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We synthesized four kinds of magnetite particles having average sizes of 7, 18, 40, and 80nm and investigated their heating ability when they were dispersed in an agar gel and exposed to an ac magnetic field at 120kHz, a noninvasive frequency for anticancer hyperthermia. The particles 18nm in average diameter gave the highest heating ability, though they exhibited narrow hysteresis loops as compared to the particles having average diameters of 40 and 80nm. This indicates that hysteresis loss does not contribute much to the heat rise by the 120kHz ac field, and Néel relaxation is dominantly contributing to the heat rise by the 18nm sized particles. A calculation based on Néel relaxation loss gave a plausible explanation.

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