LAUSR

Abstract The monodisperse CoFe 2 O 4 nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO 2 on the surface of the CoFe 2 O 4 nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures ( T c ) of the monodisperse CoFe 2 O 4 nanoparticles can be accurately measured because the SiO 2 shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe 2 O 4 @SiO 2 nanoparticles with core-shell structure in a wide temperature range (300～950 K) were investigated. It is remarkable that the coercive field ( H c ) of CoFe 2 O 4 nanoparticles increased from about 760 Oe to 1806 Oe after being coated with SiO 2 , which increased by 137.6% compared to the uncoated samples at 300 K. The saturation magnetization ( M s ) of the CoFe 2 O 4 @SiO 2 nanoparticles is 34.59 emu/g, which is about 52% of the naked CoFe 2 O 4 nanoparticles value (66.51 emu/g) at 300 K. The hysteresis loops of the CoFe 2 O 4 @SiO 2 nanoparticles showed an orderly magnetic behavior at high temperature, such as the M s , remanence magnetization ( M r ) and H c decreased as temperature increasing, being equal to zero near T c . This is a good indication that the CoFe 2 O 4 @SiO 2 nanoparticles are suitable for a wide variety of technological applications at high temperature.