Magnetic Properties of Nickel–Zinc Ferrite Toroids Prepared from Nanoparticles
Authors: Naughton, Brian T.; Majewski, Pawel; Clarke, David R.
Source: Journal of the American Ceramic Society, Volume 90, Number 11, November 2007 , pp. 3547-3553(7)
Abstract:Toroids comprised of silica-coated 10 nm diameter nickel–zinc (Ni–Fe) ferrite nanoparticles (Ni0.5Zn0.5Fe2O4) 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.
Document Type: Research Article
Affiliations: Materials Department, College of Engineering, University of California, Santa Barbara, California 93106-5050
Publication date: November 1, 2007