Magnetic cores usable in gigahertz range: permalloy/Ni-Zn ferrite microcomposite made by low-temperature wet process

Permalloy (Ni₄₇Fe₅₃) microspheres of 4 and 8 μm in average diameter were encapsulated with Ni-Zn ferrite by ultrasound enhanced ferrite plating from an aqueous solution at 80°C at pH=6-9. The ferrite-encapsulated microspheres were compacted to toroidal cores, on which complex permeability (μ=μ´-jμ´´) was measured. The natural ferromagnetic resonance frequency f_r (criterion for usable frequency), and μ´ of the cores exceeded Snoek´s limit for Ni-Zn ferrite bulk by one order of magnitude. The cores exhibited one-turn-coil impedance exceeding 40 Ω at 1-3 GHz, suggesting that they will be applied to gigahertz electromagnetic compatibility bead inductors to suppress electromagnetic noises. In addition, the cores will be applied to low-loss inductors operated at tens of megahertz, since they have Q=μ´/μ´´>100 up to 45 MHz when the microspheres were plated at pH=6.