Microwave composites filled with ferromagnetic films

For many technical applications, magnets with high microwave permeability are needed. For such magnets, both static permeability μ_s and the ferromagnetic resonance frequency f_r must be high. These values depend greatly on the manufacturing process of the magnet, though are tightly related each to other. The microwave performance of bulk magnets is governed by well known Snoek´s law. According to computations, thin films are better than bulk magnets in their potential microwave performance. Also, thin ferromagnetic films may have the highest microwave permeability values of all magnets. However, bulk samples are necessary for most applications. As ferromagnets are conductors, the high-frequency behavior of thick ferromagnetic films may be due to the effect of eddy currents rather than due to the ferromagnetic resonance. For normally incident waves, dielectric response arising because of the conductivity is dominant over the magnetic response. These drawbacks are overcame with the use of magnetic composites filled with pieces of thin ferromagnetic films. In this case, μ_s and f_r are still related, with the volume fraction of the magnetic films, p, and a proper randomization factor being introduced as a factor. It is easily estimated that even at small p, e.g., p=0.1 and with the values of f_r and 4πM_o, μ≈20 that is still larger than the-values typical for microwave ferrites.