| Abstract
| - Mechanical milling of pre-prepared layered double hydroxide carbonate, CoFe2(OH)4(CO3)2·nH2O, leads to the formation of cobalt ferrite, CoFe2O4, nanoparticles. The blocking temperature and coercive field increase with milling time. The hysteresis loop shift is associated with finite size effects and canting of surface moments.
- Cobalt ferrite, CoFe2O4, nanoparticles have been obtained from pre-prepared layered doublehydroxide carbonate, LDH-CO3, by mechanical milling. X-ray powder diffraction shows theonly product of the milling for 5 h of the LDH-CO3 is cobalt ferrite nanoparticles. Longer-term milling induces particle growth characterized by sharpening of the Bragg peaks andan increase of the blocking temperature, while prolonged milling results in the formation ofsome cobalt metal. Mössbauer spectra and temperature dependence of the magnetization ofthe 5-h milled sample suggest that it consists of nanoparticles of size less than 10 nm withblocking temperature of 200 K, in good agreement with microscopy showing an average sizeof 6 nm. The magnetic properties exhibit a strong dependence on the particle size as a resultof an unusual cation distribution and of surface effect. The saturation magnetization at 5 Kand the squareness of the hysteresis loops increase with the average particle size. Thedifference between zero-field-cooled and field-cooled hysteresis loops is correlated with thepresence of randomness of the moments at the surface.
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