Stability of (Active) Bilayer Skyrmions in Synthetic Antiferromagnets

Abstract

Synthetic antiferromagnetic (SAF) skyrmions are nanoscale composite textures that exhibit high-speed, Hall-free current-driven motion and recently demonstrated self-propulsion. These remarkable properties rely on the stability of the SAF skyrmion's topological bound state, whose underlying mechanisms remain unclear. Here, using an atomistic spin model, we analyze the collapse pathways of bilayer SAF skyrmions in homochiral systems, where both ferromagnetic layers share the same Dzyaloshinskii-Moriya interaction (DMI) vectors, and in heterochiral systems, where the DMI vectors have opposite directions. We find that pair destruction occurs either by decoupling or by sequential collapse into the homogeneous antiferromagnetic state, so the activation energy is set by the smaller of these two barriers. By examining how these barriers vary with DMI strength, anisotropy, magnetic field, and interlayer exchange, we identify regimes of enhanced stability. In particular, increasing interlayer coupling strengthens homochiral skyrmions but weakens heterochiral ones, while reducing the anisotropy constant effectively stabilizes heterochiral SAF skyrmions. These results outline viable strategies to optimize SAF heterostructures for enhanced skyrmion stability in racetrack devices and emerging active skyrmionic systems.

Figures (10)

• Figure 1: Schematic illustration of (a) homochiral and (b) heterochiral synthetic antiferromagnets, where the spin chirality of each FM layer (indicated by the circular arrows) is induced by interfacial DMI, which changes sign depending on the ferromagnet--heavy-metal (FM-HM) stacking order. (c)-(e) Cross-sectional view of the spin configuration for coaxial (core) binding in the (c) homochiral and (d) heterochiral cases, and for non-coaxial (domain-wall) binding (e).
• Figure 2: (a) MEPs for the collapse of an isolated SAF skyrmion for different values of the interlayer exchange coupling, $J_\text{int}$. The reaction coordinate, $x$, defines the normalized (geodesic) displacement along the collapse path, with $x=0$ representing the SAF skyrmion state and $x=1$ the homogeneous saturated SAF state. (b) Activation energy, $E_a$, for the collapse of the first skyrmion as a function of $J_\text{int}$. (c) Snapshots of the magnetic configurations at each layer along the MEP, for $J_\text{int}=0.2$ meV. The fist and second saddle-point (SP) configurations are indicated.
• Figure 3: (a,c,e) MEPs for the collapse of a SAF skyrmion, calculated for $J_\text{int}=0.2$ meV and different values of the DMI strength $D$ (a), anisotropy constant $K$ (c), and applied field $B$ (e). Reference values are indicated by the red asterisk. (b,d,f) Activation energy $E_a$ for the collapse of the first skyrmion as a function of $D$ (b), $K$ (d), and $B$ (f), calculated for $J_\text{int}=0.2$, $0.4$, and $1.0$ meV.
• Figure 4: (a,b) Magnetic phase diagrams obtained from simulations for a bilayer SAF film as a function of the interlayer exchange coupling $J_\text{int}$ and (a) the DMI ratio $D_2/D_1$ between the two layers, for $D_1 = 1.5$ meV and $K=0.279$ meV, and (b) the anisotropy constant $K$ for the case $D_2 = -D_1 = -1.5$ meV. (c) Corresponding snapshots of the stabilized magnetic configurations within the phase diagram for both layers of the sample. Colors represent the $z$-component of the magnetization, as in Fig. \ref{['fig1']}(c).
• Figure 5: (a) Skyrmion radius $R$, (b) domain wall width $w$, and (c) $R/w$ ratio as a function of perpendicular anisotropy $K$ for a bilayer skyrmion in symmetric heterochiral SAFs with different interlayer exchange constants $J_\text{int}$and $D_2=-D_1=-1.5$ meV. Error bars indicate standard errors of the fitting parameters $R$ and $w$ and propagated errors of $R/w$. The arrows in (c) indicate for each $J_\text{int}$ the upper limit of the coaxial arrangement, which is followed by the non-coaxial configuration. The dashed line indicates the $R=2w$ condition below which the estimated domain-wall frustration energy of the coaxial configuration exceeds the AFM core energy (see text).