Novel frequency dispersion of permeability in nickel ferrites with different amounts of cobalt doping

The effect of cobalt doping on permeability spectra of nickel ferrites with excess iron (Ni_0.743-xMn_0.057Co_xFe _2.20O₄, x = 0-0.027) has been studied. With increasing amounts of Co-doping, the static permeability μ_i decreases at first, then increases to the local maximum at Co substitution of x = 0.018, and finally decreases again. The permeability spectra go through a modification of relaxation-resonance-relaxation dispersions. The samples with Co substitution of x = 0.018 and 0.021 have the static permeability of ∼13, resonance frequency of 400 MHz, and sufficiently low magnetic loss tangents, over 1-250 MHz, make them promising candidates for high frequency applications.