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Noise absorbing properties of Fe3O4 thin films have been analyzed in a microstrip line using an electromagnetic field simulator which employs the finite element method (FEM). With a simulation model of a microstrip line attached by an Fe3O4 thin film of good electrical conductivity (~ 10 Ω-1cm-1) and high magnetic loss (μ"r ~ 35), the S parameters and power absorption were calculated in the frequency range from 0.05 to 2.8 GHz. The simulation results of magnetic loss effect indicate that the S parameters and power absorption are dominantly controlled by the electrical properties of the thin film. Although the film has a large magnetic loss value due to ferromagnetic resonance, it is predicted that the power dissipation by magnetic loss is negligibly small. Sheet resistance controlled by film thickness is then the most important parameter for maximum power absorption. At the film thickness of 10 μm (corresponding sheet resistance is 100 Ω), the maximum power absorption was predicted to be about 80% at 1 GHz. For the conductive and magnetic Fe3O4 thin film, it is concluded that the dominant power loss mechanism is eddy current loss for the magnetic field or Ohmic loss for the electric field around the strip conductor.