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Magnetic Properties of Nickel–Zinc Ferrite Toroids Prepared from Nanoparticles
Author(s) -
Naughton Brian T.,
Majewski Pawel,
Clarke David R.
Publication year - 2007
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2007.01981.x
Subject(s) - materials science , sintering , composite material , nanoparticle , coating , magnetization , coercivity , remanence , ferrite (magnet) , metallurgy , magnetic field , nanotechnology , condensed matter physics , physics , quantum mechanics
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.