When using the bottom-up approach with anisotropic building-blocks, an important goal is to find simple methods to elaborate nanocomposite materials with a truly macroscopic anisotropy. Here, micrometer size colloidal mesoporous particles with a highly anisotropic rod-like shape (aspect ratio ≈ 10) have been fabricated from silica (SiO ) and iron oxide (Fe O ). When dispersed in a solvent, these particles can be easily oriented using a magnetic field (≈200 mT). A macroscopic orientation of the particles is achieved, with their long axis parallel to the field, due to the shape anisotropy of the magnetic component of the particles. The iron oxide nanocrystals are confined inside the porosity and they form columns in the nanochannels. Two different polymorphs of Fe O iron oxide have been stabilized, the superparamagnetic γ-phase and the rarest multiferroic ε-phase. The phase transformation between these two polymorphs occurs around 900 °C. Because growth occurs under confinement, a preferred crystallographic orientation of iron oxide is obtained, and structural relationships between the two polymorphs are revealed. These findings open completely new possibilities for the design of macroscopically oriented mesoporous nanocomposites, using such strongly anisotropic Fe O /silica particles. Moreover, in the case of the ε-phase, nanocomposites with original anisotropic magnetic properties are in view.

The ϵ‐Fe2O3 polymorph of iron oxide, which has outstanding physical properties, is successfully stabilized inside mesoporous silica particles with tailored shapes. Using mesoporous silica particles with three different morphologies of rod, platelet and donut, we obtain ϵ‐Fe2O3/mesoporous silica nanocomposites. Iron oxide is loaded inside the porosity using a two steps impregnation cycle: solvent‐free impregnation followed by oxidation under 1000 °C. The amount of loaded iron oxide can be enhanced using two successive impregnation cycles. We fully characterise these nanocomposite particles with a wide panel of techniques to establish the exact amount of loaded iron inside the porosity, the nature of the iron oxide phase and the size of the nanocrystals. As a result, we conclude that ϵ‐Fe2O3 nanocrystals can be confined and stabilized in all types of morphologies, even inside the donut morphology which possesses closed mesopores.