Permeability of hybrid soft magnetic composites

The magnetic permeability of a three-phase hybrid soft magnetic composite (SMC) is formulated, extending Eshelby’s equivalent inclusion method to account for the three-phase microstructure of this SMC. The composite contains two kinds of ellipsoidal fillers dispersed in a matrix. All of the components have distinct magnetic properties. For the first time, we found that the magnetic permeability, μ, of the SMC strongly depends on whether the matrix is magnetic or non-magnetic so that, for a given composition, μ becomes much smaller for the case of a non-magnetic matrix than for a magnetic matrix. By increasing the volume fraction of the non-magnetic phase, the effective permeability of the SMC decreases almost linearly for a magnetic matrix. However, if the matrix is non-magnetic, the permeability of the SMC decreases very strongly with a small amount of the non-magnetic phase, even if fillers with very-high permeability are included in the SMC. For a SMC with non-magnetic matrix, the only way to keep the permeability at a useful level is to reduce the amount of the non-magnetic phase to less than 5 vol.%. The simulated results are compared with the experimental permeability measured for SMCs containing two types of fillers: Fe-based amorphous alloy and crystalline Fe. The magnetic fillers are surrounded by insulating materials.