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The temperature rise of NiFe204 nanoparticles having diameters of 242 and 7.7 nm was measured. The results of the measurement were analyzed by comparing the areas of the hysteresis loops in order to clarify the mechanism of heat dissipation in the samples. The hysteresis loops were obtained by applying both ac and dc magnetic fields. It was found that the contribution of magnetic relaxation losses to the heat dissipation was negligible in the case of NiFe204 nanoparticle of diameter 242 nm. The contribution of the NCel relaxation to the heat dissipation in the case of NiFe204 nanoparticle of diameter 7.7 nm was observed as the difference between the areas of ac and dc hysteresis loops. From the dependences of temperature rise and hysteresis loops of the nanoparticles on the intensity and frequency of an applied magnetic field, the relaxation time for NiFe204 nanoparticle of diameter 7.7 nm was obtained as approximately 0.5-0.7 ps. O 201 0 American Institute of Physics.