Heating of magnetic powders in the ferromagnetic resonance mode at a frequency of 8.9 GHz

Nickel ferrite nanoparticles 4 nm in size were synthesized by chemical deposition. Subsequent annealing at $T$ = 700$^\circ$C for 5 h led to an increase in the particle size to 63 nm. The Mössbauer spectra and the frequency-field dependences of ferromagnetic resonance have been measured. It has been shown that freshly prepared powders are superparamagnetic at room temperature. The kinetic dependences of the heating of nanoparticles in the ferromagnetic resonance mode at a frequency of 8.9 GHz were measured. It was found that the maximum rate of temperature increase in this mode for a ferromagnetic powder is an order of magnitude greater than for the superparamagnetic state (1.2 and 0.13 K/s, respectively). The latter is determined by the saturation magnetization of the studied powders.