Topological magnetic textures, characterized by their topological charge (Q), have garnered significant attention due to their particle-like properties and potential for technological applications. Magnetic skyrmions (SKs), with integer Q values, are particularly interesting due to their topological protection – their spin texture cannot be continuously transformed into a saturated ferromagnetic state without singularities. Two primary mechanisms stabilize SKs: short-range interactions (e.g., in chiral magnets, requiring cryogenic temperatures and single-crystal growth) and the competition between long-range dipolar and short-range exchange interactions (in thin ferrimagnetic films with perpendicular magnetic anisotropy, enabling room-temperature fabrication). While high-Q SKs (those with |Q|>1) promise novel physics and enhanced applications, their observation has been limited. This study focuses on directly observing and characterizing dipolar SKs and antiskyrmions (ASKs) with arbitrary topological charge at room temperature, opening new avenues for skyrmionics research and device development.

Prior research extensively explored SKs with Q = -1 in chiral magnets, arising from the interplay of exchange and Dzyaloshinskii-Moriya interactions. However, these systems often require cryogenic temperatures and are challenging to scale industrially. Studies on dipolar SK stabilization in thin films are more recent, demonstrating the coexistence of SKs (Q = -1), bubbles (Q = 0), and ASKs (Q = 1) under specific material parameters. Reports of higher-order ASKs (Q = 2) in Fe/Gd multilayers also exist but were limited in size (<200 nm). Theoretical work described extra Bloch and Néel wall iterations in dipolar-stabilized hard magnetic bubbles, though their size typically exceeded typical dipolar-SK sizes. This study builds on these prior findings by investigating high-order SKs and ASKs with significantly larger topological charges at room temperature.

The authors fabricated a series of [Co(0.2 nm)/Ni(0.7 nm)]n multilayers with varying bilayer repetition numbers (n = 4–11) using dc magnetron sputtering. These multilayers exhibited perpendicular magnetic anisotropy. Magnetic textures were investigated using Lorentz transmission electron microscopy (LTEM) at room temperature, which is sensitive to in-plane magnetic induction. Accompanying micromagnetic simulations, performed using the magnum.np code, provided detailed characterization of the magnetic solitons and their topological charge. The topological charge was calculated using a contour integral of the azimuthal angle of magnetization. SQUID-VSM and ferromagnetic resonance measurements were used to determine material parameters such as saturation magnetization (Ms) and uniaxial magnetic anisotropy (K). Micromagnetic simulations were also used to investigate the current-induced motion of spin objects via spin-transfer torque, applying a constant current density and out-of-plane bias field.

The study directly observed dipolar SKs and ASKs with topological charges ranging from Q = -5 to Q = 5, and even up to |Q| = 10 in some cases. LTEM images revealed a variety of coexisting spin objects with different charges. The size of these objects increased with topological charge, but remained below 500 nm. Micromagnetic simulations confirmed the existence of these high-order (A)SKs, showing that they are enclosed by a single domain boundary with multiple Bloch and Néel segments. The energy of (A)SKs increased nearly linearly with topological charge at a fixed magnetic field. The study found that high-order spin objects nucleate from domain walls containing vertical Bloch lines (VBLs). As the out-of-plane magnetic field increased, domain walls with numerous VBLs shrunk, ultimately leading to isolated high-order (A)SKs. The stability of high-order (A)SKs depended on material parameters (Ms and K). A phase diagram indicated that a magnetic quality factor Ku/Ka ≈ 1 is crucial for the coexistence of stable solutions with different topological charges. Finally, simulations demonstrated current-driven motion of these spin objects, showing a reduction in the Skyrmion Hall effect with increasing topological charge.

This work significantly advances the understanding of skyrmion stability and dynamics. The observation of high-order SKs and ASKs at room temperature in readily fabricated Co/Ni multilayers opens exciting possibilities for technological applications. The dependence of stability on material parameters allows for tailoring of the system to optimize the presence of desired topological charges. The reduced Skyrmion Hall effect observed in higher-order spin objects is particularly significant for device applications requiring controlled current-driven motion. The study’s findings also provide valuable insights into the nucleation process, suggesting that controlling domain wall structures and VBLs could be a pathway to manipulate the formation of specific topological textures.

This study presents the first observation, to the authors’ knowledge, of high-order SKs and ASKs with Q values up to 10 in Co/Ni multilayers at room temperature. These structures originate from domain walls with VBLs and their stability is governed by the material’s magnetic anisotropy and saturation magnetization. The reduced Skyrmion Hall effect in higher-order objects suggests promising implications for skyrmionic devices. Future research could focus on further exploring the parameter space for optimizing high-order (A)SK stability, investigating their interactions, and developing novel device concepts based on these findings.

While the study provides a comprehensive analysis, some limitations exist. The LTEM contrast did not allow confident classification of all observed spin objects. The micromagnetic simulations used effective temperature-dependent material parameters instead of explicit thermal fluctuations. Future work could utilize more sophisticated methods to address these aspects and investigate the impact of thermal fluctuations on stability and dynamics.