We developed cobalt-containing Fe3O4 particles whose particle sizes and magnetic properties were precisely controlled so that they could be used as hysteresis-loss heating materials for thermoablation in cancer therapy. The (Co, Fe)3O4 particles were synthesized through the formation of crystalline nuclei from an alkali solution containing Co 2+ , Fe 2+ , and Fe 3+ ions, and then through crystalline growth using hydrothermal treatment. The coercivity was adjusted in the range from 150 to 490 Oe

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... by changing the Co content. The saturation magnetization was in the range of 76-80 emu/g. The crystalline size obtained from the (311) plane by XRD was almost constant at about 18 nm. The particle size observed in TEM photos was about 20 nm, irrespective of the cobalt content. SiO2 was coated on the surface of the (Co, Fe)3O4 particles to add dispersibility in a water medium. The dispersant of the SiO2-coated (Co, Fe)3O4 particles showed excellent dispersibility, and no precipitation of particles was observed after the solution had been standing for one month.