Natural ferromagnetic resonance in $\gamma$-Fe$_2$O$_3$ and CoFe$_2$O$_4$ nanopowders

$\gamma$-Fe$_2$O$_3$ and CoFe$_2$O$_4$ nanopowders were prepared by the chemical precipitation method. Magnetization curves, ferromagnetic resonance curves, and dependences of the temperature of the powders on time were measured during the absorption of microwave field energy at a frequency of 8.9 GHz at different values of the magnetic field strength. It was found that the greatest temperature increment of cobalt ferrite powders is achieved in the absence of an applied constant magnetic field – at natural ferromagnetic resonance. The frequencies of natural ferromagnetic resonance of the prepared powders were determined. Heating of magnetic nanoparticles in the field of their magnetic anisotropy (natural ferromagnetic resonance) can find application in biomedicine.