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

Permalloy (Ni/sub 47/Fe/sub 53/) microspheres of 4 and 8 /spl mu/m in average diameter were encapsulated with Ni-Zn ferrite by ultrasound enhanced ferrite plating from an aqueous solution at 80/spl deg/C at pH=6-9. The ferrite-encapsulated microspheres were compacted to toroidal cores, on which complex permeability (/spl mu/=/spl mu/'-j/spl mu/'') was measured. The natural ferromagnetic resonance frequency f/sub r/ (criterion for usable frequency), and /spl mu/' 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 /spl Omega/ 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=/spl mu/'//spl mu/''>100 up to 45 MHz when the microspheres were plated at pH=6.