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The frequency behavior of the initial permeability of MnZn ferrites is generally described by Snoek’s law related to gyromagnetic resonance. However, in polycrystalline systems, for the same initial permeability, the frequency stability can be tailored by modifying the specific resistivity of the grain boundaries. This is achieved through the accumulation of high resistivity doping phases along the grain boundaries. The grain boundary specific resistivity may decrease or increase with increasing the average grain size depending on the ferrite composition and the extent of dopant segregation. In order to explain the experimental results and to account for the permeability dependency on the specific resistivity, a modification of the nonmagnetic grain boundary model is proposed to a less magnetic grain boundary model, where not the thickness but the initial permeability of the grain boundary layer is the main variable that is influenced by the process parameters.