Ferromagnetic resonance in the complex permeability of an Fe3O4-based ferrofluid at radio and microwave frequencies

The complex permeability of an iron-oxide nanoparticle suspension has been measured as a function of magnetic field strength at radio and microwave frequencies using a loop-gap resonator. The system studied was a water-based ferrofluid containing Dextran-coated Fe₃O₄ particles with an average diameter of 8 nm. The real part of the permeability increased sharply beyond a frequency-dependent threshold value of the static magnetic field before saturating. Just beyond this threshold field, there was a peak in the imaginary part of the permeability. The permeability measurements, which exhibited features associated with ferromagnetic resonance, were used to determine the dependence of the microwave absorption on static magnetic field strength. Using the absorption data, the g-factor of the ferrofluid was found to be 1.86 ± 0.07.