The magnetic properties and hyperthermia effect were studied in a magnetorheological fluid (MRF) containing iron particles of \(1 \upmu \mathrm{m}\, \text{ to}\, 5 \,\upmu \mathrm{m}\) in diameter. The measurements showed that the magnetization in the saturation state reaches a value of 171 \(\text{ A}\cdot \text{ m}^{2}\cdot \mathrm{kg}^{-1}\) with very small values of coercivity and remanence. They also showed the ferromagnetic behavior in the system together with a value of the magnetic susceptibility of 1.7. Theoretical and experimental results of the calorimetric effect investigation under a changeable magnetic field of high frequency (\(f = 504\) kHz) in an MRF will be presented in the article. The sample was subjected to an alternating magnetic field of different strengths (\(H = 0\) to 4 \(\text{ kA}\cdot \text{ m}^{-1})\). It results from a theoretical analysis that the heat power density (released in the MRF sample) referenced to the eddy current is proportional to the square of frequency, the magnetic field amplitude, and the iron grain diameter. Experimental results indicate that there are some reasons for the released heat energy such as: energy losses from magnetic hysteresis and eddy currents induced in the iron grains. If the magnetic field intensity amplitude grows, the participation of losses connected with magnetic hysteresis is increased. From the calorimetric measurements, the conclusion is as follows: for a magnetic field \(H<1946\,\text{ A}\cdot \mathrm{m}^{-1}\), the eddy current processes dominate in the heat generation mechanism, whereas hysteresis processes for the total release of thermal energy dominate for higher magnetic fields. Both mechanisms take equal parts in heating the tested sample at a magnetic field intensity amplitude \(H= 1946\,\text{ A}\cdot \mathrm{m}^{-1}\). The specific absorption rate referenced to the mass unit of the MRF sample at the amplitude of the magnetic field strength 4 \(\text{ kA}\cdot \mathrm{m}^{-1}\) equals 24.94 \(\text{ W} \cdot \mathrm{kg}^{-1}\) at a frequency \(f\) = 504 kHz.

The heating ability of mineral oil-based magnetic fluids with different magnetic particle concentrations is studied. The calorimetric measurements were carried out in an alternating magnetic field of 500 A · m⁻¹ to 2500 A · m⁻¹ amplitude and of 1500 kHz frequency. The revealed H ⁿ law-type dependence of the temperature increase rate, (dT/dt) _t=0, on the amplitude of the magnetic field indicates the presence of superparamagnetic and partially ferromagnetic particles in the tested samples since n > 2. The specific absorption rate (SAR) defined as the rate of energy absorption per unit mass increases with a decrease of the volume fraction of the dispersed phase. This can be explained by the formation of aggregates in the samples with a higher concentration of magnetic particles.