Artificial Double-Helix for Geometrical Control of Magnetic Chirality

Abstract

Chirality plays a major role in nature, from particle physics to DNA, and its control is much sought-after due to the scientific and technological opportunities it unlocks. For magnetic materials, chiral interactions between spins promote the formation of sophisticated swirling magnetic states such as skyrmions, with rich topological properties and great potential for future technologies. Currently, chiral magnetism requires either a restricted group of natural materials or synthetic thin-film systems that exploit interfacial effects. Here, using state-of-the-art nanofabrication and magnetic X-ray microscopy, we demonstrate the imprinting of complex chiral spin states via three-dimensional geometric effects at the nanoscale. By balancing dipolar and exchange interactions in an artificial ferromagnetic double-helix nanostructure, we create magnetic domains and domain walls with a well-defined spin chirality, determined solely by the chiral geometry. We further demonstrate the ability to create confined 3D spin textures and topological defects by locally interfacing geometries of opposite chirality. The ability to create chiral spin textures via 3D nanopatterning alone enables exquisite control over the properties and location of complex topological magnetic states, of great importance for the development of future metamaterials and devices in which chirality provides enhanced functionality.

Keywords: 3D; X-ray; chirality; double-helix; nanomagnetic; nanoprinting; topological.

Figure 1

Artificial double-helix nanomagnets. (a–c) Competing magnetic interactions in an artificial double-helix. Colors separate different regions of the system (two strands in green and magenta and core in yellow), with no difference in material composition. Arrow insets indicate spin alignment inside the dashed boxes. (a) Dipolar stray field minimization promotes an antiparallel state between strands. (b) Exchange coupling in the overlap region (yellow) favors the parallel alignment of spins. (c) Geometric chirality favors a given sense of spin rotation in a strand via shape anisotropy. (d) 3D-printing of a cobalt nanohelix by FEBID. After injection of Co₂(CO)₈ into the chamber of a scanning electron microscope (SEM) using a gas injection system (GIS), the focused electron beam (in green and magenta) alternatively exposes the two helix strands. (e) Colored SEM image of the nanostructure under investigation, consisting of two double-helices of opposite chirality joined at the tendril perversion marked with *. Scale bar 250 nm, image tilt 45°.

Figure 2

Geometric imprinting of magnetic chirality. (a) Schematic setup for synchrotron X-ray magnetic microscopy. X-rays are focused by a capillary condenser onto the sample. A magnified image is formed on a CCD by a Fresnel Zone plate. Two exposures with different polarization (selected by a slit) are taken to extract X-ray magnetic circular dichroism (XMCD) contrast, which is proportional to the x component of the nanostructure’s magnetization. (b) XMCD image of the double-helix studied, which changes geometric chirality at the *. Image at zero field, after application of a saturating field along −z. (c, d) Micromagnetic simulations of a RH (top) and LH (bottom) double-helix, after application of a saturating external field along −z. (c) Magnetic state for strands and core. (d) Computed XMCD signal from simulations in panel c. (e) XMCD image of the double-helix under study in the as-grown state. (f–h) Micromagnetic simulations for a RH (top) and LH (bottom) double helix with antiparallel magnetic alignment of the strands. (f) Internal spin structure for strands and core. (g) Corresponding calculated XMCD contrast. (h) Selected xy cross sections at different heights, revealing a chiral interstrand Bloch domain wall. (i) Volumetric representation of the 3D spin configuration of a single LH double helix with antiparallel magnetic alignment of the strands (gray arrows) and a helical Bloch domain wall in the core (color arrows). Scale bars in panels b and e, 200 nm.

Figure 3

Localized 3D spin texture and topological defect in a chirality interface. (a) Schematics of the experimental procedure by which the double-helix is rotated to measure X-ray magnetic images at different angles. (b) Reference electronic-contrast X-ray transmission image. Each double-helix half-pitch is marked from I to IX (tendril perversion located at IV*). (c) XMCD measurements taken at several rotation angles. A vortex-like spin state is identified from the angular evolution of the magnetic contrast in region *. (d) Simulated XMCD contrast for the spin structure in panel e, reproducing the magnetic pattern experimentally observed upon rotation. (e) Micromagnetic simulations of the linkage between two double-helices of opposite chirality with antiparallel magnetic alignment of their strands (blue and red arrows). LH (bottom) and RH (top) Bloch domain walls are present in the core (white arrows), and an asymmetric 3D vortex state emerges at the tendril perversion *. Represented viewpoint corresponds to 0° measurements. (f) Subset of spins in panel e after untwisting the magnetization state. Line cross sections are taken between the centers of the two helix strands at different heights and rotated into the xy plane revealing that the change in magnetic chirality (transition from a RH to a LH Bloch wall) takes place via the formation of an asymmetric vortex (gray area). The Néel-type defect mediating opposite chiralities (i), the vortex core (ii), and the deformed Bloch-wall (iii) discussed in the main text are indicated. Scale bar in panel c, 200 nm.