Formation of spherical and rod-shaped ε-Fe2O3 nanocrystals with a large coercive field

epsilon-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 epsilon-Fe2O3 nanoparticle of 16 nm diameter was obtained at a sintering temperature of 1050 degreesC. When an appropriate amount of IIA metal ion (Sr2+ or Ba2+) was added to this system, epsilon-Fe2O3 particles of larger particle size up to 100-200 nm were produced with rod-like shape under a 1000 degreesC sintering temperature. Magnetic measurements showed that the rod-like epsilon-Fe2O3 particles exhibited a giant coercive field up to 2.0 T (20 kOe). The reason why such a rod-like epsilon-Fe2O3 particle is obtained is that (1) the formation of IIA metal ion-containing components around the epsilon-Fe2O3 surface helps to stabilize and extend the existence of epsilon-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 epsilon-Fe2O3 nanocrystals.