Fe/ferrite composite magnetic cores far exceeding Snoek's limit fabricated by simplified ferrite plating in open air

Iron spheres 2.0 μm in diameter in distilled water was successfully encapsulated in dense Ni-Zn ferrite layer 15-20 nm thick by simplified ferrite plating method. In this method, a reaction solution of FeCl₂+NiCl₂+ZnCl₂ and a pH adjusting solution of KOH were added to the water. The solution was stirred at 25°C in open air. The dissolved oxygen worked to oxidize a part of Fe²⁺ to Fe³⁺. The deposited Ni-Zn ferrite layer on Fe microsphere surface was composed of fine grains 10-15 nm in diameter. Since the Ni-Zn ferrite layer was very thin and dense, a large saturation magnetization M_s=210 emu/g, almost the same as that of bare Fe, was obtained. The compacted cores of these Fe/ferrite microspheres exhibited excellent frequency characteristics exceeding Snoek's limit for Ni-Zn ferrite cores by factor 4.