Control of morphology and formation of highly geometrically confined magnetic skyrmions

Abstract

The ability to controllably manipulate magnetic skyrmions, small magnetic whirls with particle-like properties, in nanostructured elements is a prerequisite for incorporating them into spintronic devices. Here, we use state-of-the-art electron holographic imaging to directly visualize the morphology and nucleation of magnetic skyrmions in a wedge-shaped FeGe nanostripe that has a width in the range of 45–150 nm. We find that geometrically-confined skyrmions are able to adopt a wide range of sizes and ellipticities in a nanostripe that are absent in both thin films and bulk materials and can be created from a helical magnetic state with a distorted edge twist in a simple and efficient manner. We perform a theoretical analysis based on a three-dimensional general model of isotropic chiral magnets to confirm our experimental results. The flexibility and ease of formation of geometrically confined magnetic skyrmions may help to optimize the design of skyrmion-based memory devices.

Quantification of Magnetic Surface and Edge States in an FeGe Nanostripe by Off-Axis Electron Holography

“…4(e)], (ii) in higher applied magnetic fields, the Skyrmions become round, with diameters that are larger than the 80 nm characteristic diameter of Skyrmions in an extended thin film [35]. The formation of elliptically deformed Skyrmions with large diameters can be ascribed to a strong confinement effect, as discussed recently for a single Skyrmion chain in an FeGe nanostripe [22]. In the present study, the width of the nanostripe (∼330 nm) spans a pair of edge twists and three rows of Skyrmions.…”

“…4(b). The elongation is thought to result from the geometrical confinement effect, with the Skyrmions flexibly changing their shape in response to the specimen geometry [19,20,22]. The elongated distortions became less pronounced with increasing applied magnetic field: the Skyrmions adopted more circular shapes and decreased in size in a 200 mT field [ Fig.…”

“…3(b), 3(c), and (iv) the edge states persist up to very high applied magnetic fields, at which the Skyrmions are transformed to a conical or a field-polarized ferromagnetic state [ Fig. 3(h)] [22].…”

“…In particular, the LM technique is not suitable for studying magnetic nanostructures that have sizes far below 100 nm because the formation of Fresnel fringes arising from changes in specimen thickness at the boundaries of nanostructures complicates interpretation of the magnetic contribution. Off-axis EH provides the phase of the electron wave function that has passed through a sample, allowing the magnetic signal to be separated more easily than using other TEM-based techniques when studying Skyrmions in confined nanostructures [22,23].…”

“…Experimental details.-A thin nanostripe of noncentrosymmetric single crystalline B20 FeGe was prepared using focused ion beam milling, as described elsewhere [12,22]. Structural characterization was performed using an FEI F20 TEM operated at 200 kV and is given in the Supplemental Material [24] (Figs.…”