Observation of domain wall bimerons in chiral magnets

Abstract

Topological defects embedded in or combined with domain walls have been proposed in various systems, some of which are referred to as domain wall skyrmions or domain wall bimerons. However, the experimental observation of such topological defects remains an ongoing challenge. Here, using Lorentz transmission electron microscopy, we report the experimental discovery of domain wall bimerons in chiral magnet Co-Zn-Mn(110) thin films. By applying a magnetic field, multidomain structures develop, and simultaneously, chained or isolated bimerons arise as the localized state between the domains with the opposite in-plane components of net magnetization. The multidomain formation is attributed to magnetic anisotropy and dipolar interaction, and domain wall bimerons are stabilized by the Dzyaloshinskii-Moriya interaction. In addition, micromagnetic simulations show that domain wall bimerons appear for a wide range of conditions in chiral magnets with cubic magnetic anisotropy. Our results promote further study in various fields of physics.

Controllable inter-skyrmion attractions and resulting skyrmion-lattice structures in two-dimensional chiral magnets with in-plane anisotropy

“…The small-sized (∼ 2 nm in diameter) skyrmions observed in these materials draw attention not only for the novel mechanism of stabilizing skyrmions but also for possible applications to high-density integration of magnetic storage. We also note that anti-skyrmions with charge N = −1 [32,33] and merons with charge N = 1/2 [34,35], in addition to the Bloch and Néel skyrmions with charge N = 1, have been demonstrated.…”

“…We further investigate the SkX configurations and find unconventional states associated with the attractive couplings: a bimeron lattice formed on a background stripe domain pattern in the ground state and a one-dimensional (1D) skyrmion chain as an excitation in the FM phase. Domain wall skyrmions and bimerons are already discussed and observed in the previous works [35,60,61]. We here survey the optimal lattice structures in detail as a function of the external magnetic field and the strength of the magneto-crystalline anisotropy.…”

“…From our calculations, |A|/B cr2 ≥ 0.5 is required to observe the large attractive interaction and the domain wall skyrmions. The observed values in real materials for the ratio are |A|/B cr2 ∼ 0.00364 in a Cu 2 OSeO 3 thin film at 5 K [15,68,69], |A|/B cr2 ∼ 0.385 in a Fe 0.7 Co 0.3 Si thin film at 5 K [70,71], and |A|/B cr2 ∼ 1.59 in a Co 8.5 Zn 7.5 Mn 4 thin film at 330 K [35,39]. In the last material, the domain wall skyrmions (or bimerons) indeed appear in a thin film with the thickness ∼ 50 nm.…”

“…Strictly speaking, the topological object that appears on a domain wall in Region (v) is not a skyrmion but a bimeron [35]. Here, a bimeron is a pair of merons and has the same topological charge as a skyrmion.…”

“…Based on the analytical expression, we consider two anisotropic effects that deform the skyrmion shape, (i) an in-plane magnetic field and (ii) the magneto-crystalline anisotropy, and numerically demonstrate that the inter-skyrmion attractions indeed appear. In general, the magnetocrystalline anisotropy depends on the crystal plane direction to the film [6,34,35,39]. We consider a (011) film to break the symmetry in spin space to create distorted skyrmions.…”