Nickel ions inducing growth of high-index faceted α-Fe2O3 and their facet-controlled magnetic properties

Abstract

In this study, single-crystalline α-Fe₂O₃ polyhedrons with high-indexed facets exposed were controllably synthesized through a simple hydrothermal route with Ni²⁺ ions as the structure-directing agent. The polyhedral crystals are quasi-cubic with an angle of approximately 86° and bound by (012), (10–2) and (1–12) facets. With insufficient Ni²⁺ ions added, only concave α-Fe₂O₃ crystals can be got while only irregular α-Fe₂O₃ crystals were obtained without the addition of Ni²⁺ ions. The growth process of α-Fe₂O₃ quasi-cubes was investigated and possible growth mechanism was proposed. Magnetic measurements confirm that the α-Fe₂O₃ nanocrystals with different morphologies display shape and surface-dependent magnetic behaviors. As the morphology of α-Fe₂O₃ crystals change from irregular particles to concave cube and then quasi-cubes with the increase of added Ni²⁺ amounts, the magnetism of the obtained α-Fe₂O₃ alters from spin-canted ferromagnetism, coexistent spin-canted and defect magnetism and then defect magnetism at room temperature.