Fig. 1.

Suspension of Fe${}_3$O${}_4$ nanoparticles (a) at the beginning and (b) at the end of the ligand exchange process. (c) Alignment and synthesis of MNBs was performed in a custom-made holder with a neodymium magnet next to the vial. (d) Representative electron micrographs of individual MNBs (white scale bars indicate the stated lengths). (e) Length distribution, $p(\ell )$, obtained from seven different batches. Beyond a peak around $\ell \approx 1\mathrm{\mu }$m a roughly exponential decay is observed (ensemble mean: $⟨\ell ⟩\approx 3.6\mathrm{\mu }$m).