Nucleation of magnetic skyrmions on curvilinear surfaces using local magnetic fields

TL;DR

This work investigates how curvature influences skyrmion formation by depositing Pt/Co/Ta multilayers on self-assembled polystyrene spheres and comparing them to planar films. Using STXM for planar samples and high-sensitivity in-vacuum MFM for spheres, the authors map maze-like domains on planar films and 3D spiraling stripe states on curved surfaces. They demonstrate that local magnetic fields from a high-moment MFM tip can rupture spiraling stripes on spheres, nucleating metastable skyrmions at the apex through repeated scans. The results establish curvilinear films as accessible platforms to study curvature-induced DMI and write/skyrmion nucleation on 3D magnetic architectures, with potential implications for 3D spintronic devices and neuromorphic computation.

Abstract

Magnetic skyrmions stabilized by interfacial Dzyaloshinskii-Moriya interactions (DMI) are promising candidates for applications in memory, logic, and neuromorphic computing. Beyond planar films, theoretical studies predict that curvature can influence skyrmion stability by introducing effective chiral interactions. Here, we investigate skyrmion formation on self-assembled polystyrene particles coated with Pt/Co/Ta multilayers by magnetron sputtering. Vibrating sample magnetometry reveals clear differences in the magnetic reversal behavior of the curvilinear film compared to that of the planar counterpart. Using non-invasive imaging methods such as scanning transmission X-ray microscopy and high-sensitivity in-vacuum magnetic force microscopy (MFM) with low moment magnetic tipcs, we observe a maze domain pattern for the planar films while the curvilinear film reveals three-dimensional spiraling stripe states. By employing a conventional MFM operating under ambient conditions requiring a tip with a higher magnetic moment, we demonstrate that these stripe states can rupture into metastable skyrmions located at the top of the spherical particles by applying consecutive scans. Our results demonstrate that curvilinear films offer an accessible platform for stabilizing single skyrmions using local magnetic field stimuli, opening pathways to study the interplay between interfacial and curvature-induced DMIs and enabling controlled skyrmion writing on three-dimensional magnetic architectures.

Figures (4)

• Figure 1: (a) Illustration of the multilayer stack sub./${\rm [Pt/Co/Ta]_{12}}$. (b) Schematic of the multilayer stack sputtered onto the particle monolayer with a zoomed-in cross-section, which highlights the existence of a thickness gradient across the surface of the spherical particle. (c) SEM image of the particle assembly (averaged particle diameter: $919nm$).
• Figure 2: Comparison of magnetic properties of planar and curvilinear films, where we show the M-H hysteresis loop as a function of applied oop field $\mu_0H_z$ in (a). The measurement geometry is explained in (b), for the loops presented in (c, planar), and (d, spheres), where we measured the loops for different angles of applied magnetic fields.
• Figure 3: (a-e) Magnetic domain configuration on planar films under different oop magnetic fields imaged by STXM at the Co $L_3$ edge, where the black contrast shows domains that are magnetized up, and white contrast indicates that domains that are magnetized down. (f-j) The magnetic states on the spheres are imaged using a single-pass MFM, where the distance between the tip and the surface is kept constant, thus, following the topography. The colors indicate the frequency shift, $\Delta f_0$, which is a measure of the local stray field gradients.
• Figure 4: (a) Remanent magnetic domain configuration on the spheres imaged by convential MFM at $\mu_0H_z = 0mT$. The red arrows in (a) indicate the stripes which show sudden relaxation lines and distortions. An oop magnetic field $\mu_0H_z = 35mT$ is applied by a using a permanent magnet in (b,c,d). The stripe domains in (b) appear to become unstable, as they exhibit many horizontal lines originating from tip-sample interactions. By further scanning same area (Scan 2, c) and (Scan 3, (e)) the stripes rupture into isolated single skyrmions at the top of the spheres. The MFM contrast is given by the phase shift $\Delta \phi_0$ of the cantilever. The red and green arrows in show where a stripe collapses into a skyrmions in (b to c) and (b, c to d), respectively.