The theory of ferromagnetic resonance (FMR) in an assembly of single-domain uniaxially anisotropic particles is being applied to suspensions. The approach used has, as its basis, the independent-grain model once proposed for the description of FMR in polycrystals. We surmise that the basic assumptions of this model are also appropriate for composites of fine magnetic particles. With regard to the specific properties of the systems in question, we extend the model taking into account (i) the thermofluctuational effects (superparamagnetism) and (ii) the orientational mobility of the particles which leads to the change of the orientational distribution of their anisotropy axes under the influence of the applied field. The joint action of these effects imparts some specific features to the FMR spectra of a magnetic suspension. For example, in a certain temperature range two peak absorption lines must occur.

On the basis of Brown’s kinetic equation a consistent study of the regimes of cyclic magnetization reversal of single-domain particles with a uniaxial anisotropy is performed. The applied field is harmonic and linearly polarized, its amplitude equals the maximal coercive force of a Stoner-Wohlfarth particle. The dynamic magnetic hysteresis loops are obtained for a particle (oriented particle ensemble), whose easy-magnetization axis is tilted to the field direction under an arbitrary angle. It is shown that the Stoner-Wohlfarth regime (often termed as quasistatic) is able to describe the behavior of a nanoparticle only in a quite limited material and external parameter range. The developed approach has at least two major merits: it enables one to consider dynamic magnetic hysteresis in the temperature-frequency domains inaccessible with the aid of approximate methods, and provides a tool to test the accuracy of the latter.