  We prepared very high-coercivity cobalt ferrite nanoparticles using
short-time high-energy mechanical milling. After the milling process, the
coercivity of the nanoparticles reached 3.75 kOe - a value almost five times
higher than that obtained for the non-milled sample. We performed a thermal
treatment on the milled sample at 300, 400, and 600 oC for 30 and 180 mins, and
studied the changes in the magnetic parameters due to the thermal treatment
using the hysteresis curves, Williamson-Hall analysis, and transmission
electron microscopy. The thermal treatment at 600 oC causes a decrease in the
microstructural strain and density of structural defects resulting in a
significant decrease in coercivity. Furthermore, this thermal treatment
increases the size of the nanoparticles and, as a consequence, there is a
substantial increase in the saturation magnetization. The coercivity and the
saturation magnetization are less affected by the thermal treatment at 300 and
400 oC.
