Vortex Domain Wall Imaging in a 3D Magnetic Nanotube Grown by Focused Electron Beam Induced Deposition

Theoretical predictions present the magnetic nanotube (MNT) as an ideal physical implementation for 3D spintronic devices due to the unique magnetization dynamics, with stable and fast domain wall (DW) motion beyond the Walker limit, given by its coreless curved topology. However, due to this heterogeneous architecture and the need to combine different synthetic methods, experimental realizations of nanometer-size MNTs are scarce and technically complex. Here, we report for the first time the fabrication of nanometer-scale 3D Co MNTs, synthesized by a single growth technique, namely focused electron beam induced deposition (FEBID), and thermally annealed in vacuum. Quantitative magnetic imaging of the remanent state and DW structure of a 10-nm-thick cylindrical Co nanotube grown on a 70-nm-wide vertical Pt-C nanowire template, carried out by off-axis electron holography in an aberration-corrected Lorentz (field-free) transmission electron microscope, reveals the nucleation and pinning of a head-to-head tilted vortex DW, whose structure has been confirmed by detailed micromagnetic simulations. The direct experimental evidence of such spin configuration stabilized in MNTs opens new venues for the investigation of curvature-driven spin textures and new technologies based on 3D nanomagnets.