Universal method for magnetic skyrmion bubble generation by controlling the stripe domain instability

Magnetic films with a quality factor exceeding 1 show perpendicular magnetic anisotropy (PMA) that favors magnetization in two directions (up or down) perpendicular to the film. Recently, the formation of magnetic topological defects in PMA systems has drawn much interest because topological defects can present a topologically protected magnetization texture known as a magnetic skyrmion. Because of their distinct topological properties, skyrmions are treated as ideal information carriers for memory, logic, and neuromorphic devices. Skyrmions have been observed in various magnetic systems, even at room temperature. Furthermore, skyrmions have been intentionally created using diverse methods, including a pulsed local magnetic field or spin-orbit torque-based perturbation. However, to date, skyrmions have only been discovered in extremely narrow ranges of material parameters. For example, more than ten multilayers of [heavy metal/ferromagnet/heavy metal] are required to induce a large dipole field, and the ability to control the thickness within 0.1 nm or less is essential for achieving a particular PMA value. Moreover, most reported creations of magnetic skyrmions are based on local nucleation at defects with reduced anisotropy, which disturbs their motion and increases the critical threshold for driving skyrmions. Creation of multiple bubble domains after applying a tilted magnetic field has also been reported, but the mechanism remained unclear. Although nearly perfect skyrmion lattices have been demonstrated in helimagnets, a universal way of creating a large number of skyrmions in a typical PMA film system has been lacking. Here, the authors present a universal method for magnetic skyrmion generation with experimental proof of the microscopic mechanism. The method applies to a broad range of PMA and DMI parameters and enables generation of many skyrmions even in high-PMA samples that usually show large domains and do not produce individual skyrmions.

Samples: The stack was SiO2/W (5 nm)/Co2FeB2 (1.3 nm)/Ta (0.12–0.14 nm wedge)/MgO (1 nm)/Ta (2 nm) on SiO2. PMA originates from the CoFeB/MgO interface. A Ta wedge layer inserted between CoFeB and MgO was used to tune PMA across the wafer. The summed magnetization is dominated by CoFeB volume, minimally affected by the Ta insertion. DMI arises primarily at the W/CoFeB interface, and the thin Ta insertion between CoFeB and MgO does not significantly modify DMI. PMA and saturation magnetization variations and DMI were characterized (details in supplementary, referenced). Imaging and fields: Magnetic domain patterns were observed using a magneto-optic Kerr effect microscope. Two magnetic field components were applied: a perpendicular field Hz and an in-plane field Hx. Initial zero-field states exhibited stripe domains with characteristic stripe width λ0 determined by FFT of domain images. Protocols: 1) Perpendicular-field sweeps: For various sample positions (i.e., different λ0 due to PMA variation), Hz was increased to determine the minimum field to erase stripes (Hz,s) and, starting from bubble states, the bubble disappearance field (Hz,B). These fields were plotted versus λ0 to map global minimum energy (GME) regimes for stripe, bubble, and uniform magnetization states. Sequential imaging documented transitions for narrow stripes (e.g., λ0 = 1.3 µm) and absence of bubble states for wide stripes (e.g., λ0 = 2.6 µm). 2) In-plane-field compression: From initial stripe states at zero field, Hx was applied incrementally while imaging. Stripe width λ (measured along y) decreased with Hx and recovered upon field removal, indicating reversible compression for modest Hx. Above a threshold (~0.45–0.56 kOe depending on position), discontinuous changes appeared with formation of isolated domains. The logarithm of stripe width varied linearly with Hx. At multiple positions with different initial λ0, the onset of isolated domains always occurred when λ was compressed to ~1.5–1.8 µm, indicating a universal critical width (~1.7 µm) for stripe instability. 3) Combined fields for bubble creation: Simultaneous application of Hx (to compress stripes below the instability) and a small Hz selected one polarity via Zeeman energy, yielding dense, circular bubble-filled states upon field removal. Field matrices were applied (e.g., Hx ≥ 0.54 kOe combined with varying Hz) for 1 s while preparing well-connected stripe states before each trial. 4) Time dependence and equilibration: Starting from three initial conditions (connected stripes, +z bubble-filled, −z bubble-filled), a field set (μ0Hz ≈ 2.8 Oe, μ0Hx ≈ 0.52 kOe) was applied for variable durations. After removal, the numbers of ±z isolated domains (N±) were counted; the normalized asymmetry (N−−N+)/Nmax converged exponentially within ~2 s to a common value irrespective of initial state, indicating approach to thermal equilibrium under the applied fields. Mechanistic interpretation: Thermal fluctuations perturb domain walls; for wide stripes, wall collisions are rare. Hx compresses stripes, increasing collision probability. Domain-wall magnetization has DMI-imposed chirality (from W/CoFeB), which stabilizes stripe connectivity. When Hx exceeds the effective DMI field (HDMI), domain-wall magnetizations align with Hx, reducing chiral stabilization. Collisions can proceed via two topologically distinct reconnection pathways; a small Hz breaks symmetry to favor one polarity, creating isolated domains of the selected core magnetization. Upon reducing Hx and Hz, DMI reasserts chiral structure, domain walls develop Bloch lines, and with sufficient HDMI or thermal activation, isolated bubbles attain full skyrmion spin texture. Demonstration: A wide-stripe state (λ0 ≈ 4.9 µm) was compressed by Hx to λ ≈ 1.2 µm, then a small Hz induced breakup into −z bubbles; simultaneous removal of fields left a bubble-filled state. Quantitative thresholds and example values: − Narrow stripes (<1.7 µm) under Hz alone undergo stripe→bubble→uniform transitions; wide stripes (>1.7 µm) do not produce bubble states under Hz alone because bubbles disappear at fields below stripe erasure (Hz,B < Hz,s). − Isolated domain creation consistently occurs when λ is reduced to ~1.5–1.8 µm via Hx; example thresholds: onset near μ0Hx ≈ 0.45–0.56 kOe; for μ0Hx < 0.54 kOe, increasing Hz tends to drive uniform states, whereas for μ0Hx ≥ 0.54 kOe, bubble-filled states are stabilized. − Time to equilibrium of isolated domain asymmetry under combined fields is ~2 s.

- Universal stripe-width criterion: A critical stripe width of approximately 1.7 µm governs the instability of stripe domains, independent of other material parameters (PMA, DMI). Isolated domains form when the in-plane field compresses stripes to λ ≈ 1.5–1.8 µm. - Limitation of Hz-only approach: For wide stripes (λ0 > 1.7 µm), increasing Hz cannot create a stable bubble regime; bubbles vanish at fields much lower than the field required to erase stripes (Hz,B ≪ Hz,s), so only stripe→uniform transition occurs. For narrow stripes (λ0 < 1.7 µm), canonical stripe→bubble→uniform transitions are observed. - In-plane field compression: The stripe width decreases with Hx (log λ linear in Hx), reversibly for modest fields. Above a threshold (∼0.45–0.56 kOe), discontinuous changes occur, producing isolated domains. - Bubble generation in wide-stripe samples: Simultaneous application of Hx (to reach the instability) and a small Hz selects one polarity via Zeeman energy and yields dense bubble-filled states upon field removal. Example: Hx ≥ 0.54 kOe with small Hz produces bubble populations, while Hx below this value does not. - Thermal equilibration: The difference in populations of ±z isolated domains converges exponentially to a common value within ~2 s under fixed fields, independent of initial state, indicating a thermally equilibrated bubble population. - Mechanism: Hx reduces stripe width and overcomes DMI-induced chiral stabilization (Hx > HDMI), increasing wall collision events. With Hz breaking symmetry, reconnection yields isolated domains of a chosen polarity; upon field reduction, DMI reinstates chiral domain-wall magnetization, forming Bloch lines and completing skyrmion texture via HDMI or thermal activation. - Applicability: The method generates many skyrmions even in high-PMA films that typically do not produce individual skyrmions, demonstrating generality across a broad parameter space.

The work addresses the central challenge of robust, universal skyrmion generation in PMA films by identifying stripe width as the governing parameter for stripe-domain instability and skyrmion (bubble) nucleation. By using an in-plane field to deterministically compress stripes to a universal critical width (~1.7 µm), the authors bypass the narrow materials parameter windows and defect-mediated nucleation common in prior approaches. The addition of a small perpendicular field selects the bubble polarity and stabilizes dense bubble arrays, even in systems with high PMA that ordinarily favor large stripe domains. This framework also clarifies prior reports of bubble creation under tilted fields by revealing the underlying sequence: stripe compression, domain-wall collision and reconnection, symmetry breaking by Hz, and DMI-governed relaxation to skyrmion textures. The findings thus provide a broadly applicable route to generate large skyrmion populations suitable for device concepts and enable controlled studies of skyrmion physics, including generation, stability, and annihilation mechanisms.

The study presents a universal method to generate magnetic skyrmion bubbles by controlling stripe-domain instability via an in-plane magnetic field. It demonstrates that stripe width—not specific material parameters—sets the criterion for instability at a critical value near 1.7 µm. Combining stripe compression (Hx) with a small perpendicular field (Hz) yields dense, polarity-selected bubble states and enables skyrmion formation even in high-PMA films. This general approach overcomes limitations of defect-mediated, locally nucleated skyrmions and resolves mechanisms behind tilted-field-induced bubble creation. The results open pathways for scalable skyrmion-based device architectures and systematic exploration of skyrmion generation and dynamics. Potential future directions include quantifying HDMI and its role across materials, optimizing spatiotemporal field protocols for on-demand, localized skyrmion writing, and integrating the method with current-induced manipulation in device geometries.