Hard magnetism in structurally engineered silica nanocomposite

Abstract

Creation of structural complexity by simple experimental control will be an attractive approach for the preparation of nanomaterials, as a classical bottom-up method is supplemented by a more efficient and more direct artificial engineering method. In this study, structural manipulation of MCM-41 type mesoporous silica is investigated by generating and imbedding hard magnetic CoFe₂O₄ nanoparticles into mesoporous silica. Depending on the heating rate and target temperature, mesoporous silica undergoes a transformation in shape to form hollow silica, framed silica with interior voids, or melted silica with intact mesostructures. Magnetism is governed by the major CoFe₂O₄ phase, and it is affected by antiferromagnetic hematite (α-Fe₂O₃) and olivine-type cobalt silicate (Co₂SiO₄), as seen in its paramagnetic behavior at the annealing temperature of 430 °C. The early formation of Co₂SiO₄ than what is usually observed implies the effect of the partial substitution of Fe in the sites of Co. Under slow heating (2.5 °C min⁻¹) mesostructures are preserved, but with significantly smaller mesopores (d₁₀₀ = 1.5 nm). In addition, nonstoichiometric Co_xFe_1−xO with metal vacancies at 600 °C, and spinel Co₃O₄ at 700 °C accompany major CoFe₂O₄. The amorphous nature of silica matrix is thought to contribute significantly to these structurally diverse and rich phases, enabled by off-stoichiometry between Si and O, and accelerated by the diffusion of metal cations into SiO₄ polyhedra at an elevated temperature.