This study deals with the noteworthy effects of inner demagnetizing effects on the ferrimagnetic resonance (FMR) of disordered heterostructures, in particular, close to their percolation concentration (C p) in magnetic material. A recognized experimental fact is that the FMR increases with decreasing soft ferrite content in such composite materials. We have undertaken a systematic experimental study of Ni 1Àx Zn x Fe 2 O 4 composite materials, at the low frequency range and at their gyroresonance. Our interpretation of the data, based on the Effective Medium Theory, uses a reciprocity principle developed for heterostrucures. At concentrations below C p magnetic gaps, which originate from the magnetic dilution, may cause a cutoff of the magnetic flux-paths in the composite medium, leading to inner demagnetizing effects that increase the magnetic anisotropy. Whereas above C p ; the topology of the percolating cluster allows a magnetic flux path to be continuous throughout the material, here the inner demagnetizing effects vanish, leading the FMR to be that of the bulk itself in the concentration range going from C p to unity. r