Two series of nanosized cobalt spinel ferrites CoFe2O4 are synthesized from metal salts using high-energy ball milling with the addition of NaCl as a growth agent (series CFO-NaCl), and without (CFO Series). The particle properties are characterized using atomic force microscopy, as well as magnetic and calorimetric measurements. It is shown that the average sizes of the nanoparticles were ∼5.6 and ∼10.3 nm for CFO and CFO-NaCl series, respectively. We performed magnetostatic measurements and determined the parameters that are required to analyze the magnetic state and remagnetization processes of the nanoparticles. It is shown that the blocking temperature is ≈160 K for CFO samples and ≈300 K for the CFO-NaCl series. It was concluded that at 293 K the CFO series particles exhibit a superparamagnetic state, whereas the CFO-NaCl series are in the blocked state. The specific loss power that is scattered by the synthesized nanoparticle ensembles placed in an alternating magnetic field, is measured experimentally and theoretically assessed. The nature of the processes that determine the thermal characteristics of the nanoparticles is analyzed.

The magnetic hysteresis loops in fields ranging from 0 to ± 2.5 kOe and the dielectric permittivity ɛ (measured at 1 kHz, 1 MHz, and 8.3 GHz) of composite multiferroic materials (1–х)BiFeO3–xYMnO3 (0 ≤ x ≤ 0,5) have been studied. Ceramic solid solution samples were prepared using nitrate technology. It was found that the (1–x)BiFeO3–xYMnO3 (0.1 < x < 0.5) compounds are magnetically soft ferromagnetic materials that are sensitive to weak magnetic fields. The coercive field Hc is ∼20–32 Oe. Maximum magnetization was observed for samples with х = 0.35. For samples with 0 ≤ x ≤ 0.4 at Т = 300 K and a frequency of 1 kHz, ɛ ∼17.6–248.6, at a frequency of 1 MHz ɛ ∼7.8–63.2, and at a frequency of 8.3 GHz, ɛ ∼4 ± 0.3.