Nanosized MgFe2O4-based ferrite powder having heat generation ability in an AC magnetic field was prepared by bead milling and studied for thermal coagulation therapy applications. The crystal size and the particle size significantly decreased by bead milling. The heat generation ability in an AC magnetic field improved with the milling time, i.e. a decrease in crystal size. However, the heat generation ability decreased for excessively milled samples with crystal sizes of less than 5.5nm. The highest heat ability (Delta T34 degrees C) in the AC magnetic field (370kHz, 1.77kA/m) was obtained for fine MgFe2O4 powder having a ca. 6 nm crystalsize (the samples were milled for 6-8 h using 0.1 mm phi beads). The heat generation of the samples was closely related to hysteresis loss, a B-H magnetic property. The reason for the high heat generation properties of the samples milled for 6-8 h using 0.1 mm f beads was ascribed to the increase in hysteresis loss by the formation of a single domain. Moreover, the improvement in heating ability was obtained by calcination of the bead-milled sample at low temperature. In this case, the maximum heat generation (Delta=41 degrees C) ability was obtained for a ca. 11 nm crystal size sample was prepared by crystal growth during the sample calcination. On the other hand, the Delta T value for Mg0.5Ca0.5Fe2O4 was synthesized using a reverse precipitation method decreased by bead milling. (C) 2010 Elsevier B.V. All rights reserved.