Chirality Imprinting and Spin-texture Tunability in Conformally Coated 3D Magnetic Nanostructured Metamaterials

Three-dimensional (3D) magnetic nanostructures offer unprecedented opportunities for engineering emergent spin textures, but controlling their configuration remains a central challenge. Here we show that conformally coated Ni Nanotubes arranged in a woodpile geometry with lattice spacings ranging from 800 to 1200 nm, realised by two-photon lithography and atomic layer deposition, exhibit a geometry-tuneable balance between chiral and axial states. Magnetic force microscopy on the top layer of the 3D woodpile reveals that few-layer systems exhibit a chiral contrast whilst increasing the number of stacked layers drives a transition to an axial configuration with the change in state populations depending strongly on lattice spacing. Micromagnetic simulations demonstrate that chirality is not intrinsic to isolated tubes but is imprinted by spin textures formed in the substrate sheet film, which couple into the 3D network. As the sheet-film influence diminishes with increasing layer number, dipolar interactions dominate and stabilise the axial state. This two-stage mechanism of chirality imprinting followed by increasingly dominant dipolar interactions, provides clear control parameters for tailoring spin-texture populations. Our results establish conformally coated woodpiles as a reconfigurable 3D ferromagnetic metamaterial platform that can be exploited for data storage, magnonics, and neuromorphic computing.

💡 Research Summary

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In this work the authors present a novel three‑dimensional (3D) magnetic metamaterial platform based on conformally coated nickel (Ni) nanotubes arranged in a woodpile geometry. The structures are fabricated by combining two‑photon lithography (TPL) to create a scaffold of stacked nanowires with atomic‑layer deposition (ALD) that uniformly coats the scaffold with a thin Ni layer. By varying the lattice spacing between 800 nm and 1200 nm and by controlling the number of stacked layers from a single layer up to many layers, the authors demonstrate a systematic, geometry‑driven transition between chiral (handed) spin textures and axial (straight‑axis) spin textures.

Magnetic force microscopy (MFM) performed on the top surface of the woodpile reveals that thin, few‑layer systems display a clear chiral contrast, whereas increasing the number of layers progressively suppresses the chiral signal and yields an axial configuration. The authors attribute the initial chiral state to an “imprinting” effect: the underlying thin film (the substrate sheet) hosts a spiral spin texture that couples into the Ni‑coated tubes, imposing a handedness on the 3D network. Micromagnetic simulations corroborate this picture, showing that the substrate‑induced texture dominates the energy landscape for low‑layer counts. As the stack height grows, the influence of the substrate diminishes and dipolar interactions between neighboring tubes become the dominant term. These dipolar couplings favor a uniform axial alignment, thereby stabilising the axial state.

The paper therefore identifies a two‑stage mechanism for spin‑texture control: (1) a chirality‑imprinting stage governed by the substrate’s spiral domains, and (2) a dipolar‑dominance stage that emerges with increasing layer number. By tuning lattice spacing, layer count, Ni coating thickness, and material parameters such as saturation magnetisation and anisotropy, one can precisely set the relative populations of chiral versus axial domains.

This controllability opens up several technological opportunities. In data‑storage concepts, the coexistence of chiral and axial domains could be exploited for multi‑bit encoding within a single nanostructure. In magnonics, the ability to pattern handed versus straight spin textures provides a route to engineer spin‑wave propagation paths, non‑reciprocal behaviour, and band‑structure tailoring. In neuromorphic computing, the gradual tuning of domain populations can emulate synaptic weight adjustment in spin‑based artificial neurons.

Beyond the experimental demonstrations, the authors emphasize the predictive power of micromagnetic modelling. Simulations reproduce the MFM observations across the full range of geometries, enabling a priori design of woodpile parameters that yield a desired spin‑texture distribution. This reduces the need for extensive trial‑and‑error fabrication, thereby saving time and resources.

In summary, the study establishes conformally coated Ni woodpile structures as a reconfigurable 3D ferromagnetic metamaterial platform. The identified two‑stage mechanism—chirality imprinting followed by dipolar‑driven axial ordering—provides clear, tunable design knobs for engineering emergent spin textures. The work paves the way for exploiting 3D magnetic metamaterials in high‑density storage, advanced magnonic circuitry, and spin‑based neuromorphic architectures.