Nonequilibrium synthesis of silica-supported magnetite tubes and mechanical control of their magnetic properties.

Materials synthesis far from thermodynamic equilibrium can yield hierarchical order that spans from molecular to macroscopic length scales. Here we report the nonequilibrium formation of millimeter-scale iron oxide-silica tubes in experiments that tightly control the tube radius and growth speed. The experiments involve the hydrodynamic injection of an iron (II,III) solution into a large volume of solution containing sodium silicate and ammonium hydroxide. The forming tubes are pinned to a motorized glass rod that moves at a predetermined speed. X-ray diffraction and electron microscopy, as well as Raman and Mössbauer spectroscopy, reveal magnetite nanoparticles in the range of 5-15 nm. Optical data suggest that the magnetite particles follow first-order nucleation-growth kinetics. The hollow tubes exhibit superparamagnetic behavior at room temperature, with a transition to a blocked state at T(B) = 95 K for an applied field of 200 Oe. Heat capacity measurements yield evidence for the Verwey transition at 20 K. Finally, we show a remarkable dependence of the tubes' magnetic properties on the speed of the pinning rod and the injection rate employed during synthesis.