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.

• Received 23 March 1994

DOI:https://doi.org/10.1103/PhysRevB.50.6250