Magnetic Bimeron Traveling on the Domain Wall

TL;DR

This paper investigates the statics and damping-like spin-orbit-torque–driven dynamics of Néel-type domain-wall bimerons (DWBMs) stabilized inside domain walls of in-plane magnets. It combines analytical Thiele balance with MuMax3 micromagnetic simulations in a bilayer ferromagnet/heavy metal system to capture current-driven motion and the role of domain walls as tracks. Two dynamical regimes emerge depending on spin-current polarization: along the domain wall (X polarization), the wall confines the bimeron and suppresses the skyrmion Hall effect; perpendicular to the wall (Y polarization), the domain wall drives rapid bimeron sliding by leveraging the skyrmion Hall effect, achieving mobility about $40\times$ that of skyrmions or bimeron solitons. DWBMs remain stable over a wide range of DMI values ($D_i \in [1,6]\,\mathrm{mJ/m^2}$), and damping reduces the effective Magnus force along the wall, enabling energy-efficient operation. The results suggest that the skyrmion Hall effect can be harnessed as a driving mechanism to enhance domain-wall-based spintronic devices.

Abstract

Domain wall bimerons (DWBMs) are nanoscale spin textures residing within the magnetic domain walls of in-plane magnets. In this study, we employ both numerical and analytical methods to explore the stabilization of Néel-type domain wall bimerons and their dynamics when excited by spin-orbit torque. Our findings reveal two unique and intriguing dynamic mechanisms, which depend on the polarization direction of the spin current: In the first scenario, the magnetic domain wall serves as a track that confines the motion of the bimeron and effectively suppresses the skyrmion Hall effect. In the second scenario, pushing the magnetic domain wall triggers a rapid sliding of the bimeron along the wall. This process significantly enhances the dynamics of the bimeron, resulting in a velocity increase of approximately 40 times compared to skyrmions and bimeron solitons. Our results highlight the potential advantages of the skyrmion Hall effect in developing energy-efficient spintronic devices based on domain wall bimerons.

Figures (5)

• Figure 1: Structure and statics of the ferromagnetic DWBM with topological number Q = 1. Distribution of (a) spin vectors, (b)-(d) the magnetization components $m_x$, $m_y$, and $m_z$ and the (e) topological charge density, respectively. The color scales represent the quantities noted in bottom right. (f) Total energy of four different magnetic states varies with the Dzyaloshinskii-Moriya interaction constant $D_i$. The energy of the system is recorded after the magnetization is fully relaxed. The inset shows the initial state used for simulation, and the color scale represents the magnetization component $m_z$.
• Figure 2: Dynamic mechanism of DWBM driven by spin current with X polarization. (a) Schematic diagram of the equivalent forces involved, including the damping force of the domain wall $\boldsymbol{F}_{\alpha,\text{DW}}$, and the bimeron $\boldsymbol{F}_{\alpha,\text{BM}}$, the Magnus force $\boldsymbol{F}_G$ and the driving force from SOT $\boldsymbol{F}_{\text{SOT}}$. The red part and the blue part denote the domain wall and the bimeron, respectively. The blue and green arrows in the right corner represent the direction of the DWBM motion and the current polarization, while the red dot indicates the domain wall is static. (b) Distribution of the force density components of SOT, $\rho_{\text{SOT},i}$, and magnetic damping, $\rho_{\alpha,i}$, with the applied current density $j_c$= 10$^{10}$A/m$^2$. The black arrows indicate the direction of the resultant force.
• Figure 3: Bimeron velocities introduced by spin current with X polarization. (a) Velocities as functions of current density $j_c$ with damping constant $\alpha = 0.3$ (red) and $0.2$ (blue). (b) Velocities as functions of $\alpha$, with $j_c$= 10$^{10}$A/m$^2$.
• Figure 4: Dynamic mechanism of DWBM driven by the spin current with Y polarization. (a) Schematic diagram of the equivalent forces involved. Note that the SOT forces for the domain wall and DWBM have opposite directions. The blue, red, and green arrows in the right corner demonstrate the direction of the DWBM motion, the domain wall motion, and the current polarization, respectively. (b) Distribution of the force density components of SOT with the applied current density $j_c$= 10$^{10}$A/m$^2$. The damping force densities remain the same with Figure \ref{['FIG2']}(b).
• Figure 5: DWBM velocities introduced by spin current with Y polarization. (a) Velocities as functions of current density $j_c$ with damping constant $\alpha = 0.3$. $v_y$ manifests the skyrmion Hall effect of the bimeron. (b) Speed $|v|$ of domain wall bimeron driven by current with X (DWBMX) and Y (DWBMY) polarization, isolated bimeron soliton (BMS), and skyrmion (SK), as functions of $\alpha$, with $j_c$= 10$^{10}$A/m$^2$. For the simulation of skyrmion, we used perpendicular magnetic anisotropy and a DMI constant of 3 mJ/m$^2$, while the other parameters are the same as that of DWBM. The inset zooms in the bottom part of figure to show the speed of spin textures directly driven by SOT. Snapshots of (c) magnetization component $m_z$ and (d) topological charge density $q$ when current with $j_c$= 10$^{10}$A/m$^2$ is applied. The damping constant is set at 0.05.