Thermal effect on magnetic parameters of high-coercivity cobalt ferrite
  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. 
E. F. Chagas, A. S. Ponce, R. J.Prado, G. M. Silva, J. Bettini and E.   Baggio-Saitovitch
cond-mat.mes-hall cond-mat.mtrl-sci