Sintered Ni_0.4Zn_0.6Fe₂O₄ ferrite with various additives, namely, CoO, Cu₂O, SiO₂, CaCO₃, and MgO was prepared by conventional ceramic processes. The formation of a single-phase cubic spinel structure was confirmed by powder X-ray diffraction. The microstructure and magnetic properties of the samples were significantly changed by the type of additive. In the frequency-dependent complex permeability (μ', μ") measurement, it was found that Cu₂O and CoO were more effective than the other additives for modulating the μ', μ" spectra. This was because a larger spectrum shift, either to a lower or higher frequency, could be achieved by the same doping amount of 1 wt.%. When the doping amount of CoO increased up to 3 wt.%, the μ', μ" spectra gradually shifted to a higher frequency following Snoek’s limitation law. The co-doping of 1.5 wt.% CoO and 0.5 wt.% Cu₂O was more effective for radio-frequency identification (RFID) applications than any individual doping because a relatively large real part of the permeability (μ' = 125.3) and small loss factor (μ " = 5.8) could be achieved simultaneously at the frequency of 13.56 MHz.