Geometry-guided vortex-to-axial magnetization switching in 3D nickel conical nanowires.

Three-dimensional magnetic nanostructures with tailored geometry offer unprecedented opportunities for controlling magnetization dynamics at the nanoscale. Here, we report the large-scale fabrication of vertically aligned, mechanically robust nickel conical nanowires via galvanostatic pulse electrodeposition into asymmetrically etched ion-track polycarbonate templates. By systematically varying the methanol concentration in the etchant solution (40% vs. 70%), we achieve precise control over the conical geometry, with base diameters ranging from 2.9 to 3.5 μm, tip diameters from 67 to 96 nm, and aspect ratios up to 7.2. Combining vibrating sample magnetometry with advanced micromagnetic simulations using Mumax3-employing periodic boundary conditions to account for dipolar interactions in dense arrays-we elucidate the complex magnetization reversal mechanisms in these tapered structures. Our simulations reveal that at remanence, the magnetization adopts a vortex-like flux-closure configuration at the base while remaining axially aligned at the tip. Under applied fields, reversal nucleates at the base and propagates upward via a curling mechanism, governed by the geometry-induced spatial variation of shape anisotropy. The larger base diameter (70% methanol sample) stabilizes vortex states, manifesting as step-like features in experimental hysteresis loops-a signature of intermediate vortex formation during switching. The smaller cones (40% methanol) exhibit more coherent reversal with higher squareness ratios (Mr/Ms = 0.07 parallel vs. 0.05 perpendicular), demonstrating enhanced shape anisotropy dominance. Our work establishes conical nanowires as a promising model system for studying geometry-engineered magnetization dynamics, with potential implications for three-dimensional spintronic devices, neuromorphic computing concepts, and high-density magnetic memory.