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

Li, Liping and Li, Guangshe and Smith, R. L. and Inomata, H. (2000) Microstructural Evolution and Magnetic Properties of NiFe_2O_4 Nanocrystals Dispersed in Amorphous Silica. Chemistry of Materials, 12 (12). pp. 3705-3714. ISSN 0897-4756. doi:10.1021/cm000481l.

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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.

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Additional Information:© 2000 American Chemical Society. Received 13 June 2000. Published online 22 November 2000. Published in print 1 December 2000. This project was financially supported by a fund from NSFC (Grant No.19804005) (L.L.) of P. R. China.
Funding AgencyGrant Number
National Natural Science Foundation of China19804005
Issue or Number:12
Record Number:CaltechAUTHORS:20170201-100155193
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Official Citation:Microstructural Evolution and Magnetic Properties of NiFe2O4 Nanocrystals Dispersed in Amorphous Silica Liping Li, Guangshe Li, R. L. Smith, Jr., and H. Inomata Chemistry of Materials 2000 12 (12), 3705-3714 DOI: 10.1021/cm000481l
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73922
Deposited By: Ruth Sustaita
Deposited On:01 Feb 2017 20:03
Last Modified:11 Nov 2021 05:23

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