Microstructural Evolution and Magnetic Properties of NiFe_2O_4 Nanocrystals Dispersed in Amorphous Silica

Abstract

NiFe_2O_4 nanocrystals were dispersed in silica by a sol−gel route. The dried gel was amorphous, in which isolated Fe^(3+) ions had a weak interaction with silica matrix, as characterized by a weak IR absorption at ca. 580 cm^(-1). Heat treatment at 400 °C resulted in nickel ferrite clusters being partially formed, and these clusters were observed to interact with the matrix through Si−O−Fe bonds. This interaction reached its maximum with the complete formation of NiFe_2O_4 clusters as the temperature was raised to 600 °C. Above this temperature, NiFe_2O_4 clusters grew larger into nanocrystals, while the interaction between the nanocrystals and silica matrix disappeared with breakage of Si−O−Fe bonds. The grain growth for magnetic nanoparticles was accompanied with rearrangement of amorphous silica network. The preference of forming NiFe_2O_4 nanocrystals eliminated the possibility of precipitation of crystallite component oxides, e.g., NiO, γ-Fe_2O_3, or Fe_3O_4 in amorphous silica matrix, or crystalline silica, e.g., cristobalite or quartz, even when the treatment temperature was 1100 °C. Fe ions in silica glasses were determined by Mössbauer spectroscopy to be present exclusively as Fe^(3+) ions in a high-spin state at octahedral coordination, and the chemical environment of the Fe^(3+) ions seemed to remain unchanged until the nickel ferrite clusters crystallized. The formation mechanism for NiFe_2O_4 nanocrystals can be explained in terms of Ni^(2+) ions shifting from the tetrahedral centers to undistorted octahedral sites in the spinel lattice and the partial transformation of FeO_6 octahedron to FeO_4 tetrahedron. The critical dimension for the NiFe_2O_4 nanocrystals in silica was detected as ca. 9 nm. Below the critical size, NiFe_2O_4 nanocrystals had a superparamagnetic single-domain structure, while the nanocrystals with particle sizes larger than the critical size exhibited bulklike behavior.