Controlling bubble and skyrmion lattice order and dynamics via stripe domain engineering in ferrimagnetic Fe/Gd multilayers

Ferrimagnetic Fe/Gd multilayers host maze-like stripe domains that transform into a disordered bubble/skyrmion lattice under out-of-plane magnetic fields at ambient temperature. Femtosecond magneto-optics distinguishes these textures via their distinct coherent breathing dynamics. Crucially, applying a brief in-plane ``set’’ magnetic field to the stripe state enhances both frequency and amplitude of the bubble/skyrmion lattice breathing mode. Lorentz transmission electron microscopy, magnetic force microscopy, and micromagnetic simulations reveal that this enhancement arises from field-aligned stripes nucleating a dense, near-hexagonal bubble/skyrmion lattice upon out-of-plane field application, with strong indications for a pure skyrmion lattice. Thus, modifying the initial domain configuration by in-plane fields enables precise control of coherent magnetization dynamics on picosecond to nanosecond timescales and potentially even of topology.

💡 Research Summary

This paper investigates how the magnetic domain configuration in ferrimagnetic Fe/Gd multilayers can be deliberately engineered to control the formation, ordering, and dynamics of bubble and skyrmion lattices. In their as‑grown state, these multilayers exhibit maze‑like stripe domains that are either chiral (with a uniform sense of rotation across both domain walls) or non‑chiral (with opposite rotation senses). When an out‑of‑plane (OOP) magnetic field is applied, the chiral stripes evolve into clockwise (CW) or counter‑clockwise (CCW) Bloch‑type skyrmions (topological charge Q = ±1), while the non‑chiral stripes give rise to topologically trivial bubbles (Q = 0).

The authors introduce a brief in‑plane (IP) “set” magnetic field before ramping up the OOP field. This IP pulse aligns the stripe domains, forcing the domain walls to run parallel to the field direction. Lorentz transmission electron microscopy (L‑TEM) shows that after the IP field the stripes become uniformly oriented, and magnetic force microscopy (MFM) confirms a markedly higher density of cylindrical spin objects when the OOP field is subsequently increased. Micromagnetic simulations reproduce these observations: a modest IP field of ~50 mT is sufficient to produce a parallel‑aligned stripe pattern, which, under an OOP field of ~220 mT, nucleates a mixed bubble‑skyrmion lattice with a density about 20 % larger than in the unaligned case. Moreover, the simulations reveal that the initially generated bubbles are unstable under repeated laser excitation and convert into skyrmions, leading to a lattice that is close to a perfect hexagonal arrangement.

Time‑resolved femtosecond magneto‑optical Kerr effect (MOKE) measurements probe the coherent “breathing” dynamics of the lattice—periodic expansion and contraction of the core of each bubble or skyrmion. Without the IP set field, the breathing mode appears at a characteristic frequency (≈ 2 GHz) with modest amplitude. When the IP field is applied, the breathing frequency shifts upward by ~0.2 GHz and the amplitude increases dramatically. Fourier analysis identifies a threshold IP field of ~55 mT for this enhancement, and fields above ~100 mT produce a full transformation of the static texture. Dynamic micromagnetic simulations, which model the laser‑induced heating and subsequent magnetization dynamics, confirm that the higher density of skyrmions (as opposed to bubbles) accounts for the increased frequency and amplitude: skyrmions exhibit faster core dynamics than bubbles.

The combined experimental and computational evidence demonstrates that a simple, brief in‑plane magnetic pulse can pre‑condition the magnetic landscape, steering the system toward a dense, nearly pure skyrmion lattice. This “domain engineering” provides a powerful, low‑energy route to control both the topology (through the proportion of Q = ±1 objects) and the ultrafast dynamical response of the material. The findings have direct implications for skyrmion‑based spintronic and magnonic devices, where precise control of skyrmion density, ordering, and resonant frequencies is essential for applications such as high‑frequency oscillators, magnonic crystals, and neuromorphic computing elements. Future work will likely explore the optimization of IP pulse shape, temperature dependence, and multilayer thickness to further refine the control of topological spin textures in technologically relevant platforms.