Preparation of fine Mg1−XCaXFe2O4 powder using reverse coprecipitation method for thermal coagulation therapy in an ac magnetic field

New magnetic materials having a potentially high heat generation ability in an ac magnetic field were studied for application in thermal coagulation therapy. The Mg1−XCaXFe2O4 ferrites with X varied from 0 to 1.0 were synthesized using a reverse coprecipitation method. The obtained precursors were calcined at various temperatures in the range of 300–1000 °C with the intent to obtain fine ferrite powders. The heat generation in an ac magnetic field was very poor for X = 0. However, the heat generation improved with partial Ca2+ substitution of the Mg2+ sites. The maximum rise in temperature (ΔT) in the ac magnetic field (370 kHz, 1.77 kA/m) was ca. 50 °C for the samples with 1.0 g in weight and 0.2–0.8 in X of Mg1−XCaXFe2O4. Especially, the samples calcined at 800 °C showed the highest heat generation. When the Ca2+-substituted samples were calcined at 900 °C, the heat generation was reduced due to decomposition into the two phases of the MgFe2O4-type cubic and CaFe2O4-type orthorhombic. In addition, the samples calcined at 300 °C also showed a high heat generation. Although the crystal and particle sizes increased with the calcination temperature, they did not influence the heat generation of this system. The heat generation of the samples was closely related to the hysteresis loss. The reason for the high heat generation properties of the samples calcined at 300 and 800 °C is ascribed to the increase in the hysteresis loss by distortion of the cubic crystal structure.