Hard magnetism in structurally engineered silica nanocomposite

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 CoFe 2 O 4 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 CoFe 2 O 4 phase, and it is affected by antiferromagnetic hematite (-Fe 2 O 3 ) and olivine-type cobalt silicate (Co 2 SiO 4 ), as seen in its paramagnetic behavior at the annealing temperature of 430 C. The early formation of Co 2 SiO 4 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 ( d 100 = 1.5 nm). In addition, nonstoichiometric Co x Fe 1 x O with metal vacancies at 600 C, and spinel Co 3 O 4 at 700 C accompany major CoFe 2 O 4 . 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 SiO 4 polyhedra at an elevated temperature. Architectural engineering of silica nanocomposites with structural intricacy and desirable hard magnetism is achieved by rapid thermal process.