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Magnetic Properties of Nickel–Zinc Ferrite Toroids Prepared from Nanoparticles

2007; Wiley; Volume: 90; Issue: 11 Linguagem: Inglês

10.1111/j.1551-2916.2007.01981.x

1551-2916

Brian Naughton, Paweł W. Majewski, David R. Clarke,

Characterization and Applications of Magnetic Nanoparticles

Toroids comprised of silica‐coated 10 nm diameter nickel–zinc (Ni–Fe) ferrite nanoparticles (Ni 0.5 Zn 0.5 Fe 2 O 4 ) have been fabricated by careful control of both the coating process and subsequent densification by viscous sintering. A narrow processing window is identified between a maximum temperature at which the nanoparticles coarsen, losing their super‐paramagnetic properties, and a lower temperature required for viscous flow densification. Key to the successful fabrication was drying and cold isostatic pressing of the silica‐coated nanoparticles; other routes invariably led to cracking during either drying or sintering. The super‐paramagnetic blocking temperature, the coercive field, and remanent magnetization could all be controlled over a wide range by varying the thickness of the silica coating from 1 to 15 nm. The dipole–dipole coupling distance is estimated to be 4 nm. The high‐frequency (1–500 MHz) properties were sensitive to the sintering temperature as well as the thickness of the silica coating. Toroids sintered at 1000°C or less exhibited no high‐frequency magnetic losses and their permeability decreased with increasing temperature, suggesting that the permeability was controlled by thermally activated magnetization relaxation.