Initial Permeability Processes in Nickel-Cobalt Ferrites of Various Densities

Initial permeability has been measured at a frequency of 10 Mc/sec on ferrites of composition ${\mathrm{Ni}}_{\text{1−α}}{\mathrm{Co}}_{\alpha }{\mathrm{Mn}}_{\text{0.02}}{\mathrm{Fe}}_{\text{1.9}}{\mathrm{O}}_{\text{4±}}$ with varying densities. Here $\text{α\hspace{0.17em}=\hspace{0.17em}0,\hspace{0.17em}0.005,\hspace{0.17em}0.01,\hspace{0.17em}0.015,\hspace{0.17em}0.02,\hspace{0.17em}0.025,\hspace{0.17em}0.03,\hspace{0.17em}0.04,\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em} 0.06}$ , and density is varied for each composition between 3.8 g/cm³ and 5.1 g/cm³. The data are presented in a family of curves of permeability, $\mathrm{\mu \prime }$ , versus density with $\alpha$ as a parameter, and in a family of curves of $\mathrm{\mu \prime }$ versus $\alpha$ with density as a parameter. For $\text{α\hspace{0.17em}=\hspace{0.17em}0}$ , the extremely rapid increase of $\mathrm{\mu \prime }$ at high density is indicative of an increase in wall motion, the increase being much too sharp to explain by a rotational model. For $\text{α≥0.04}$ , this rapid increase is not seen, suggesting that the walls are almost completely pinned at these cobalt contents. For high densities, the curve of $\mathrm{\mu \prime }$ versus $\alpha$ shows a sharp decrease with the first cobalt addition, then rises to a maximum near $\text{α\hspace{0.17em}=\hspace{0.17em}0.03}$ , and decreases for further cobalt additions. But ferromagnetic resonance line width measurements show that the crystalline anisotropy is decreased by the first cobalt addition, reaches a minimum near $\text{α\hspace{0.17em}=\hspace{0.17em}0.03}$ , and subsequently increases again. Thus it is concluded that rotational contributions to $\mathrm{\mu \prime }$ must have been increased by the first cobalt addition; therefore some other mechanism of magnetization, presumably domain wall motion, existed in the beginning, and its contribution to $\mathrm{\mu \prime }$ was decreased by more than the rotational contribution was increased.