Temperature dependence of the FMR absorption lines in viscoelastic magnetic materials

The magnetic properties of the viscoelastic materials filled with magnetic nanoparticles strongly depend on the viscosity of these materials. The time–temperature changes of the viscosity can affect the orientation ordering of the anisotropy axes of the magnetic nanoparticles after an external magnetic field is applied. In consequence, the absorption lines obtained in the ferromagnetic resonance experiment (FMR) will possess an additional temperature dependence through the viscosity of the materials under consideration. The particular case of the temperature dependence of the FMR signal detected from the γ-Fe2O3 (maghemite) magnetic nanoparticles dispersed at low concentration (0.1 wt.%) in a poly(ether-ester) multiblock copolymer (PEN-block-PTMO)) matrix has been investigated. A strong increase of the resonance line amplitude is observed with increasing temperature. We note also two qualitatively different temperature dependences of the magnetic resonance field at low and high temperatures. A simplified theoretical model of a single cluster consisting of N magnetic nanoparticles in an elastic polymer matrix is introduced to explain these temperature dependences of the absorption lines. The model is based on the stochastic version of the Lifshitz–Landau equation for nanoparticle magnetization.