Formation of spherical and rod-shaped ε-Fe2O3nanocrystals with a large coercive field

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Abstract
ε-Fe2O3/SiO2 composites were prepared by combination of reverse micelle and sol–gel methods. The samples were characterized by X-ray powder diffraction and transmission electron microscopy. A spherical ε-Fe2O3 nanoparticle of 16 nm diameter was obtained at a sintering temperature of 1050 °C. When an appropriate amount of IIA metal ion (Sr2+ or Ba2+) was added to this system, ε-Fe2O3 particles of larger particle size up to 100–200 nm were produced with rod-like shape under a 1000 °C sintering temperature. Magnetic measurements showed that the rod-like ε-Fe2O3 particles exhibited a giant coercive field up to 2.0 T (20 kOe). The reason why such a rod-like ε-Fe2O3 particle is obtained is that (1) the formation of IIA metal ion-containing components around the ε-Fe2O3 surface helps to stabilize and extend the existence of ε-Fe2O3 as metastable phase during sintering, and (2) the selective adsorption of IIA metal ions on the tetrahedral site of the Fe2O3 surface leads to the anisotropic crystal growth of ε-Fe2O3 nanocrystals.