Mesoporous Cobalt Ferrite Thin Films
Ferrimagnetic frameworks with tunable coercivity and orientation
Periodic nanoporous cobalt ferrite (CFO) exhibits tunable room temperature ferrimagnetism. The porous cubic CFO frameworks are fabricated by coassembly of inorganic precursors with a large amphiphilic diblock copolymer, referred to as KLE. The inverse spinel framework boasts an ordered open network of pores averaging 14 nm in diameter. The domain sizes of the crystallites are tunable from 6 to 15 nm, a control which comes at little cost to the ordering of the mesostructure. Increases in crystalline domain size directly correlate with increases in room temperature coercivity. In addition, these materials show a strong preference for out-of-plane oriented magnetization, which is unique in a thin film system. The preference is explained by in-plane tensile strain, combined with relaxation of the out-of-plane strain through flexing of the mesopores. It is envisioned that the pores of this ferrimagnet could facilitate the formation of a diverse range of exchange coupled composite materials.
High-angle XRD experiments where the angle between the plane of the substrate and the incident beam, χ, was varied from 90 to 180°. The results show that the lattice constant in the plane of the film is larger than the lattice constant normal to the substrate, indicating an in-plane tensile strain. Annealing at higher temperature relaxes the expansive strain. As CoFe2
is magnetostrictive, these results can be used to explain both the propensity for out-of-plane magnetization and the reduction in this effect with increased thermal annealing.