X-ray diffraction of the different CNT. X-ray diffraction patterns of modified CNT before (panel a, PS-CNT) and after length reduction (panel b, S-CNT), and L-CNT (panel c), powder-like samples being placed in capillaries. The corresponding diffraction diagrams are drawn in (d). The solid and dotted lines indicate positions of diffraction peaks characteristic of iron oxide Fe3O4 nanoparticles and of inter-wall distance in MWCNT, respectively. The solid and dotted arrows in the inset point towards diffraction peaks characteristic of γ and α-iron nanoparticles, while their most intense diffraction peaks are located around 3 Å-1, where CNT contribution is also found. The broad peak below the 002 CNT peak (around 1.83 Å-1) is due to scattering from the glass capillary; its relative intensity with respect to other peaks is meaningless since it only reflects the density of the powder in the capillary.