Magnetic properties of nickel-zinc ferrite toroids prepared from nanoparticles

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 degrees 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.