The use of ferromagnetic nanoparticles for hyperthermia and thermoablation therapies has shown great promise in the field of nanobiomedicine. Even local hyperthermia offers numerous advantages as a novel cancer therapy; however, it requires a remarkably high heating power of more than 1 kW g−1 for heat agents. As a candidate for high heat generation, we focus on ferromagnetic nanoparticles and compare their physical properties with those of superparamagnetic substances. Numerical simulations for ideal single-domain ferromagnetic nanoparticles with cubic and uniaxial magnetic symmetries were carried out and MH curves together with minor loops were obtained. From the simulation, the efficient use of an alternating magnetic field (AMF) having a limited amplitude was discussed. Co-ferrite nanoparticles with various magnitudes of coercive force were produced by co-precipitation and a hydrothermal process. A maximum specific loss power of 420 W g−1 was obtained using an AMF at 117 kHz with H 0 = 51.4 kA m−1 (640 Oe). The relaxation behaviour in the ferromagnetic state below the superparamagnetic blocking temperature was examined by Mössbauer spectroscopy.

Heating characteristics of Fe oxide nanoparticles designed for hyperthermia were examined. Samples with coercive forces from 50 to 280 Oe(codoped magnetite) were produced with a coprecipitation technique following by hydrothermal reaction. The maximum specific loss powers (SLPs) of 420 W/g was obtained at 117 kHz (640 Oe) for a dispersant sample with coercive force of 280 Oe (ATH9D). SLPs measured on dry powder samples at 17 kHz and measured at 117 kHz on dispersant samples were compared. The measured SLP amplitudes are lower for 17 kHz and higher for 117 kHz than those expected from ferromagnetic dc minor loops. For the 117 kHz case, friction of particles in a carrier fluid (similar mechanism to Brown relaxation in superparamagnetic dispersant samples) is considered to contribute to the heating mechanism.

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 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.

We developed cobalt-containing Fe3O4 particles, (Co, Fe)3O4, whose particle sizes and magnetic properties were precisely controlled to apply hysteresis-loss heating materials in thermoablation. The crcivity of the (Co, Fe)3O4 particles was adjusted in the range of 19.1 to 42.1 kA/m by changing the content of Co, while the particle size was maintained about 20 nm. SiO2-coated (Co, Fe)3O4 particles were dispersed in water. The temperature increase characteristics were examined for the dispersant containing (Co, Fe)3O4 particles with the crcivity of 24.0 kA/m by applying a 120kHz ac magnetic field with an amplitude up to 50.1 kA/m. The temperature increase ΔT/Δt was proportional to the amplitude of magnetic field, but was not saturated even when a magnetic field of 50.1 kA/m was applied. The specific loss power (SLP) calculated from the ΔT/Δt and the particle content of the dispersant did not show a clear dependence on the coercivity, but a maximum value of over 1000 W/g was obtained for a dispersant containing (Co, Fe)3O4 particles with a coercivity of 24.0 kA/m under an ac magnetic field of 50.1 kA/m.