The dielectric and magnetic properties of Ni 1Àx Co x Fe 1.98 O 4 (with x 5 0.005, 0.01, 0.02, and 0.05) ferrite ceramics, doped with 5% Bi 2 O 3 as a sintering aid, were investigated. X-ray diffraction results indicated that there is no obvious reaction between Bi 2 O 3 and the ferrites within the temperature range. All samples were fully densified after sintering at temperatures of ! 10001C for 2 h. Densification behavior, microstructural development, and grain growth of the Ni 1Àx Co x Fe 1.98 O 4 ferrite ceramics are independent of the concentration of Co. The appropriate sintering temperature range for achieving a low dielectric loss tangent is 10001-10501C. The high dielectric loss tangent of the samples sintered at temperatures of o10001C is due mainly to the conduction losses caused by their porous microstructures, while that of the samples sintered at temperatures of 410501C is not readily explainable at the moment. The variation in the magnetic properties (static permeability) can be understood in terms of the magnetocrystalline anisotropic constant combined with the single-ion model. Promising magneto-dielectric properties have been obtained in Ni 0.99 Co 0.01 Fe 1.98 O 4 and Ni 0.98 Co 0.02 Fe 1.98 O 4 , both sintered at 10501C for 2 h, over 3-30 (high frequency) and 30-90 MHz, respectively. Such materials might find potential applications such as in antenna miniaturizations.
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