Metal ions induce growth and magnetism alternation of α-Fe ₂O ₃ crystals bound by high-index facets

In this study, quasi-cubic and hexagonal bipyramid α-Fe ₂O ₃ polyhedrons with high-index facets exposed were controllably synthesized by applying metal ions Zn ²⁺ or Cu ²⁺ as structure-directing agents. The growth of the α-Fe ₂O ₃ nanostructures with high-index facets were induced by metal ions without the addition of any other surfactants. The quasi-cubic form controlled by Zn ²⁺ looks like a cube but has an angle of approximately 86° bound by (012), (10-2), and (1-12) facets, whereas the hexagonal bipyramid form controlled by Cu ²⁺ has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape- and surface-dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi-cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal-ion-directing technique could provide a universal method for shape- and surface-controlled synthesis of nanocrystals with high-index facets exposed.