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

TL;DR

The study addresses how to control density, ordering, and topology of dipolar-stabilized spin textures in Fe/Gd ferrimagnetic multilayers by engineering the initial stripe-domain state. It combines in-plane set-field preconditioning, out-of-plane field ramps, time-resolved Kerr effect measurements, LTEM/MFM imaging, and micromagnetic simulations to link static domain configurations with dynamic breathing behavior. The key findings show that an in-plane prealignment yields a dense, near-hexagonal skyrmion lattice and substantially enhances the breathing mode frequency and amplitude (e.g., a shift of about $\\Delta f \approx 0.2$ GHz), with Bloch-point–mediated transformation of bubbles into skyrmions driving the dynamics. This work provides a practical route to actively control density and topology of spin textures in multilayer films, with implications for magnonics and topology-based data encoding.

Abstract

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.

Figures (7)

• Figure 1: Ground state magnetic spin textures hosted by the [Fe($0.35$ nm)/Gd($0.40$ nm)]$_{120}$ multilayer film for LTEM studies. At low magnetic fields, the system exhibits two distinct types of stripe domains: chiral and non-chiral stripe domains. At high out-of-plane magnetic fields, the chiral stripe domains have been transformed into clockwise (CW) and counterclockwise (CCW) skyrmions (orange), while topologically trivial bubbles have emerged from the non-chiral ones (gray).
• Figure 2: The effect of initial in-plane magnetic set fields on the coherent magnetization dynamics.a) Laser-induced magnetization dynamics using an out-of-plane magnetic field $\mu_0H_{\mathrm{oop}}=196$ mT. In the case of the red squares, an in-plane magnetic set field $\mu_0H_{\mathrm{ip}}=151$ mT was switched on prior to the measurement to align the demagnetized stripe domain state. b) Fourier analysis of the breathing mode frequency for various in-plane magnetic set fields which were applied prior to the constant out-of-plane magnetic field $\mu_0H_{\mathrm{oop}}=196$ mT.
• Figure 3: The effect of in-plane magnetic set fields on the magnetic spin textures.a) Magnetic spin textures obtained from micromagnetic simulations, LTEM, and MFM considering various out-of-plane magnetic fields as given in the center of the image. b) An in-plane magnetic set field was applied prior to increasing the out-of-plane magnetic field to align the stripe domain state. For the region shown in the LTEM images, which is approximately the same for all images, membrane buckling effects are minimized. The LTEM measurements were carried out on a [Fe($0.35$ nm)/Gd($0.40$ nm)]$_{120}$ sample, while the MFM measurements and micromagnetic simulations were performed on the [Fe($0.35$ nm)/Gd($0.40$ nm)]$_{160}$ sample which was used for the optical experiments.
• Figure 4: Coherent magnetization dynamics obtained from micromagnetic simulations.a) Coherent magnetization dynamics after 1 and 8 excitations of the initial state created without applying an in-plane magnetic field and b) of the initial state for which an in-plane magnetic set field has been applied prior to increasing the out-of-plane magnetic field.
• Figure 5: Transformation of topologically trivial bubbles into non-trivial skyrmions.a) The initial state in the simulation is created by briefly applying an in-plane magnetic set field of $\mu_0 H_{\mathrm{ip}}=50$ mT before increasing the out-of-plane magnetic field to $\mu_0 H_{\mathrm{oop}}=220$ mT. Most of the topologically trivial bubbles (highlighted by gray circles) transform into non-trivial skyrmions (orange circles) upon laser excitation. b) 3D representation of two topologically trivial bubbles. While the bubble in the upper panel remains unchanged, the bubble in the lower panel transforms into a non-trivial skyrmion by introducing a Bloch point at around 0.5 ns after laser excitation. The transformation process is completed at approximately 1.5 ns.