Figure Presented Magnetic carbon nanotube (CNT) arrays are grown by in situ encapsulation of ferromagnetic iron carbide nanoparticles within the tube walls (see figure). The resulting structures, combining the inherently unique characteristics of aligned and open-ended CNTs with the fact that magnetic functionality is provided from "inside the walls", are promising functional units in highly efficient flow systems, for example, as parallel magnetic nanoreactors and in biological separations.