Abstract: Magnetic hyperthermia is a mechanism that allows the conversion of electromagnetic field energy into heat by utilizing magnetic nanoparticles. To this, an external ac magnetic field, typically in the radiofrequency range, is applied to magnetic colloids containing magnetic monodomain nanoparticles resulting in the dissipation of heat energy into the surrounding medium. This work focuses on investigating iron oxide magnetic nanoparticles (IONPs), specifically Fe3O4 (magnetite), with diameters of 17 nm and 25 nm. Magnetic hyperthermia experiments, conducted at frequencies between 150 kHz and 488 kHz with magnetic field amplitudes up to 550 Oe, revealed notable values of specific absorption rate (SAR), reaching 405 W/g for the 25 nm sample and 642 W/g for the 17 nm sample. Intriguingly, a reproducible increase in SAR was observed over time when applying the magnetic field, peaking at 768 W/g for the 17 nm sample, prompting ongoing investigation into its origin Ver menos