Fig. 1: Schematic illustrations of the synthesis and the measurement concept.

a The synthesis protocol for obtaining core-shell nanorods is presented, which involves three steps: the hydrolysis of iron(III) chloride hexahydrate (FeCl₃ ∙ 6 H₂O) to akaganeite (β-FeOOH), the modification with sodium polyacrylate (PAA), and the deposition of a thin silica layer (SiO₂). In a subsequent hydrothermal step, these precursor particles are converted into cobalt-doped ferrite nanoparticles (Co_xFe_3-xO₄). The boxes visualize the surface coating of the nanoparticles in each step, respectively (orange: akaganeite with hydroxy (–OH) groups, grey: PAA layer with carboxy (–COOH) groups, blue: silica layer with –OH groups). b The SAXS images of the random (left) or chain-like (right) particle arrangements in dependence on the magnetic field strength H, which is perpendicular to the X-ray beam, are visualized. The chain-like orientation will manifest as an anisotropic pattern, whereas random orientations are generally more isotropic. The patterns were analyzed over all azimuthal angles φ, perpendicular, and parallel to the field to assess the extent of the anisotropy and to examine the chain-like structures in terms of width and length.