless, the exact physical mechanisms behind these phase transitions remain unresolved. Here, we show numerically that it is possible to collectively reverse the polarity of a skyrmion lattice in a field-induced first-order phase transition via a transient antiskyrmion-lattice state. We thus propose a new type of phase transformation where a skyrmion lattice inverts to another one due to topological constraints. In the presence of even a single defect, the process becomes a second-order phase transition with gradual topological-charge melting. This radical change in the system’s behavior from a first-order to a second-order phase transition demonstrates that defects in real materials could prevent us from observing collective topological phenomena. We have systematically compared ultra-thin films with isotropic and anisotropic Dzyaloshinskii-Moriya interactions (DMIs), and demonstrated a nearly identical behavior for such technologically relevant interfacial systems.

Spontaneous creation and annihilation dynamics and strain-limited stability of magnetic skyrmions

“…This antiskyrmion also fulfils topological charge conservation, at the precise moment of skyrmion merging. The reverse mechanism is similar to one suggested for skyrmion lattice inversion during polarity switching of an applied external magnetic field, whereby antiskyrmions are formed at the point of skyrmion core merging 18 .…”

“…Therefore, the results may only give approximations to the true pathway. We chose ultra-thin films at zero temperature in order for the simulations to be valid, given the thickness-temperature-field phase space of B20 FeGe 20 . Finally, the micromagnetic simulations were performed to support insight into the mechanistic pathways which are currently inaccessible experimentally due to timescales.…”

Magnetic Bloch-point hopping in multilayer skyrmions and associated emergent electromagnetic signatures

“…Furthermore, in realistic conditions, disordered pinning sites which generally exist in sputtered films may widen the range of critical fields for skyrmion annihilation, or may energetically favour worm-like textures [28]. In such cases the system undergoes gradual metamagnetic transitions which are reflected in a smooth variation of ρ TH xy − B ext loops [28,53,54] rather than the sharp jumps shown in Fig. 4b.…”

Elliptical skyrmions: theory and nucleation by a magnetic tip in an antiskyrmion-hosting material

“…Karube et al 55 have also observed antiskyrmions in a chiral magnet with S 4 symmetry. Pierobon et al 56 have shown numerically that the application of a magnetic field to a skyrmion lattice can produce an "inverted" lattice phase by producing antiskyrmions. Directly relevant to this work, Jena et al 1 and Peng 2 have observed both antiskyrmions and elliptically distorted skyrmions in the material Mn 1.4 Pt 0.9 Pd 0.1 Sn which also has the D 2d crystal symmetry.…”